DRBD.CONF(5)

📦

version 9.34.0 · 3 versions

9.34.0 · alpine 3.23, alpine 3.24, fedora 43, fedora 44, nixos 26.05, nixpkgs unstable (latest)
9.15.0 · ubuntu jammy
9.28.0 · alpine 3.22, debian bookworm, debian trixie, epel 9, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

, in alpine 3.23, alpine 3.24, fedora 43, fedora 44, nixos 26.05, nixpkgs unstable

💅 drbd.conf(5) — Configuration Files 💅

okay bestie, sit down, hydrate, because this is drbd.conf(5) and we are going to understand it together. no cap. 🫶

🌸 NAME 🌸

drbd.conf — the configuration file for DRBD's devices. short, sweet, iconic. ✨

🦋 INTRODUCTION 🦋

okay so the file /etc/drbd.conf is the one drbdadm reads. that's the whole relationship. 💖

The file format was literally designed so you can have a verbatim copy of the file on both nodes of the cluster. and bestie, please do that, it is highly recommended — it's how you keep your config manageable and not a chaotic mess. The file /etc/drbd.conf should be the same on both nodes of the cluster. 🎀

heads up: changes to /etc/drbd.conf do not apply immediately. it's giving "no instant gratification". 😭

By convention the main config contains two include statements. The first one includes the file /etc/drbd.d/global_common.conf, and the second one all files 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;
     }
}

so 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:

okay so the symlink layout it hands you looks a lil something like this:

# 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

and if you drop this parameter into 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 bestie, we do recommend you enable it. no cap. 🫶

usage-count val — pls pls pls participate in DRBD's online usage counter[2] 🥺 the comfiest way to do that is to set this option to yes. valid options are: yes, no and ask.
protocol prot-id — this is the protocol that gets used on the TCP/IP link. 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. (this one is the vibe, just saying)
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. like, not even a little. 🙅‍♀️

you can either omit the name or 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 omitted 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] — where the meta-data lives, three flavors:

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 DRBD 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 which 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 switchover 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, like, so many aspects to discard/trim/unmap support on linux block devices, bestie. 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). It's giving chaos energy ngl.

So 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). And then online verify would potentially report tons of spurious differences. While probably harmless for most use cases (fstrim on a file system), DRBD cannot have that. Not on our watch. 💅

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. sneaky little things. 🦋

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. slay, we fixed it ourselves. ✨

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. the audacity, honestly. 😭

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. heads up bestie, don't just assume the default. ⭐

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!

Like, genuinely: crash your nodes, corrupt your data. This is the dangerous one. Do not freestyle this. 💀

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. the name literally warns you. violently. as0p. bestie. 🔥
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 — okay so the possible policies are:
- disconnect — No automatic resynchronization, simply disconnect. periodt 💅
- 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. 🚨 bestie, do NOT play with this one 🚨
- 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. it's giving "you're fired, reboot" energy 💀
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.) lowkey messy but okay ✨

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. vibe check: who's primary? who's secondary? let's fix it
- disconnect — No automatic resynchronization, simply disconnect. bye felicia 👋
- 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. literally do not 💀
- 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. it's giving "reboot to fix your role" energy 🔄
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. you gotta opt in, bestie 🫶

✨ 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 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 📝

DRBD User's Guide — http://www.drbd.org/users-guide/
DRBD's online usage counter — http://usage.drbd.org
DRBD web site — http://www.drbd.org/

And that's the whole DRBD.CONF(5) vibe, bestie! This page is from DRBD 8.4.0, dated 6 May 2011, and it's the DRBD.CONF(5) man page. Stay slay, keep your data consistent, and may your resyncs always be quick. 💖✨

📦 where this page comes from

alpine 3.23: drbd-utils-doc 9.33.0-r0 · GPL-2.0-or-later
alpine 3.24: drbd-utils-doc 9.34.0-r0 · GPL-2.0-or-later
fedora 43: drbd-utils 9.32.0-1.fc43 · GPL-2.0-or-later
fedora 43: drbd-utils 9.34.0-2.fc43 · GPL-2.0-or-later
fedora 44: drbd-utils 9.34.0-1.fc44 · GPL-2.0-or-later
fedora 44: drbd-utils 9.34.0-2.fc44 · GPL-2.0-or-later
nixos 26.05: drbd 9.33.0
nixpkgs unstable: drbd 9.34.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms