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drbd.conf-8.4 - Man Page

Configuration file for DRBD's devices


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;
	on bob {
		volume 0 {
			device    minor 1;
			disk      /dev/sda7;
			meta-disk internal;

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.



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'.


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.


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 {
			disk /dev/mapper/some-san;
		on charlie {
			disk /dev/mapper/other-san;
		on daisy {
			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

		# longer form, only device inherited
		floating {
			disk /dev/sdb;
			meta-disk /dev/sdc8;

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.


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.


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.


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.


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.


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.


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.


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

# explicit volume definition: volume 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:


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


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.


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.


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:


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.


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.


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.


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.


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


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.


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.


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.


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.


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:


No automatic resynchronization, simply disconnect.


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


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


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.


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


Auto sync to the named node.

after-sb-1pri policy

possible policies are:


No automatic resynchronization, simply disconnect.


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.


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 the secondary's version.


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:


No automatic resynchronization, simply disconnect.


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 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.


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.


No automatic resynchronization, simply disconnect.


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 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.


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.


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).


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.


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.


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


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>.

See Also

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


  1. DRBD User's Guide
  2. DRBD's online usage counter
  3. DRBD web site

Referenced By

drbd-8.3(8), drbd-8.4(8), drbd-9.0(8), drbdadm-8.3(8), drbdadm-8.4(8), drbdadm-9.0(8), drbddisk-8.3(8), drbddisk-8.4(8), drbdmeta-9.0(8), drbdmon-9.0(8), drbdsetup-8.3(8), drbdsetup-8.4(8), drbdsetup-9.0(8).

The man page drbd.conf(5) is an alias of drbd.conf-8.4(5).

6 May 2011 DRBD 8.4.0 Configuration Files