drbd-reactor.promoter

title: drbd-reactor.promoter(5)

summary: drbd-reactor promoter plugin

text:
# drbd-reactor.promoter(5)

## NAME
drbd-reactor.promoter — drbd-reactor promoter plugin

## DESCRIPTION
drbd-reactor.promoter is the promoter plugin for drbd-reactor(1).

The promoter plugin monitors events on resources an executes systemd units. This plugin can be used for simple high-availability.

By default the plugin generates a series of systemd service overrides (i.e., what systemd calls "Drop-In") and a systemd target unit that contains dependencies on these generated services. The services, and their order, is defined via the list specified in start.

The plugin generates two implicit extra units:

- a drbd-promote@ override that promotes the DRBD resource (i.e., switches it to Primary). This is a dependency for all the other units from start (according overrides are generated).

- a drbd-services@ target that subsumes all the generated dependencies from the start list.

Let's look at a simple example to see which overrides get generated from a dummy start list like this:

```
[[promoter]]
[promoter.resources.foo]
start = [ "a.service", "b.service", "c.service" ]
```

- /var/run/systemd/system/drbd-promote@foo.d/reactor.conf containing the necessary pieces to wait for the backing devices of DRBD resource “foo” and to promote it to Primary.

- /var/run/systemd/system/a.service.d/reactor.conf containing a dependency on drbd-promote@foo

- /var/run/systemd/system/b.service.d/reactor.conf containing dependencies on drbd-promote@foo and on a.service.

- /var/run/systemd/system/c.service.d/reactor.conf containing dependencies on drbd-promote@foo and on b.service.

- /var/run/systemd/system/drbd-services@foo.target.d/reactor.conf containing dependencies on a.service, b.service, and c.service.

If a DRBD resource changes its state to “may promote”, the plugin (i.e., all plugins on all nodes in the cluster) start the generated systemd target (e.g., drbd-services@foo.target ). All will try to start the drbd-promote@ unit first, but only one will succeed and continue to start the rest of the services. All the others will fail intentionally.

In order to promote the resource with the best disk state, the plugin evaluates its local disk states and waits a short period of time, which might give a promoter on another host the chance (i.e., because its sleep time is shorter) to promote the resource first. Sleep times are calculated as follows:

DiskState

Sleep time in seconds

Diskless

6

Attaching

6

Detaching

6

Failed

6

Negotiating

6

Unknown

6

Inconsistent

3

Outdated

2

Consistent

1

UpToDate

0

The actual sleep time is calculated as the worst case of all the volumes in a resource and can be scaled by setting sleep-before-promote-factor.

If a resource loses “quorum”, it stops the systemd drbd-services@ target and all the dependencies. Stopping services on the node that lost quorum is the standard behavior one would expect from a cluster manger. There might be scenarios where it is preferable to freeze the started service until quorum is gained again. As this requires multiple prerequisites to hold true, freezing a resource on quorum loss is described in FREEZING RESOURCES.

The plugin's configuration can contain an action that is executed if a stop action fails (e.g., triggering a reboot). Start actions in start are interpreted as systemd.unit(5). and have to have an according postfix like .service or .mount. ocf resource agents are supported via the ocf.rs@ service. Please see OCF RESOURCE AGENTS for a detailed overview.

The configuration can contain a setting that specifies that resources are stopped whenever the plugin exits (e.g., on service restart).

It also contains a runner that can be set to shell. Then the items in start are interpreted as shell scripts and started in order (no explicit targets or anything) and stopped in reverse order or as defined via stop. This can be used on systems without systemd and might be useful for Windows systems in the future. If you can, use the default systemd method, it is the preferred one.

In order for DRBD to use quorum as needed by this plugin, make sure the resource file of the DRBD resource contains the following option (this is the default for LINSTOR, but your resources needs to qualify for quorum).

```
options {
   auto-promote no;
   quorum majority;
   on-suspended-primary-outdated force-secondary;
   on-no-quorum io-error; # for the default drbd-reactor on-quorum-loss policy (i.e., Shutdown)
   # on-no-quorum suspend-io; # for freezing resources
   on-no-data-accessible io-error # always set this to the value of on-no-quorum!
   # on-no-data-accessible suspend-io # for freezing, always set this to the value of on-no-quorum!
}
# net { rr-conflict retry-connect; } # for freezing resources
```

drbd-reactor itself is pretty relaxed about these settings, don't expect too much hand holding or even auto-configuration, you as the admin are the one that should understand your system, but it checks properties and writes warnings to the log (file/journal) if misconfiguration is detected.

## SERVICE DEPENDENCIES
Let's get back to our simple example:

```
[[promoter]]
[promoter.resources.foo]
start = [ "a.service", "b.service", "c.service" ]
```

As we noted in previously we generate a dependency chain for these services (i.e., all depend on drbd-promote@ as well as on the previous services). The strictness of these dependencies can be set via dependencies-as , where the default is Requires systemd.unit(5)

We also generate the mentioned drbd-services@.target , which lists all the services from start. The dependencies for that are generated via the value set in target-as.

Especially when one debugs services it might make sense to lower these defaults to for example Wants. Otherwise a failed service might prohibit a successful start of the drbd-services@.target , which then triggers a stop of the target and its dependencies, which might again trigger a start because the resource is DRBD promotable again and so on.

It is really up to you and how strict/hard you want your dependencies and what their outcome should be. Requires should be a good default, you might lower or increase the strictness depending on the scenario.

## OCF RESOURCE AGENTS
It is possible to use resource agents in the start list of services via ocf:$vendor:$agent instance-id name=value ... The instance-id is user defined and gets postfixed with _$resourcename. For example the generated systemd unit for an instance-id of “p_iscsi_demo1” for a DRBD resource “foo” would be ocf.rs@p_iscsi_demo1_foo. name / value pairs are passed to the unit as environment variables prefixed with OCF_RESKEY_. In a concrete example using the “heartbeat:IPaddr2” agent this could look like this:

```
start = [
  "foo.service",
  "ocf:heartbeat:IPaddr2 p_iscsi_demo1_ip ip=10.43.7.223 cidr_netmask=16",
  "bar.service"
]
```

## FREEZING RESOURCES
The default behavior when a DRBD Primary loses quorum is to immediately stop the generated target unit and hope that other nodes still having quorum will successfully start the service. This works well if services can be failed over/started on another node in reasonable time. Unfortunately there are services that take a very long time to start, for example huge data bases.

When a DRBD Primary loses its quorum we basically have two possibilities:

- the rest of the nodes, or at least parts of it still have quorum: Then these have to start the service, they are the only ones with quorum, but still we could keep the old Primary in a frozen state. And then, when the nodes with quorum come into contact with the old Primary, then it should stop the service and its storage should become in sync with the other nodes.

- the rest of the nodes are not able to form a partition with quorum. In such a scenario there are no alternatives anyways, we would need to keep the Primary frozen. But if the nodes eventually join the old Primary again, and quorum would be restored, we could just unfreeze/thaw the old Primary (which is also the new Primary).

There are several requirements for this to work properly:

- A system with unified cgroups. If the file /sys/fs/cgroup/cgroup.controllers exists you should be fine. That requires a relatively “new” kernel. Note that “even” RHEL8 for example needs the addition of systemd.unified_cgroup_hierarchy on the kernel command line.

- a service that can tolerate to be frozen

- DRBD option on-suspended-primary-outdated set to force-secondary

- DRBD option on-no-quorum set to suspend-io

- DRBD option on-no-data-accessible set to suspend-io

- DRBD net option rr-conflict set to retry-connect

If these requirements are fulfilled, then one can set the promoter option on-quorum-loss to freeze.

## PREFERRED NODES
While in a HA cluster that deserves the name every node needs to be able to run all services, some users like to add preferences for nodes. This can be done by setting a list of preferred-nodes. On resource startup a delay based on the node's position in the list is added. Node names need to match the output of uname -n. Nodes with a lower preference will sleep longer. By default, if a node joins on DRBD level, and that peer's disk becomes UpToDate, and the peer has a higher preference, then the active node stops the services locally. As it will then have a higher sleep penalty as the preferred node, the preferred one will take over the service (if it can). If preferred-nodes-policy is set to always the system behaves as described above. If it is set to start-only, then preferred nodes are only taken into account when a service can be startet, but it will not fall back to a more preferred node when the service is already running on a lower priority node and a higher priority nodes joins later.

## TWO NODE CLUSTERS WITH FENCING
If possible the promoter plugin should be used in setups that allow for DRBD quorum. Two node clusters with properly configured fencing are the exception. We only support fencing as split-brain avoidance mechanism in two node clusters, everything else should use DRBD quorum. The policy gets auto detected as follows:

- if DRBD quorum is something other than “off”, quorum is used

- if DRBD quorum is “off” and DRBD fencing is something other than “dont-care”, fencing is used

Before a secondary attempts to promote, it has to give the primary some time to fence the secondary. This should usually be fast (i.e., remotely switching off power). In fencing mode the value of fencing-promote-delay is used to sleep the given number of seconds before the promote attempt.

## ADJUSTING RESOURCES ON START
When the promoter plugin starts, it runs drbdadm adjust on all configured resources by default. This ensures that the DRBD resource configuration on disk matches the running kernel state. The plugin waits for the backing devices to become available before running adjust.

This behavior can be disabled per resource by setting adjust-resource-on-start to false. Disabling it may be useful if drbdadm adjust is managed externally, or if its execution during startup causes undesirable side effects in a specific setup.

## LOCAL DISK FAILURE
If the local DRBD disk on a Primary node gets detached (i.e., becomes Diskless), the behavior is controlled by the on-disk-detach policy. By default the policy is set to “ignore”, which means no action is taken. If set to “fail-over”, the promoter will stop services if an UpToDate peer is found, triggering a failover to that peer.

## FILES
/usr/lib/ocf/resource.d/

Path used for for OCF agents.

## EXAMPLES
This configures a promoter for resource “foo”. If the resource might be promoted the “foo.mount” and “foo.service” units get started. On DRBD demote failures a systemd "reboot" is triggered.

```
[promoter.resources.foo]
start = ["foo.mount", "foo.service"]
on-drbd-demote-failure =  "reboot"
stop-services-on-exit = false
preferred-nodes = ["nodeA", "nodeB" ]
```

## SEE ALSO
For further design notes and handled failure scenarios see doc/promoter.md in the source code. An online version is available at promoter.md

## AUTHORS
The drbd-reactor(1) program was written by the following authors:

Roland Kammerer <rck@linbit.com>

Moritz Wanzenböck <moritz.wanzenboeck@linbit.com>


drbd-reactor.toml

package: drbd-reactor

title: drbd-reactor.toml(5)

summary: DRBD-REACTOR daemon configuration file

text:
# drbd-reactor.toml(5)

## NAME
drbd-reactor.toml — DRBD-REACTOR daemon configuration file

## DESCRIPTION
drbd-reactor.toml is the configuration file for the DRBD-REACTOR daemon, drbd-reactor(1). The file has to be valid toml file.

The configuration file consists of sections for global settings, logging, and the actual plugin configuration. While this is rarely used and should be used with caution, it is possible to have multiple instances of a plugin as well as multiple loggers. The program ships with a commented version of the configuration file, please refer to that for further details.

Note that plugin settings are shown here for demonstration purposes only, plugin configuration should be stored in /etc/drbd-reactor.d/

## EXAMPLES

```
snippets = "/etc/drbd-reactor.d"
# update statistics every 45 seconds
statistics-poll-interval = 45

[[log]]
level = "debug"

# Configure a promoter
[[promoter]]
[promoter.resources.foo]
start = ["foo.mount", "foo.service"]
on-drbd-demote-failure = "reboot"

# Configure a user mode helper filter
[[umh]]
[[umh.resource]]
command = "drbd-reactor-role-change.sh"
event-type = "Change"
old.role = { operator = "NotEquals", value = "Primary" }
new.role = "Primary"

# Configure a prometheus exporter
[[prometheus]]
enums = true

# Configure an agentx subagent
[[agentx]]
peer-states = false
```

## AUTHORS
The drbd-reactor(1) program was written by the following authors:

Roland Kammerer <rck@linbit.com>

Moritz Wanzenböck <moritz.wanzenboeck@linbit.com>

## CAVEATS
Please note that the path specified in snippets gets path-joined with “*.toml” and the result is then subject to glob expansion.

branch: sisyphus


drbd-reactor.umh

title: drbd-reactor.umh(5)

summary: drbd-reactor user mode helper plugin

text:
# drbd-reactor.umh(5)

## NAME
drbd-reactor.umh — drbd-reactor user mode helper plugin

## DESCRIPTION
drbd-reactor.umh is the user mode helper plugin for drbd-reactor(1).

The user mode helper is a plugin that allows users to specify filters matched on DRBD state changes. If a filter rule matches, a user defined script gets executed. Information is passed to scripts via environment variables. For every event all rules are evaluated, so if multiple rules match, multiple actions are executed (concurrently).

There are 4 different types a rule can be defined for:

- RESOURCE RULES

- DEVICE RULES

- PEER DEVICE RULES

- CONNECTION RULES

It is important to note that fields not specified do not restrict the filter. Think of them as “don't care”. Make sure to read and understand the implied CAVEATS section.

## COMMON FIELDS
Every rule type (e.g., resource, device,...) has a set of common fields

Common fields

Description

Type

Mandatory

name

Name of the rule written to logs

String

no

command

Command/script to execute via sh -c

String

yes

env

User defined env variables

Map

no

Every type also has a common set of filters that can be used for matching.

Common filters

Description

Type

resource-name

Name of the DRBD resource

String

event-type

Type of the event

EventType

## STATE CHANGES
Every field shown in this section marked as state change can be set on the old and/or new section of a rule. This can be used to filter state changes (e.g., from Secondary to Primary ).

### RESOURCE RULES
Fields

Description

Type

State change

role

Role of the resource

Role

yes

may-promote

If the resource may be promoted

Boolean

yes

A match on such a rule calls the specified command and sets the following environment variables:

DRBD_RES_NAME

Name of the DRBD resource

DRBD_{OLD,NEW}_ROLE

Role of the resource

DRBD_{OLD,NEW}_MAY_PROMOTE

Resource may be promoted to Primary

### DEVICE RULES
Fields

Description

Type

State change

volume

Volume number

Integer

no

disk-state

Disk state of the device

DiskState

yes

client

Device is a DRBD client

Boolean

yes

quorum

Device has DRBD quorum

Boolean

yes

open

Device is opened

Boolean

yes

A match on such a rule calls the specified command and sets the following environment variables:

DRBD_RES_NAME

Name of the DRBD resource

DRBD_MINOR

Minor number of the device

DRBD_MINOR_$volume

Minor number of the device by volume

DRBD_VOLUME

volume (number) of the device

DRBD_BACKING_DEV

Block device path to backing device or “none”

DRBD_BACKING_DEV_$volume

Block device path to backing device or “none” by volume

DRBD_{OLD,NEW}_DISK_STATE

DiskState of the device

DRBD_{OLD,NEW}_CLIENT

Device was/is a DRBD client

DRBD_{OLD,NEW}_QUORUM

Device had/has DRBD qourum

DRBD_{OLD,NEW}_OPEN

Device was/is opened

### PEER DEVICE RULES
Fields

Description

Type

State change

volume

Volume number

Integer

no

peer-node-id

Node ID of the Peer

Integer

no

peer-disk-state

Disk state of the peer-device

DiskState

yes

peer-client

Peer-device is a DRBD client

Boolean

yes

resync-suspended

DRBD resync is suspended

Boolean

yes

replication-state

Replication state

ReplicationState

yes

A match on such a rule calls the specified command and sets the following environment variables:

DRBD_{OLD,NEW}_DISK_STATE

DiskState of the device

DRBD_RES_NAME

Name of the DRBD resource

DRBD_MINOR

Minor number of the device

DRBD_MINOR_$volume

Minor number of the device by volume

DRBD_VOLUME

volume (number) of the device

DRBD_BACKING_DEV

Block device path to backing device or “none”

DRBD_BACKING_DEV_$volume

Block device path to backing device or “none” by volume

DRBD_PEER_NODE_ID

Node ID of the peer

DRBD_{OLD,NEW}_PEER_DISK_STATE

DiskState of the peer-device

DRBD_{OLD,NEW}_PEER_CLIENT

Peer-device was/is a DRBD client

DRBD_{OLD,NEW}_PEER_RESYNC_SUSPENDED

Resync was/is suspended

DRBD_{OLD,NEW}_PEER_REPLICATION_STATE

ReplicationState

A note on DRBD_BACKING_DEV* : DRBD does not know the backing device path of its peer, so the device set in these variables is the local backing device path! Usually the backing device names on all peers are the same for diskful nodes, but it is not strictly required. This was not invented by drbd-reactor(1), this is how these variables always have been set when DRBD kernel called user mode helpers from kernel space. So this might be unexpected, but that is what it always was.

### CONNECTION RULES
Fields

Description

Type

State change

peer-node-id

Node ID of the Peer

Integer

no

conn-name

Name of the connection

String

yes

connection-state

Connection state

ConnectionState

yes

peer-role

Peer role

Role

yes

congested

Connection is congested

Boolean

yes

A match on such a rule calls the specified command and sets the following environment variables:

DRBD_RES_NAME

Name of the DRBD resource

DRBD_PEER_NODE_ID

Node ID of the peer

DRBD_CSTATE

Connection state

DRBD_{OLD,NEW}_CONN_NAME

Conneciton name

DRBD_{OLD,NEW}_CONN_STATE

Connection state

DRBD_{OLD,NEW}_PEER_ROLE

Peer role

DRBD_{OLD,NEW}_CONGESTED

Connection was/is congested

## ENVIRONMENT
Before a command is executed its environment gets deleted. Then the common environment variables in this section are set, followed by the filter type specific environment variables, and the user defined ones.

HOME

“/”

TERM

“Linux”

PATH

“/sbin:/usr/sbin:/bin:/usr/bin”

## OPERATORS
Currently filters that are set are compared for equality with the value received in a state update. One handy operator is "not equal", meaning everthing except the given value. We have to play within the boundaries of toml, and we did not want to sacrifice type safety we get for free from the parser by inventing our own “filter language”.

The default comparison operator is Equals

old.role = Primary # compares for equality

If another operator should be used, one has to specify the value and the operator

```
old.role = { operator = "NotEquals", value = "Primary" }
# which is toml equivalent to these two lines:
old.role.operator = "NotEquals"
old.role.value = "Primary"
```

It is not possible to mix and match these two notations:

```
old.role.operator = "NotEquals"
old.role = "Primary"  # fails. it requires a .value in this case
```

The allowed operators are:

- Equals (the default)

- NotEquals

## TYPES

### EVENT TYPES

- Exists

- Create

- Destroy

- Change

### RESOURCE ROLES

- Unknown

- Primary

- Secondary

### DISK STATES

- Diskless

- Attaching

- Detaching

- Failed

- Negotiating

- Inconsistent

- Outdated

- DUnknown

- Consistent

- UpToDate

### REPLICATION STATES

- Off

- Established

- StartingSyncS

- StartingSyncT

- WFBitMapS

- WFBitMapT

- WFSyncUUID

- SyncSource

- SyncTarget

- VerifyS

- VerifyT

- PausedSyncS

- PausedSyncT

- Ahead

- Behind

### CONNECTION STATES

- StandAlone

- Disconnecting

- Unconnected

- Timeout

- BrokenPipe

- NetworkFailure

- ProtocolError

- TearDown

- Connecting

- Connected

## EXAMPLES
Configure a user mode helper filter of type resource.

```
[[umh]]
[[umh.resource]]
command = "slack.sh $DRBD_RES_NAME on $(uname -n) from $DRBD_OLD_ROLE to $DRBD_NEW_ROLE"
event-type = "Change"
old.role = { operator = "NotEquals", value = "Primary" }
new.role = "Primary"
```

## AUTHORS
The drbd-reactor(1) program was written by the following authors:

Roland Kammerer <rck@linbit.com>

Moritz Wanzenböck <moritz.wanzenboeck@linbit.com>

## CAVEATS
As it was mentioned before, fields that are not set are not taken into consideration when matching the filter. Let's look at how one might write a filter:

```
[[umh.resource]]
command = "/usr/local/bin/primary.sh"
resource-name = "foo"
new.role = "Primary"
```

What this means is that this filter does *not* care about the state of the old role. So if the resource changes for whatever reason, not related to it's role, an update is sent and the current state is matched against the rule. In this case it would trigger as the the role "changes" from the old state “don't care” (i.e., already Primary) to current/new state Primary.

Most rules are written to match specific state changes anyways, so a natural fit would be:

```
[[umh.resource]]
command = "/usr/local/bin/primary.sh"
resource-name = "foo"
old.role = "Secondary"
new.role = "Primary"
```

Another possibility for more complex fields than a resource's role, capturing everthing besides a given target value look like this. See section OPERATORS for details:

```
[[umh.resource]]
command = "/usr/local/bin/primary.sh"
resource-name = "foo"
old.role = { operator = "NotEquals", value = "Primary" }
new.role = "Primary"
```


package: drbd-utils

title: drbd.conf-8.4(5)

summary: Configuration file for DRBD\*(Aqs devices

text:
# drbd.conf-8.4(5)

## NAME
drbd.conf - Configuration file for DRBD's devices

## INTRODUCTION
The file /etc/drbd.conf is read by drbdadm.

The file format was designed as to allow to have a verbatim copy of the file on both nodes of the cluster. It is highly recommended to do so in order to keep your configuration manageable. The file /etc/drbd.conf should be the same on both nodes of the cluster. Changes to /etc/drbd.conf do not apply immediately.

By convention the main config contains two include statements. The first one includes the file /etc/drbd.d/global_common.conf, the second one all file with a .res suffix.

```
resource r0 {
	net {
		protocol C;
		cram-hmac-alg sha1;
		shared-secret "FooFunFactory";
	}
	disk {
		resync-rate 10M;
	}
	on alice {
		volume 0 {
			device    minor 1;
			disk      /dev/sda7;
			meta-disk internal;
		}
		address   10.1.1.31:7789;
	}
	on bob {
		volume 0 {
			device    minor 1;
			disk      /dev/sda7;
			meta-disk internal;
		}
		address   10.1.1.32:7789;
	}
}
```

In this example, there is a single DRBD resource (called r0) which uses protocol C for the connection between its devices. It contains a single volume which runs on host alice uses /dev/drbd1 as devices for its application, and /dev/sda7 as low-level storage for the data. The IP addresses are used to specify the networking interfaces to be used. An eventually running resync process should use about 10MByte/second of IO bandwidth. This resync-rate statement is valid for volume 0, but would also be valid for further volumes. In this example it assigns full 10MByte/second to each volume.

There may be multiple resource sections in a single drbd.conf file. For more examples, please have a look at the DRBD User's Guide[1].

## FILE FORMAT
The file consists of sections and parameters. A section begins with a keyword, sometimes an additional name, and an opening brace (“{”). A section ends with a closing brace (“}”. The braces enclose the parameters.

section [name] { parameter value; [...] }

A parameter starts with the identifier of the parameter followed by whitespace. Every subsequent character is considered as part of the parameter's value. A special case are Boolean parameters which consist only of the identifier. Parameters are terminated by a semicolon (“;”).

Some parameter values have default units which might be overruled by K, M or G. These units are defined in the usual way (K = 2^10 = 1024, M = 1024 K, G = 1024 M).

Comments may be placed into the configuration file and must begin with a hash sign (“#”). Subsequent characters are ignored until the end of the line.

### Sections
skip

Comments out chunks of text, even spanning more than one line. Characters between the keyword skip and the opening brace (“{”) are ignored. Everything enclosed by the braces is skipped. This comes in handy, if you just want to comment out some 'resource [name] {...}' section: just precede it with 'skip'.

global

Configures some global parameters. Currently only minor-count, dialog-refresh, disable-ip-verification and usage-count are allowed here. You may only have one global section, preferably as the first section.

common

All resources inherit the options set in this section. The common section might have a startup, a options, a handlers, a net and a disk section.

resource name

Configures a DRBD resource. Each resource section needs to have two (or more) on host sections and may have a startup, a options, a handlers, a net and a disk section. It might contain volumes sections.

on host-name

Carries the necessary configuration parameters for a DRBD device of the enclosing resource. host-name is mandatory and must match the Linux host name (uname -n) of one of the nodes. You may list more than one host name here, in case you want to use the same parameters on several hosts (you'd have to move the IP around usually). Or you may list more than two such sections.

```
	resource r1 {
		protocol C;
		device minor 1;
		meta-disk internal;
		on alice bob {
			address 10.2.2.100:7801;
			disk /dev/mapper/some-san;
		}
		on charlie {
			address 10.2.2.101:7801;
			disk /dev/mapper/other-san;
		}
		on daisy {
			address 10.2.2.103:7801;
			disk /dev/mapper/other-san-as-seen-from-daisy;
		}
	}

```

See also the floating section keyword. Required statements in this section: address and volume. Note for backward compatibility and convenience it is valid to embed the statements of a single volume directly into the host section.

volume vnr

Defines a volume within a connection. The minor numbers of a replicated volume might be different on different hosts, the volume number (vnr) is what groups them together. Required parameters in this section: device, disk, meta-disk.

stacked-on-top-of resource

For a stacked DRBD setup (3 or 4 nodes), a stacked-on-top-of is used instead of an on section. Required parameters in this section: device and address.

floating AF addr:port

Carries the necessary configuration parameters for a DRBD device of the enclosing resource. This section is very similar to the on section. The difference to the on section is that the matching of the host sections to machines is done by the IP-address instead of the node name. Required parameters in this section: device, disk, meta-disk, all of which may be inherited from the resource section, in which case you may shorten this section down to just the address identifier.

```
	resource r2 {
		protocol C;
		device minor 2;
		disk      /dev/sda7;
		meta-disk internal;
		# short form, device, disk and meta-disk inherited
		floating 10.1.1.31:7802;
		# longer form, only device inherited
		floating 10.1.1.32:7802 {
			disk /dev/sdb;
			meta-disk /dev/sdc8;
		}
	}

```

disk

This section is used to fine tune DRBD's properties in respect to the low level storage. Please refer to drbdsetup(8) for detailed description of the parameters. Optional parameters: on-io-error, size, fencing, disk-barrier, disk-flushes, disk-drain, md-flushes, max-bio-bvecs, resync-rate, resync-after, al-extents, al-updates, c-plan-ahead, c-fill-target, c-delay-target, c-max-rate, c-min-rate, disk-timeout, discard-zeroes-if-aligned, rs-discard-granularity, read-balancing.

net

This section is used to fine tune DRBD's properties. Please refer to drbdsetup(8) for a detailed description of this section's parameters. Optional parameters: protocol, sndbuf-size, rcvbuf-size, timeout, connect-int, ping-int, ping-timeout, max-buffers, max-epoch-size, ko-count, allow-two-primaries, cram-hmac-alg, shared-secret, after-sb-0pri, after-sb-1pri, after-sb-2pri, data-integrity-alg, no-tcp-cork, on-congestion, congestion-fill, congestion-extents, verify-alg, use-rle, csums-alg, socket-check-timeout.

startup

This section is used to fine tune DRBD's properties. Please refer to drbdsetup(8) for a detailed description of this section's parameters. Optional parameters: wfc-timeout, degr-wfc-timeout, outdated-wfc-timeout, wait-after-sb, stacked-timeouts and become-primary-on.

options

This section is used to fine tune the behaviour of the resource object. Please refer to drbdsetup(8) for a detailed description of this section's parameters. Optional parameters: cpu-mask, and on-no-data-accessible.

handlers

In this section you can define handlers (executables) that are started by the DRBD system in response to certain events. Optional parameters: pri-on-incon-degr, pri-lost-after-sb, pri-lost, fence-peer (formerly oudate-peer), local-io-error, initial-split-brain, split-brain, before-resync-target, after-resync-target.

The interface is done via environment variables:

•DRBD_RESOURCE is the name of the resource

•DRBD_MINOR is the minor number of the DRBD device, in decimal.

•DRBD_CONF is the path to the primary configuration file; if you split your configuration into multiple files (e.g. in /etc/drbd.conf.d/), this will not be helpful.

•DRBD_PEER_AF , DRBD_PEER_ADDRESS , DRBD_PEERS are the address family (e.g. ipv6), the peer's address and hostnames.

DRBD_PEER is deprecated.

Please note that not all of these might be set for all handlers, and that some values might not be useable for a floating definition.

### Parameters
minor-count count

count may be a number from 1 to 1048575. Minor-count is a sizing hint for DRBD. It helps to right-size various memory pools. It should be set in the in the same order of magnitude than the actual number of minors you use. Per default the module loads with 11 more resources than you have currently in your config but at least 32.

dialog-refresh time

time may be 0 or a positive number. The user dialog redraws the second count every time seconds (or does no redraws if time is 0). The default value is 1.

disable-ip-verification

Use disable-ip-verification if, for some obscure reasons, drbdadm can/might not use ip or ifconfig to do a sanity check for the IP address. You can disable the IP verification with this option.

udev-always-use-vnr

When udev asks drbdadm for a list of device related symlinks, drbdadm would suggest symlinks with differing naming conventions, depending on whether the resource has explicit volume VNR { } definitions, or only one single volume with the implicit volume number 0:

```
# implicit single volume without "volume 0 {}" block
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>
# explicit volume definition: volume VNR { }
DEVICE=drbd<minor>
SYMLINK_BY_RES=drbd/by-res/<resource-name>/VNR
```

If you define this parameter in the global section, drbdadm will always add the .../VNR part, and will not care for whether the volume definition was implicit or explicit.

For legacy backward compatibility, this is off by default, but we do recommend to enable it.

usage-count val

Please participate in DRBD's online usage counter[2]. The most convenient way to do so is to set this option to yes. Valid options are: yes, no and ask.

protocol prot-id

On the TCP/IP link the specified protocol is used. Valid protocol specifiers are A, B, and C. Protocol A: write IO is reported as completed, if it has reached local disk and local TCP send buffer.

Protocol B: write IO is reported as completed, if it has reached local disk and remote buffer cache.

Protocol C: write IO is reported as completed, if it has reached both local and remote disk.

device name minor nr

The name of the block device node of the resource being described. You must use this device with your application (file system) and you must not use the low level block device which is specified with the disk parameter.

One can ether omit the name or minor and the minor number. If you omit the name a default of /dev/drbdminor will be used.

Udev will create additional symlinks in /dev/drbd/by-res and /dev/drbd/by-disk.

disk name

DRBD uses this block device to actually store and retrieve the data. Never access such a device while DRBD is running on top of it. This also holds true for dumpe2fs(8) and similar commands.

address AF addr:port

A resource needs one IP address per device, which is used to wait for incoming connections from the partner device respectively to reach the partner device. AF must be one of ipv4, ipv6, ssocks or sdp (for compatibility reasons sci is an alias for ssocks). It may be omited for IPv4 addresses. The actual IPv6 address that follows the ipv6 keyword must be placed inside brackets: ipv6 [fd01:2345:6789:abcd::1]:7800.

Each DRBD resource needs a TCP port which is used to connect to the node's partner device. Two different DRBD resources may not use the same addr:port combination on the same node.

meta-disk internal,

meta-disk device,

meta-disk device [index]

Internal means that the last part of the backing device is used to store the meta-data. The size of the meta-data is computed based on the size of the device.

When a device is specified, either with or without an index, DRBD stores the meta-data on this device. Without index, the size of the meta-data is determined by the size of the data device. This is usually used with LVM, which allows to have many variable sized block devices. The meta-data size is 36kB + Backing-Storage-size / 32k, rounded up to the next 4kb boundary. (Rule of the thumb: 32kByte per 1GByte of storage, rounded up to the next MB.)

When an index is specified, each index number refers to a fixed slot of meta-data of 128 MB, which allows a maximum data size of 4 TiB. This way, multiple DBRD devices can share the same meta-data device. For example, if /dev/sde6[0] and /dev/sde6[1] are used, /dev/sde6 must be at least 256 MB big. Because of the hard size limit, use of meta-disk indexes is discouraged.

on-io-error handler

handler is taken, if the lower level device reports io-errors to the upper layers. handler may be pass_on, call-local-io-error or detach.

pass_on: The node downgrades the disk status to inconsistent, marks the erroneous block as inconsistent in the bitmap and retries the IO on the remote node.

call-local-io-error: Call the handler script local-io-error.

detach: The node drops its low level device, and continues in diskless mode.

fencing fencing_policy

By fencing we understand preventive measures to avoid situations where both nodes are primary and disconnected (AKA split brain).

Valid fencing policies are:

dont-care

This is the default policy. No fencing actions are taken.

resource-only

If a node becomes a disconnected primary, it tries to fence the peer's disk. This is done by calling the fence-peer handler. The handler is supposed to reach the other node over alternative communication paths and call 'drbdadm outdate res' there.

resource-and-stonith

If a node becomes a disconnected primary, it freezes all its IO operations and calls its fence-peer handler. The fence-peer handler is supposed to reach the peer over alternative communication paths and call 'drbdadm outdate res' there. In case it cannot reach the peer it should stonith the peer. IO is resumed as soon as the situation is resolved. In case your handler fails, you can resume IO with the resume-io command.

disk-barrier,

disk-flushes,

disk-drain

DRBD has four implementations to express write-after-write dependencies to its backing storage device. DRBD will use the first method that is supported by the backing storage device and that is not disabled. By default the flush method is used. Since drbd-8.4.2 disk-barrier is disabled by default because since linux-2.6.36 (or 2.6.32 RHEL6) there is no reliable way to determine if queuing of IO-barriers works. Dangerous only enable if you are told so by one that knows for sure.

When selecting the method you should not only base your decision on the measurable performance. In case your backing storage device has a volatile write cache (plain disks, RAID of plain disks) you should use one of the first two. In case your backing storage device has battery-backed write cache you may go with option 3. Option 4 (disable everything, use "none") is dangerous on most IO stacks, may result in write-reordering, and if so, can theoretically be the reason for data corruption, or disturb the DRBD protocol, causing spurious disconnect/reconnect cycles. Do not use no-disk-drain.

Unfortunately device mapper (LVM) might not support barriers.

The letter after "wo:" in /proc/drbd indicates with method is currently in use for a device: b, f, d, n. The implementations are:

barrier

The first requires that the driver of the backing storage device support barriers (called 'tagged command queuing' in SCSI and 'native command queuing' in SATA speak). The use of this method can be enabled by setting the disk-barrier options to yes.

flush

The second requires that the backing device support disk flushes (called 'force unit access' in the drive vendors speak). The use of this method can be disabled setting disk-flushes to no.

drain

The third method is simply to let write requests drain before write requests of a new reordering domain are issued. This was the only implementation before 8.0.9.

none

The fourth method is to not express write-after-write dependencies to the backing store at all, by also specifying no-disk-drain. This is dangerous on most IO stacks, may result in write-reordering, and if so, can theoretically be the reason for data corruption, or disturb the DRBD protocol, causing spurious disconnect/reconnect cycles. Do not use no-disk-drain.

md-flushes

Disables the use of disk flushes and barrier BIOs when accessing the meta data device. See the notes on disk-flushes.

max-bio-bvecs

In some special circumstances the device mapper stack manages to pass BIOs to DRBD that violate the constraints that are set forth by DRBD's merge_bvec() function and which have more than one bvec. A known example is: phys-disk -> DRBD -> LVM -> Xen -> misaligned partition (63) -> DomU FS. Then you might see "bio would need to, but cannot, be split:" in the Dom0's kernel log. The best workaround is to proper align the partition within the VM (E.g. start it at sector 1024). This costs 480 KiB of storage. Unfortunately the default of most Linux partitioning tools is to start the first partition at an odd number (63). Therefore most distribution's install helpers for virtual linux machines will end up with misaligned partitions. The second best workaround is to limit DRBD's max bvecs per BIO (= max-bio-bvecs) to 1, but that might cost performance.

The default value of max-bio-bvecs is 0, which means that there is no user imposed limitation.

disk-timeout

If the lower-level device on which a DRBD device stores its data does not finish an I/O request within the defined disk-timeout, DRBD treats this as a failure. The lower-level device is detached, and the device's disk state advances to Diskless. If DRBD is connected to one or more peers, the failed request is passed on to one of them. This option is dangerous and may lead to kernel panic!

"Aborting" requests, or force-detaching the disk, is intended for completely blocked/hung local backing devices which do no longer complete requests at all, not even do error completions. In this situation, usually a hard-reset and failover is the only way out.

By "aborting", basically faking a local error-completion, we allow for a more graceful swichover by cleanly migrating services. Still the affected node has to be rebooted "soon".

By completing these requests, we allow the upper layers to re-use the associated data pages.

If later the local backing device "recovers", and now DMAs some data from disk into the original request pages, in the best case it will just put random data into unused pages; but typically it will corrupt meanwhile completely unrelated data, causing all sorts of damage.

Which means delayed successful completion, especially for READ requests, is a reason to panic(). We assume that a delayed *error* completion is OK, though we still will complain noisily about it.

The default value of disk-timeout is 0, which stands for an infinite timeout. Timeouts are specified in units of 0.1 seconds. This option is available since DRBD 8.3.12.

discard-zeroes-if-aligned {yes | no}

There are several aspects to discard/trim/unmap support on linux block devices. Even if discard is supported in general, it may fail silently, or may partially ignore discard requests. Devices also announce whether reading from unmapped blocks returns defined data (usually zeroes), or undefined data (possibly old data, possibly garbage).

If on different nodes, DRBD is backed by devices with differing discard characteristics, discards may lead to data divergence (old data or garbage left over on one backend, zeroes due to unmapped areas on the other backend). Online verify would now potentially report tons of spurious differences. While probably harmless for most use cases (fstrim on a file system), DRBD cannot have that.

To play safe, we have to disable discard support, if our local backend (on a Primary) does not support "discard_zeroes_data=true". We also have to translate discards to explicit zero-out on the receiving side, unless the receiving side (Secondary) supports "discard_zeroes_data=true", thereby allocating areas what were supposed to be unmapped.

There are some devices (notably the LVM/DM thin provisioning) that are capable of discard, but announce discard_zeroes_data=false. In the case of DM-thin, discards aligned to the chunk size will be unmapped, and reading from unmapped sectors will return zeroes. However, unaligned partial head or tail areas of discard requests will be silently ignored.

If we now add a helper to explicitly zero-out these unaligned partial areas, while passing on the discard of the aligned full chunks, we effectively achieve discard_zeroes_data=true on such devices.

Setting discard-zeroes-if-aligned to yes will allow DRBD to use discards, and to announce discard_zeroes_data=true, even on backends that announce discard_zeroes_data=false.

Setting discard-zeroes-if-aligned to no will cause DRBD to always fall-back to zero-out on the receiving side, and to not even announce discard capabilities on the Primary, if the respective backend announces discard_zeroes_data=false.

We used to ignore the discard_zeroes_data setting completely. To not break established and expected behaviour, and suddenly cause fstrim on thin-provisioned LVs to run out-of-space instead of freeing up space, the default value is yes.

This option is available since 8.4.7.

--disable-write-same {yes | no}

Some disks announce WRITE_SAME support to the kernel but fail with an I/O error upon actually receiving such a request. This mostly happens when using virtualized disks -- notably, this behavior has been observed with VMware's virtual disks.

When disable-write-same is set to yes, WRITE_SAME detection is manually overriden and support is disabled.

The default value of disable-write-same is no. This option is available since 8.4.7.

read-balancing method

The supported methods for load balancing of read requests are prefer-local, prefer-remote, round-robin, least-pending, when-congested-remote, 32K-striping, 64K-striping, 128K-striping, 256K-striping, 512K-striping and 1M-striping. The default value of read-balancing is prefer-local. This option is available since 8.4.1.

rs-discard-granularity byte

When rs-discard-granularity is set to a non zero, positive value then DRBD tries to do a resync operation in requests of this size. In case such a block contains only zero bytes on the sync source node, the sync target node will issue a discard/trim/unmap command for the area. The value is constrained by the discard granularity of the backing block device. In case rs-discard-granularity is not a multiplier of the discard granularity of the backing block device DRBD rounds it up. The feature only gets active if the backing block device reads back zeroes after a discard command.

The default value of rs-discard-granularity is 0. This option is available since 8.4.7.

sndbuf-size size

size is the size of the TCP socket send buffer. The default value is 0, i.e. autotune. You can specify smaller or larger values. Larger values are appropriate for reasonable write throughput with protocol A over high latency networks. Values below 32K do not make sense. Since 8.0.13 resp. 8.2.7, setting the size value to 0 means that the kernel should autotune this.

rcvbuf-size size

size is the size of the TCP socket receive buffer. The default value is 0, i.e. autotune. You can specify smaller or larger values. Usually this should be left at its default. Setting the size value to 0 means that the kernel should autotune this.

timeout time

If the partner node fails to send an expected response packet within time tenths of a second, the partner node is considered dead and therefore the TCP/IP connection is abandoned. This must be lower than connect-int and ping-int. The default value is 60 = 6 seconds, the unit 0.1 seconds.

connect-int time

In case it is not possible to connect to the remote DRBD device immediately, DRBD keeps on trying to connect. With this option you can set the time between two retries. The default value is 10 seconds, the unit is 1 second.

ping-int time

If the TCP/IP connection linking a DRBD device pair is idle for more than time seconds, DRBD will generate a keep-alive packet to check if its partner is still alive. The default is 10 seconds, the unit is 1 second.

ping-timeout time

The time the peer has time to answer to a keep-alive packet. In case the peer's reply is not received within this time period, it is considered as dead. The default value is 500ms, the default unit are tenths of a second.

max-buffers number

Limits the memory usage per DRBD minor device on the receiving side, or for internal buffers during resync or online-verify. Unit is PAGE_SIZE, which is 4 KiB on most systems. The minimum possible setting is hard coded to 32 (=128 KiB). These buffers are used to hold data blocks while they are written to/read from disk. To avoid possible distributed deadlocks on congestion, this setting is used as a throttle threshold rather than a hard limit. Once more than max-buffers pages are in use, further allocation from this pool is throttled. You want to increase max-buffers if you cannot saturate the IO backend on the receiving side.

ko-count number

In case the secondary node fails to complete a single write request for count times the timeout, it is expelled from the cluster. (I.e. the primary node will kill and restart the connection.) To disable this feature, you should explicitly set it to 0; defaults may change between versions.

max-epoch-size number

The highest number of data blocks between two write barriers. If you set this smaller than 10, you might decrease your performance.

allow-two-primaries

With this option set you may assign the primary role to both nodes. You only should use this option if you use a shared storage file system on top of DRBD. At the time of writing the only ones are: OCFS2 and GFS. If you use this option with any other file system, you are going to crash your nodes and to corrupt your data!

unplug-watermark number

This setting has no effect with recent kernels that use explicit on-stack plugging (upstream Linux kernel 2.6.39, distributions may have backported). When the number of pending write requests on the standby (secondary) node exceeds the unplug-watermark, we trigger the request processing of our backing storage device. Some storage controllers deliver better performance with small values, others deliver best performance when the value is set to the same value as max-buffers, yet others don't feel much effect at all. Minimum 16, default 128, maximum 131072.

cram-hmac-alg

You need to specify the HMAC algorithm to enable peer authentication at all. You are strongly encouraged to use peer authentication. The HMAC algorithm will be used for the challenge response authentication of the peer. You may specify any digest algorithm that is named in /proc/crypto.

shared-secret

The shared secret used in peer authentication. May be up to 64 characters. Note that peer authentication is disabled as long as no cram-hmac-alg (see above) is specified.

after-sb-0pri policy

possible policies are: disconnect

No automatic resynchronization, simply disconnect.

discard-younger-primary

Auto sync from the node that was primary before the split-brain situation happened.

discard-older-primary

Auto sync from the node that became primary as second during the split-brain situation.

discard-zero-changes

In case one node did not write anything since the split brain became evident, sync from the node that wrote something to the node that did not write anything. In case none wrote anything this policy uses a random decision to perform a "resync" of 0 blocks. In case both have written something this policy disconnects the nodes.

discard-least-changes

Auto sync from the node that touched more blocks during the split brain situation.

discard-node-NODENAME

Auto sync to the named node.

after-sb-1pri policy

possible policies are: disconnect

No automatic resynchronization, simply disconnect.

consensus

Discard the version of the secondary if the outcome of the after-sb-0pri algorithm would also destroy the current secondary's data. Otherwise disconnect.

violently-as0p

Always take the decision of the after-sb-0pri algorithm, even if that causes an erratic change of the primary's view of the data. This is only useful if you use a one-node FS (i.e. not OCFS2 or GFS) with the allow-two-primaries flag, AND if you really know what you are doing. This is DANGEROUS and MAY CRASH YOUR MACHINE if you have an FS mounted on the primary node.

discard-secondary

Discard the secondary's version.

call-pri-lost-after-sb

Always honor the outcome of the after-sb-0pri algorithm. In case it decides the current secondary has the right data, it calls the "pri-lost-after-sb" handler on the current primary.

after-sb-2pri policy

possible policies are: disconnect

No automatic resynchronization, simply disconnect.

violently-as0p

Always take the decision of the after-sb-0pri algorithm, even if that causes an erratic change of the primary's view of the data. This is only useful if you use a one-node FS (i.e. not OCFS2 or GFS) with the allow-two-primaries flag, AND if you really know what you are doing. This is DANGEROUS and MAY CRASH YOUR MACHINE if you have an FS mounted on the primary node.

call-pri-lost-after-sb

Call the "pri-lost-after-sb" helper program on one of the machines. This program is expected to reboot the machine, i.e. make it secondary.

always-asbp

Normally the automatic after-split-brain policies are only used if current states of the UUIDs do not indicate the presence of a third node. With this option you request that the automatic after-split-brain policies are used as long as the data sets of the nodes are somehow related. This might cause a full sync, if the UUIDs indicate the presence of a third node. (Or double faults led to strange UUID sets.)

rr-conflict policy

This option helps to solve the cases when the outcome of the resync decision is incompatible with the current role assignment in the cluster. disconnect

No automatic resynchronization, simply disconnect.

violently

Sync to the primary node is allowed, violating the assumption that data on a block device are stable for one of the nodes. Dangerous, do not use.

call-pri-lost

Call the pri-lost-after-sb helper program on one of the machines unless that machine can demote to secondary. The helper program is expected to reboot the machine, which brings the node into a secondary role. Which machine runs the helper program is determined by the after-sb-0pri strategy.

data-integrity-alg alg

DRBD can ensure the data integrity of the user's data on the network by comparing hash values. Normally this is ensured by the 16 bit checksums in the headers of TCP/IP packets. This option can be set to any of the kernel's data digest algorithms. In a typical kernel configuration you should have at least one of md5, sha1, and crc32c available. By default this is not enabled.

See also the notes on data integrity.

tcp-cork

DRBD usually uses the TCP socket option TCP_CORK to hint to the network stack when it can expect more data, and when it should flush out what it has in its send queue. It turned out that there is at least one network stack that performs worse when one uses this hinting method. Therefore we introducted this option. By setting tcp-cork to no you can disable the setting and clearing of the TCP_CORK socket option by DRBD.

on-congestion congestion_policy,

congestion-fill fill_threshold,

congestion-extents active_extents_threshold

By default DRBD blocks when the available TCP send queue becomes full. That means it will slow down the application that generates the write requests that cause DRBD to send more data down that TCP connection. When DRBD is deployed with DRBD-proxy it might be more desirable that DRBD goes into AHEAD/BEHIND mode shortly before the send queue becomes full. In AHEAD/BEHIND mode DRBD does no longer replicate data, but still keeps the connection open.

The advantage of the AHEAD/BEHIND mode is that the application is not slowed down, even if DRBD-proxy's buffer is not sufficient to buffer all write requests. The downside is that the peer node falls behind, and that a resync will be necessary to bring it back into sync. During that resync the peer node will have an inconsistent disk.

Available congestion_policys are block and pull-ahead. The default is block. Fill_threshold might be in the range of 0 to 10GiBytes. The default is 0 which disables the check. Active_extents_threshold has the same limits as al-extents.

The AHEAD/BEHIND mode and its settings are available since DRBD 8.3.10.

wfc-timeout time

Wait for connection timeout. The init script drbd(8) blocks the boot process until the DRBD resources are connected. When the cluster manager starts later, it does not see a resource with internal split-brain. In case you want to limit the wait time, do it here. Default is 0, which means unlimited. The unit is seconds.

degr-wfc-timeout time

Wait for connection timeout, if this node was a degraded cluster. In case a degraded cluster (= cluster with only one node left) is rebooted, this timeout value is used instead of wfc-timeout, because the peer is less likely to show up in time, if it had been dead before. Value 0 means unlimited.

outdated-wfc-timeout time

Wait for connection timeout, if the peer was outdated. In case a degraded cluster (= cluster with only one node left) with an outdated peer disk is rebooted, this timeout value is used instead of wfc-timeout, because the peer is not allowed to become primary in the meantime. Value 0 means unlimited.

wait-after-sb

By setting this option you can make the init script to continue to wait even if the device pair had a split brain situation and therefore refuses to connect.

become-primary-on node-name

Sets on which node the device should be promoted to primary role by the init script. The node-name might either be a host name or the keyword both. When this option is not set the devices stay in secondary role on both nodes. Usually one delegates the role assignment to a cluster manager (e.g. heartbeat).

stacked-timeouts

Usually wfc-timeout and degr-wfc-timeout are ignored for stacked devices, instead twice the amount of connect-int is used for the connection timeouts. With the stacked-timeouts keyword you disable this, and force DRBD to mind the wfc-timeout and degr-wfc-timeout statements. Only do that if the peer of the stacked resource is usually not available or will usually not become primary. By using this option incorrectly, you run the risk of causing unexpected split brain.

resync-rate rate

To ensure a smooth operation of the application on top of DRBD, it is possible to limit the bandwidth which may be used by background synchronizations. The default is 250 KB/sec, the default unit is KB/sec. Optional suffixes K, M, G are allowed.

use-rle

During resync-handshake, the dirty-bitmaps of the nodes are exchanged and merged (using bit-or), so the nodes will have the same understanding of which blocks are dirty. On large devices, the fine grained dirty-bitmap can become large as well, and the bitmap exchange can take quite some time on low-bandwidth links.

Because the bitmap typically contains compact areas where all bits are unset (clean) or set (dirty), a simple run-length encoding scheme can considerably reduce the network traffic necessary for the bitmap exchange.

For backward compatibility reasons, and because on fast links this possibly does not improve transfer time but consumes cpu cycles, this defaults to off.

socket-check-timeout value

In setups involving a DRBD-proxy and connections that experience a lot of buffer-bloat it might be necessary to set ping-timeout to an unusual high value. By default DRBD uses the same value to wait if a newly established TCP-connection is stable. Since the DRBD-proxy is usually located in the same data center such a long wait time may hinder DRBD's connect process.

In such setups socket-check-timeout should be set to at least to the round trip time between DRBD and DRBD-proxy. I.e. in most cases to 1.

The default unit is tenths of a second, the default value is 0 (which causes DRBD to use the value of ping-timeout instead). Introduced in 8.4.5.

resync-after res-name

By default, resynchronization of all devices would run in parallel. By defining a resync-after dependency, the resynchronization of this resource will start only if the resource res-name is already in connected state (i.e., has finished its resynchronization).

al-extents extents

DRBD automatically performs hot area detection. With this parameter you control how big the hot area (= active set) can get. Each extent marks 4M of the backing storage (= low-level device). In case a primary node leaves the cluster unexpectedly, the areas covered by the active set must be resynced upon rejoining of the failed node. The data structure is stored in the meta-data area, therefore each change of the active set is a write operation to the meta-data device. A higher number of extents gives longer resync times but less updates to the meta-data. The default number of extents is 1237. (Minimum: 7, Maximum: 65534)

Note that the effective maximum may be smaller, depending on how you created the device meta data, see also drbdmeta(8). The effective maximum is 919 * (available on-disk activity-log ring-buffer area/4kB -1), the default 32kB ring-buffer effects a maximum of 6433 (covers more than 25 GiB of data). We recommend to keep this well within the amount your backend storage and replication link are able to resync inside of about 5 minutes.

al-updates {yes | no}

DRBD's activity log transaction writing makes it possible, that after the crash of a primary node a partial (bit-map based) resync is sufficient to bring the node back to up-to-date. Setting al-updates to no might increase normal operation performance but causes DRBD to do a full resync when a crashed primary gets reconnected. The default value is yes.

verify-alg hash-alg

During online verification (as initiated by the verify sub-command), rather than doing a bit-wise comparison, DRBD applies a hash function to the contents of every block being verified, and compares that hash with the peer. This option defines the hash algorithm being used for that purpose. It can be set to any of the kernel's data digest algorithms. In a typical kernel configuration you should have at least one of md5, sha1, and crc32c available. By default this is not enabled; you must set this option explicitly in order to be able to use on-line device verification. See also the notes on data integrity.

csums-alg hash-alg

A resync process sends all marked data blocks from the source to the destination node, as long as no csums-alg is given. When one is specified the resync process exchanges hash values of all marked blocks first, and sends only those data blocks that have different hash values. This setting is useful for DRBD setups with low bandwidth links. During the restart of a crashed primary node, all blocks covered by the activity log are marked for resync. But a large part of those will actually be still in sync, therefore using csums-alg will lower the required bandwidth in exchange for CPU cycles.

c-plan-ahead plan_time,

c-fill-target fill_target,

c-delay-target delay_target,

c-max-rate max_rate

The dynamic resync speed controller gets enabled with setting plan_time to a positive value. It aims to fill the buffers along the data path with either a constant amount of data fill_target, or aims to have a constant delay time of delay_target along the path. The controller has an upper bound of max_rate. By plan_time the agility of the controller is configured. Higher values yield for slower/lower responses of the controller to deviation from the target value. It should be at least 5 times RTT. For regular data paths a fill_target in the area of 4k to 100k is appropriate. For a setup that contains drbd-proxy it is advisable to use delay_target instead. Only when fill_target is set to 0 the controller will use delay_target. 5 times RTT is a reasonable starting value. Max_rate should be set to the bandwidth available between the DRBD-hosts and the machines hosting DRBD-proxy, or to the available disk-bandwidth.

The default value of plan_time is 0, the default unit is 0.1 seconds. Fill_target has 0 and sectors as default unit. Delay_target has 1 (100ms) and 0.1 as default unit. Max_rate has 10240 (100MiB/s) and KiB/s as default unit.

The dynamic resync speed controller and its settings are available since DRBD 8.3.9.

c-min-rate min_rate

A node that is primary and sync-source has to schedule application IO requests and resync IO requests. The min_rate tells DRBD use only up to min_rate for resync IO and to dedicate all other available IO bandwidth to application requests. Note: The value 0 has a special meaning. It disables the limitation of resync IO completely, which might slow down application IO considerably. Set it to a value of 1, if you prefer that resync IO never slows down application IO.

Note: Although the name might suggest that it is a lower bound for the dynamic resync speed controller, it is not. If the DRBD-proxy buffer is full, the dynamic resync speed controller is free to lower the resync speed down to 0, completely independent of the c-min-rate setting.

The default value of min_rate is 250, in units of KiB/s

on-no-data-accessible ond-policy

This setting controls what happens to IO requests on a degraded, disk less node (I.e. no data store is reachable). The available policies are io-error and suspend-io. If ond-policy is set to suspend-io you can either resume IO by attaching/connecting the last lost data storage, or by the drbdadm resume-io res command. The latter will result in IO errors of course.

The default is io-error. This setting is available since DRBD 8.3.9.

cpu-mask cpu-mask

Sets the cpu-affinity-mask for DRBD's kernel threads of this device. The default value of cpu-mask is 0, which means that DRBD's kernel threads should be spread over all CPUs of the machine. This value must be given in hexadecimal notation. If it is too big it will be truncated.

pri-on-incon-degr cmd

This handler is called if the node is primary, degraded and if the local copy of the data is inconsistent.

pri-lost-after-sb cmd

The node is currently primary, but lost the after-split-brain auto recovery procedure. As as consequence, it should be abandoned.

pri-lost cmd

The node is currently primary, but DRBD's algorithm thinks that it should become sync target. As a consequence it should give up its primary role.

fence-peer cmd

The handler is part of the fencing mechanism. This handler is called in case the node needs to fence the peer's disk. It should use other communication paths than DRBD's network link.

local-io-error cmd

DRBD got an IO error from the local IO subsystem.

initial-split-brain cmd

DRBD has connected and detected a split brain situation. This handler can alert someone in all cases of split brain, not just those that go unresolved.

split-brain cmd

DRBD detected a split brain situation but remains unresolved. Manual recovery is necessary. This handler should alert someone on duty.

before-resync-target cmd

DRBD calls this handler just before a resync begins on the node that becomes resync target. It might be used to take a snapshot of the backing block device.

after-resync-target cmd

DRBD calls this handler just after a resync operation finished on the node whose disk just became consistent after being inconsistent for the duration of the resync. It might be used to remove a snapshot of the backing device that was created by the before-resync-target handler.

### Other Keywords
include file-pattern

Include all files matching the wildcard pattern file-pattern. The include statement is only allowed on the top level, i.e. it is not allowed inside any section.

## NOTES ON DATA INTEGRITY
There are two independent methods in DRBD to ensure the integrity of the mirrored data. The online-verify mechanism and the data-integrity-alg of the network section.

Both mechanisms might deliver false positives if the user of DRBD modifies the data which gets written to disk while the transfer goes on. This may happen for swap, or for certain append while global sync, or truncate/rewrite workloads, and not necessarily poses a problem for the integrity of the data. Usually when the initiator of the data transfer does this, it already knows that that data block will not be part of an on disk data structure, or will be resubmitted with correct data soon enough.

The data-integrity-alg causes the receiving side to log an error about "Digest integrity check FAILED: Ns +x\n", where N is the sector offset, and x is the size of the request in bytes. It will then disconnect, and reconnect, thus causing a quick resync. If the sending side at the same time detected a modification, it warns about "Digest mismatch, buffer modified by upper layers during write: Ns +x\n", which shows that this was a false positive. The sending side may detect these buffer modifications immediately after the unmodified data has been copied to the tcp buffers, in which case the receiving side won't notice it.

The most recent (2007) example of systematic corruption was an issue with the TCP offloading engine and the driver of a certain type of GBit NIC. The actual corruption happened on the DMA transfer from core memory to the card. Since the TCP checksum gets calculated on the card, this type of corruption stays undetected as long as you do not use either the online verify or the data-integrity-alg.

We suggest to use the data-integrity-alg only during a pre-production phase due to its CPU costs. Further we suggest to do online verify runs regularly e.g. once a month during a low load period.

## VERSION
This document was revised for version 8.4.0 of the DRBD distribution.

## AUTHOR
Written by Philipp Reisner <philipp.reisner@linbit.com> and Lars Ellenberg <lars.ellenberg@linbit.com>.

## REPORTING BUGS
Report bugs to <drbd-user@lists.linbit.com>.

## COPYRIGHT
Copyright 2001-2008 LINBIT Information Technologies, Philipp Reisner, Lars Ellenberg. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

## SEE ALSO
drbd(8), drbdsetup(8), drbdmeta(8), drbdadm(8), DRBD User's Guide[1], DRBD web site[3]

## NOTES
1.

DRBD User's Guide

http://www.drbd.org/users-guide/

2.

DRBD's online usage counter

http://usage.drbd.org

3.

DRBD web site

http://www.drbd.org/