DRBD.CONF(5)

📦

version 9.22.0 · 2 versions

9.22.0 · debian bookworm, debian trixie, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking (latest)
9.15.0 · ubuntu jammy

, in debian bookworm, debian trixie, ubuntu noble, ubuntu questing, ubuntu resolute, ubuntu stonking

💖 DRBD.CONF 💖 — the `drbd.conf(5)` situation, bestie edition

okay so you opened the Configuration Files section of the manual, and this is literally the whole vibe of drbd.conf. buckle up, we're doing this together 🫶

✨ NAME ✨

drbd.conf — the configuration file for DRBD's devices. that's it, that's the name, iconic.

🌸 INTRODUCTION 🌸

so the file /etc/drbd.conf gets read by drbdadm. that's the whole relationship. 📖

the file format was designed so you can have a verbatim copy of the file on both nodes of the cluster. and honestly? highly recommended, bestie, because that's how you keep your configuration manageable. /etc/drbd.conf should be the same on both nodes of the cluster. and listen — changes to /etc/drbd.conf do not apply immediately. no cap. you gotta reload. 🙈

in this example (the one from the page, not me making stuff up), there's a single DRBD resource — it's called r0 — and it uses protocol C for the connection between its devices. the device that runs on host alice uses /dev/drbd1 as its device for the application, and /dev/sda7 as low-level storage for the data. the IP addresses are what you use to specify the networking interfaces. and an eventually running resync process should use about 10MByte/second of IO bandwidth. slay. 💅

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]. go read it, i'll wait 🎀

🦋 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. it's giving nested vibes.

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. math is math, periodt. 🔢

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. classic. 🍓

🌈 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”'. so valid, so clean. ✨

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. main character energy, first in line. 👑

common

all resources inherit the options set in this section. the common section might have a startup, a syncer, a handlers, a net and a disk section. it's the shared closet everyone borrows from. 🧚

resource name

configures a DRBD resource. each resource section needs to have two (or more) on host sections, and may have a startup, a syncer, a handlers, a net and a disk section. required parameter in this section: protocol. 📌

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. look at this gorgeous example:

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 parameters in this section: device, disk, address, meta-disk, flexible-meta-disk. don't forget a single one, bestie, i'm serious 😤

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. that's the whole checklist. ✅

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 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, flexible-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. cute, efficient, we love her. 💖

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 a detailed description of the parameters. optional parameters: on-io-error, size, fencing, use-bmbv, no-disk-barrier, no-disk-flushes, no-disk-drain, no-md-flushes, max-bio-bvecs, disk-timeout. 🧊

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: 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. the networking chapter, we're obsessed. 🌐

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. 🚀

syncer

this section is used to fine tune the synchronization daemon for the device. please refer to drbdsetup(8) for a detailed description of this section's parameters. optional parameters: rate, after, al-extents, use-rle, cpu-mask, verify-alg, csums-alg, c-plan-ahead, c-fill-target, c-delay-target, c-max-rate, c-min-rate and on-no-data-accessible. syncing is her whole personality. 🔄

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 (note the singular form) is deprecated, and superseeded by DRBD_PEERS. it's giving legacy, bestie, please move on 💀

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. so don't come for me if one's missing 🙈

🎀 Parameters 🎀

minor-count `count`

count may be a number from 1 to 255.

use minor-count if you want to define massively more resources later without reloading the DRBD kernel module. per default the module loads with 11 more resources than you have currently in your config but at least 32. future-proofing queen 👑

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. obscure reasons only, no cap. 🕵️

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. go on, say yes, it's free 🥺

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. do not, i repeat, do NOT. 😭

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. cute little shortcuts, love that for us 🎀

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. i'm so serious, put the command down 🛑

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. brackets are not optional, she's dramatic like that 💅

part 1 done, bestie — more coming, don't go anywhere 🦋

okay so your DRBD resource needs a TCP port to hook up with the node's partner device — and two different DRBD resources may NOT use the same addr:port combination on the same node. no sharing, bestie, that's the whole rule 💅

meta-disk internal, flexible-meta-disk internal, meta-disk device [index], flexible-meta-disk device — this is where the meta-data lives, and there are ✨choices✨
- internal means the last part of the backing device gets used to store the meta-data. You must not use [index] with internal. Note: regardless of whether you use the meta-disk or the flexible-meta-disk keyword, it will always be of the size needed for the remaining storage size.
- You can use a single block device to store meta-data of multiple DRBD devices. E.g. use meta-disk /dev/sde6[0]; and meta-disk /dev/sde6[1]; for two different resources. In this case the meta-disk would need to be at least 256 MB in size.
- With the flexible-meta-disk keyword you specify a block device as meta-data storage. You usually use this with LVM, which lets you have many variable sized block devices. The required size of the meta-disk block device is 36kB + Backing-Storage-size / 32k. Round that number to the next 4kb boundary up and you have the exact size. Rule of the thumb: 32kByte per 1GByte of storage, round up to the next MB.

on-io-error handler — the handler that gets 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.
use-bmbv — in case the backing storage's driver has a merge_bvec_fn() function, DRBD has to pretend that it can only process IO requests in units not larger than 4KiB. (At the time of writing the only known drivers which have such a function are: md (software raid driver), dm (device mapper - LVM) and DRBD itself).
- To get the best performance out of DRBD on top of software RAID (or any other driver with a merge_bvec_fn() function) you might enable this function, if you know for sure that the merge_bvec_fn() function will deliver the same results on all nodes of your cluster. I.e. the physical disks of the software RAID are of exactly the same type. Use this option only if you know what you are doing.
no-disk-barrier, no-disk-flushes, no-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 the user.
- 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 a 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 disabled by the no-disk-barrier option. Note: since Linux-2.6.36 (or RHEL's 2.6.32) this method is disabled.
  - 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 using the no-disk-flushes option.
  - 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.
no-md-flushes — disables the use of disk flushes and barrier BIOs when accessing the meta data device. See the notes on no-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 driver of the lower_device does not finish an IO request within disk_timeout, DRBD considers the disk as failed. If DRBD is connected to a remote host, it will reissue local pending IO requests to the peer, and ship all new IO requests to the peer only. The disk state advances to diskless, as soon as the backing block device has finished all IO requests.
- The default value of disk-timeout is 0, which means that no timeout is enforced. The default unit is 100ms. This option is available since 8.3.12.
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 goes into StandAlone mode.) 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 — and the manpage literally cuts off right here, bestie 💀 the rest of this one is coming in the next chunk, stay tuned 🫶

Okay so with this option set you may assign the primary role to both nodes. But bestie — 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 one has no effect with recent kernels that use explicit on-stack plugging (that's upstream Linux kernel 2.6.39, and distributions may have backported it). 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. And you are strongly encouraged to use peer authentication, fr fr. 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" helper program on one of the machines. This program is expected to reboot the machine, i.e. make it secondary.
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. 🫶
no-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 introduced this option, which disables 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. 😬
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. 🧚

okay so real quick, the tail end of that last one: for backward-compat reasons, and because on fast links it maybe doesn't even make your transfer any quicker while it chomps through CPU cycles — this thing defaults to off. 💅

after res-name — by default, resynchronization of all devices just runs in parallel, chaos-gremlin style. But if you define a sync-after dependency, the resync of this resource won't even start until the resource res-name is already in connected state (which means it's finished its own resynchronization). 💖
al-extents extents — DRBD does hot area detection automatically, and this is you deciding how big the hot area (aka the active set) is allowed to get. Every single extent marks 4M of the backing storage (that's your low-level device, bestie). If a primary node dips out of the cluster unexpectedly, everything covered by the active set has to get resynced when it rejoins. The data structure lives in the meta-data area, so every change of the active set is a write operation to the meta-data device. Bigger number of extents = longer resync times but fewer meta-data updates. Default is 127. (Minimum: 7, Maximum: 3843.) ✨
verify-alg hash-alg — during online verification (the kind you kick off with the verify sub-command), instead of doing a bit-wise comparison, DRBD slaps a hash function on the contents of every block it's verifying and compares that hash with the peer. This option is the hash algorithm it uses for that. You can set it to any of the kernel's data digest algorithms. In a typical kernel config you should have at least one of md5, sha1, and crc32c sitting there ready. By default this is not enabled — you have to set it explicitly if you want to be able to use on-line device verification. 👑

See also the notes on data integrity (they're right below, iykyk).

csums-alg hash-alg — a resync process sends all the marked data blocks from the source to the destination node, as long as no csums-alg is given. Once you specify one, the resync process exchanges hash values of all the marked blocks first, and then only sends the data blocks that have different hash values. 🦋

This is genuinely useful for DRBD setups with low bandwidth links. When a crashed primary node restarts, every block covered by the activity log gets marked for resync — but a big chunk of those are actually still in sync, so using csums-alg lowers the bandwidth you need, and the price you pay is 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 switches on when you set plan_time to a positive value. Its whole goal: fill the buffers along the data path with either a constant amount of data (fill_target), or keep a constant delay time (delay_target) along the path. The controller has an upper bound, and that's max_rate. 🔥

plan_time is where you configure the agility of the controller. Higher values = slower/lower responses from the controller when things deviate 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 has drbd-proxy in it, it's advisable to use delay_target instead. Only when fill_target is set to 0 will the controller 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. 💜

Defaults, so you know: plan_time default is 0, and its default unit is 0.1 seconds. fill_target defaults to 0 with sectors as the default unit. delay_target defaults to 1 (100ms) with 0.1 as the default unit. max_rate defaults to 10240 (100MiB/s) with KiB/s as the default unit.

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

c-min-rate min_rate — a node that's primary and sync-source has to schedule both application IO requests and resync IO requests. min_rate tells DRBD to use only up to min_rate for resync IO, and to hand all the 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 1 if you'd rather resync IO never slows down application IO.

Note: even though the name might suggest it's 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 all the way down to 0, completely independent of the c-min-rate setting.

Min_rate has 4096 (4MiB/s) and KiB/s as default unit.

on-no-data-accessible ond-policy — this controls what happens to IO requests on a degraded, diskless node (meaning: no data store is reachable). The policies you can pick from are io-error and suspend-io. 💅

If ond-policy is set to suspend-io, you can resume IO either by attaching/connecting the last lost data storage, or with the drbdadm resume-io res command. That second one will of course result in IO errors. 🙈

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 DRBD's kernel threads should be spread over all CPUs of the machine. This value must be given in hexadecimal notation. If it's too big, it will just get truncated. ✂️
pri-on-incon-degr cmd — this handler gets called if the node is primary, degraded, and the local copy of the data is inconsistent. 😭
pri-lost-after-sb cmd — the node is currently primary, but it lost the after-split-brain auto recovery procedure. As a consequence, it should be abandoned. 💀
pri-lost cmd — the node is currently primary, but DRBD's algorithm thinks it should become sync target. As a consequence it should give up its primary role. 👋
fence-peer cmd — this handler is part of the fencing mechanism. It gets called when 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 the ones that go unresolved. 🚨
split-brain cmd — DRBD detected a split brain situation but it 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's becoming the 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 — includes all files matching the wildcard pattern file-pattern. Heads up: 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 keep your mirrored data's integrity on lock. One is the online-verify mechanism, the other is the data-integrity-alg of the network section. ✨

Both mechanisms might deliver false positives if the user of DRBD modifies the data that gets written to disk while the transfer is going on. This can happen for swap, or for certain append while global sync, or truncate/rewrite workloads — and it doesn't necessarily pose 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 won't be part of an on-disk data structure, or that it'll 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 requst 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 using the data-integrity-alg only during a pre-production phase, because of its CPU costs. Further, we suggest doing online verify runs regularly, e.g. once a month during a low load period. 🗓️

VERSION

This document was revised for version 8.3.2 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. ⚖️

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 a wrap bestie 🎀 that was the DRBD.CONF(5) page, revised for DRBD 8.3.2, dated 5 Dec 2008 — go forth and mirror your disks responsibly, you absolute icon 👑💖

📦 where this page comes from

debian bookworm: drbd-utils 9.22.0-1 · GPL-2.0-or-later
debian trixie: drbd-utils 9.22.0-1.1 · GPL-2.0-or-later
ubuntu noble: drbd-utils 9.22.0-1build1 · GPL-2.0-or-later
ubuntu questing: drbd-utils 9.22.0-1.2
ubuntu resolute: drbd-utils 9.22.0-1.2build1 · GPL-2.0-or-later
ubuntu stonking: drbd-utils 9.22.0-1.2build1 · GPL-2.0-or-later
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-18: a language model's rewrite of the original, under its terms