This guide describes DRBD version 8.4 and above. For 8.3 please look here.
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.
Example B.1. A small example.res file
resource r0 {
net {
protocol C;
cram-hmac-alg sha1;
shared-secret "FooFunFactory";
}
disk {
resync-rate 10M;
}
on alice {
volume 0 {
device minor 1;
disk /dev/sda7;
meta-disk internal;
}
address 10.1.1.31:7789;
}
on bob {
volume 0 {
device minor 1;
disk /dev/sda7;
meta-disk internal;
}
address 10.1.1.32:7789;
}
}
In this example, there is a single DRBD resource (called r0) which uses protocol C
for the connection between its devices. It contains a single volume which runs on host
alice uses /dev/drbd1 as devices for its
application, and /dev/sda7 as low-level storage for the data. The
IP addresses are used to specify the networking interfaces to be used. An eventually running
resync process should use about 10MByte/second of IO bandwidth. This sync-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.
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.
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 sections and may have a
hoststartup, 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,
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.
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.
minor-count countcount 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 timetime 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-verificationUse 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.
usage-count valPlease participate in
DRBD's online usage counter.
The most convenient way to do so is to set
this option to yes. Valid options are: yes,
no and ask.
protocol prot-idOn 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 nameDRBD 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 GB. 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 handlerhandler 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-careThis is the default policy. No fencing actions are taken.
resource-onlyIf 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-stonithIf 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-drainDRBD 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.
md-flushesDisables the use of disk flushes and barrier BIOs when accessing the meta data
device. See the notes on disk-flushes.
max-bio-bvecsIn 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 is 0, which means that no timeout is enforced. The default unit is 100ms. This option is available since 8.3.12.
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 is prefer-local.
This option is available since 8.4.1.
sndbuf-size sizesize 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 sizesize 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 timeIn 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 timeThe 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 numberMaximum number of requests to be allocated by DRBD. Unit is PAGE_SIZE, which is 4 KiB on most systems. The minimum is hard coded to 32 (=128 KiB). For high-performance installations it might help if you increase that number. These buffers are used to hold data blocks while they are written to disk.
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.) The default value is 0, which disables this
feature.
max-epoch-size numberThe highest number of data blocks between two write barriers. If you set this smaller than 10, you might decrease your performance.
allow-two-primariesWith 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 numberWhen 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. 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 policypossible policies are:
disconnectNo automatic resynchronization, simply disconnect.
discard-younger-primaryAuto sync from the node that was primary before the split-brain situation happened.
discard-older-primaryAuto sync from the node that became primary as second during the split-brain situation.
discard-zero-changesIn 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-changesAuto sync from the node that touched more blocks during the split brain situation.
discard-node-NODENAMEAuto sync to the named node.
after-sb-1pri policypossible policies are:
disconnectNo automatic resynchronization, simply disconnect.
consensusDiscard 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-as0pAlways 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-secondaryDiscard the secondary's version.
call-pri-lost-after-sbAlways 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 policypossible policies are:
disconnectNo automatic resynchronization, simply disconnect.
violently-as0pAlways 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-sbCall 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-asbpNormally 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 policyThis option helps to solve the cases when the outcome of the resync decision is incompatible with the current role assignment in the cluster.
disconnectNo automatic resynchronization, simply disconnect.
violentlySync 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-lostCall 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 algDRBD can ensure the data integrity of the user's data on the network by comparing hash values. Normally this is ensured by the 16 bit checksums in the headers of TCP/IP packets.
This option can be set to any of the kernel's data digest algorithms. In a
typical kernel configuration you should have at least one of md5,
sha1, and crc32c available. By default this is not
enabled.
See also the notes on data integrity.
tcp-corkDRBD 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_thresholdBy 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 timeWait 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 timeWait 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 timeWait 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-sbBy 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-nameSets 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-timeoutsUsually 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 rateTo 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-rleDuring 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 compatibilty reasons, and because on fast links this possibly does not improve transfer time but consumes cpu cycles, this defaults to off.
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-algDuring 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-algA 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_rateThe 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_rateA 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.
Min_rate has 4096 (4MiB/s) and KiB/s as default
unit.
on-no-data-accessible
ond-policyThis 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 cmdThis handler is called if the node is primary, degraded and if the local copy of the data is inconsistent.
pri-lost-after-sb cmdThe node is currently primary, but lost the after-split-brain auto recovery procedure. As as consequence, it should be abandoned.
pri-lost cmdThe 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 cmdinitial-split-brain cmdDRBD 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 cmdDRBD detected a split brain situation but remains unresolved. Manual recovery is necessary. This handler should alert someone on duty.
before-resync-target cmdDRBD 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.
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.
Written by Philipp Reisner <philipp.reisner@linbit.com> and Lars
Ellenberg <lars.ellenberg@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.
drbd(8), drbddisk(8), drbdsetup(8), drbdmeta(8), drbdadm(8), DRBD User's Guide, DRBD web site
This guide describes DRBD version 8.4 and above. For 8.3 please look here.