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DRBD.CONF(5) | Configuration Files | DRBD.CONF(5) |
NAME¶
drbd.conf - Configuration file for DRBD's devices .INTRODUCTION¶
The file /etc/drbd.conf is read by drbdadm. The file format was designed as to allow to have a verbatim copy of the file on both nodes of the cluster. It is highly recommended to do so in order to keep your configuration manageable. The file /etc/drbd.conf should be the same on both nodes of the cluster. Changes to /etc/drbd.conf do not apply immediately. In this example, there is a single DRBD resource (called r0) which uses protocol C for the connection between its devices. The device 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. 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¶
skipComments 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 syncer, 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 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.
See also the floating section keyword. Required parameters in this
section: device, disk, address, meta-disk,
flexible-meta-disk.
stacked-on-top-of resource
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; } }
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, 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.
disk
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; } }
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, 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
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.
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
DRBD_PEER (note the singular form) is deprecated, and superseeded by
DRBD_PEERS.
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.
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.
Parameters¶
minor-count countmay 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.
dialog-refresh 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.
usage-count val
Please participate in DRBD's online usage
counter[2]. The most convenient way to do so is to set this option to
yes. Valid options are: yes, no and ask.
protocol prot-id
On the TCP/IP link the specified
protocol is used. Valid protocol specifiers are A, B, and C.
Protocol A: write IO is reported as completed, if it has reached local disk and
local TCP send buffer.
Protocol B: write IO is reported as completed, if it has reached local disk and
remote buffer cache.
Protocol C: write IO is reported as completed, if it has reached both local and
remote disk.
device name minor nr
The name of the block device node of the
resource being described. You must use this device with your application (file
system) and you must not use the low level block device which is specified
with the disk parameter.
One can ether omit the name or minor and the minor number.
If you omit the name a default of /dev/drbd minor will be used.
Udev will create additional symlinks in /dev/drbd/by-res and
/dev/drbd/by-disk.
disk name
DRBD uses this block device to actually store
and retrieve the data. Never access such a device while DRBD is running on top
of it. This also holds true for dumpe2fs(8) and similar commands.
address AF addr:port
A resource needs one IP address per
device, which is used to wait for incoming connections from the partner device
respectively to reach the partner device. AF must be one of
ipv4, ipv6, ssocks or sdp (for compatibility
reasons sci is an alias for ssocks). It may be omited for IPv4
addresses. The actual IPv6 address that follows the ipv6 keyword must
be placed inside brackets: ipv6 [fd01:2345:6789:abcd::1]:7800.
Each DRBD resource needs a TCP port which is used to connect to the
node's partner device. Two different DRBD resources may not use the same
addr:port combination on the same node.
meta-disk internal, flexible-meta-disk
internal, meta-disk device [index],
flexible-meta-disk device
Internal means that the last part of the
backing device is 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 allows you to have
many variable sized block devices. The required size of the meta-disk block
device is 36kB + Backing-Storage-size / 32k. Round this 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
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
use-bmbv
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.
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 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
useno-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
no-md-flushes
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 useno-disk-drain.
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 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
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
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.)
The default value is 0, which disables this feature.
max-epoch-size number
The highest number of data blocks between two
write barriers. If you set this smaller than 10, you might decrease your
performance.
allow-two-primaries
With this option set you may assign the
primary role to both nodes. You only should use this option if you use a
shared storage file system on top of DRBD. At the time of writing the only
ones are: OCFS2 and GFS. If you use this option with any other file system,
you are going to crash your nodes and to corrupt your data!
unplug-watermark number
This setting has no effect with recent kernels
that use explicit on-stack plugging (upstream Linux kernel 2.6.39,
distributions may have backported).
When the number of pending write requests on the standby (secondary) node
exceeds the unplug-watermark, we trigger the request processing of our
backing storage device. Some storage controllers deliver better performance
with small values, others deliver best performance when the value is set to
the same value as max-buffers, yet others don't feel much effect at all.
Minimum 16, default 128, maximum 131072.
cram-hmac-alg
You need to specify the HMAC algorithm to
enable peer authentication at all. You are strongly encouraged to use peer
authentication. The HMAC algorithm will be used for the challenge response
authentication of the peer. You may specify any digest algorithm that is named
in /proc/crypto.
shared-secret
The shared secret used in peer authentication.
May be up to 64 characters. Note that peer authentication is disabled as long
as no cram-hmac-alg (see above) is specified.
after-sb-0pri policy
possible policies are:
disconnect
after-sb-1pri policy
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.
possible policies are:
disconnect
after-sb-2pri policy
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.
possible policies are:
disconnect
always-asbp
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.
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
data-integrity-alg alg
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.
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 introducted 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.
For backward compatibilty reasons, and because on fast links this possibly does
not improve transfer time but consumes cpu cycles, this defaults to off.
after res-name
By default, resynchronization of all devices
would run in parallel. By defining a sync-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 127. (Minimum: 7, Maximum: 3843)
verify-alg hash-alg
During online verification (as initiated by
the verify sub-command), rather than doing a bit-wise comparison, DRBD
applies a hash function to the contents of every block being verified, and
compares that hash with the peer. This option defines the hash algorithm being
used for that purpose. It can be set to any of the kernel's data digest
algorithms. In a typical kernel configuration you should have at least one of
md5, sha1, and crc32c available. By default this is not
enabled; you must set this option explicitly in order to be able to use
on-line device verification.
See also the notes on data integrity.
csums-alg hash-alg
A resync process sends all marked data blocks
from the source to the destination node, as long as no csums-alg is
given. When one is specified the resync process exchanges hash values of all
marked blocks first, and sends only those data blocks that have different hash
values.
This setting is useful for DRBD setups with low bandwidth links. During the
restart of a crashed primary node, all blocks covered by the activity log are
marked for resync. But a large part of those will actually be still in sync,
therefore using csums-alg will lower the required bandwidth in exchange
for CPU cycles.
c-plan-ahead plan_time, c-fill-target
fill_target, c-delay-target
delay_target, c-max-rate max_rate
The dynamic resync speed controller gets
enabled with setting plan_time to a positive value. It aims to fill the
buffers along the data path with either a constant amount of data
fill_target, or aims to have a constant delay time of
delay_target along the path. The controller has an upper bound of
max_rate.
By plan_time the agility of the controller is configured. Higher values
yield for slower/lower responses of the controller to deviation from the
target value. It should be at least 5 times RTT. For regular data paths a
fill_target in the area of 4k to 100k is appropriate. For a setup that
contains drbd-proxy it is advisable to use delay_target instead. Only
when fill_target is set to 0 the controller will use
delay_target. 5 times RTT is a reasonable starting value.
Max_rate should be set to the bandwidth available between the
DRBD-hosts and the machines hosting DRBD-proxy, or to the available
disk-bandwidth.
The default value of plan_time is 0, the default unit is 0.1 seconds.
Fill_target has 0 and sectors as default unit. Delay_target has
1 (100ms) and 0.1 as default unit. Max_rate has 10240 (100MiB/s) and
KiB/s as default unit.
The dynamic resync speed controller and its settings are available since DRBD
8.3.9.
c-min-rate min_rate
A node that is primary and sync-source has to
schedule application IO requests and resync IO requests. The min_rate
tells DRBD use only up to min_rate for resync IO and to dedicate all other
available IO bandwidth to application requests.
Note: The value 0 has a special meaning. It disables the limitation of resync IO
completely, which might slow down application IO considerably. Set it to a
value of 1, if you prefer that resync IO never slows down application IO.
Note: Although the name might suggest that it is a lower bound for the dynamic
resync speed controller, it is not. If the DRBD-proxy buffer is full, the
dynamic resync speed controller is free to lower the resync speed down to 0,
completely independent of the c-min-rate setting.
Min_rate has 4096 (4MiB/s) and KiB/s as default unit.
on-no-data-accessible ond-policy
This setting controls what happens to IO
requests on a degraded, disk less node (I.e. no data store is reachable). The
available policies are io-error and suspend-io.
If ond-policy is set to suspend-io you can either resume IO by
attaching/connecting the last lost data storage, or by the drbdadm
resume-io res command. The latter will result in IO errors
of course.
The default is io-error. This setting is available since DRBD
8.3.9.
cpu-mask cpu-mask
Sets the cpu-affinity-mask for DRBD's kernel
threads of this device. The default value of cpu-mask is 0, which means
that DRBD's kernel threads should be spread over all CPUs of the machine. This
value must be given in hexadecimal notation. If it is too big it will be
truncated.
pri-on-incon-degr cmd
This handler is called if the node is primary,
degraded and if the local copy of the data is inconsistent.
pri-lost-after-sb cmd
The node is currently primary, but lost the
after-split-brain auto recovery procedure. As as consequence, it should be
abandoned.
pri-lost cmd
The node is currently primary, but DRBD's
algorithm thinks that it should become sync target. As a consequence it should
give up its primary role.
fence-peer cmd
The handler is part of the fencing
mechanism. This handler is called in case the node needs to fence the peer's
disk. It should use other communication paths than DRBD's network link.
local-io-error cmd
DRBD got an IO error from the local IO
subsystem.
initial-split-brain cmd
DRBD has connected and detected a split brain
situation. This handler can alert someone in all cases of split brain, not
just those that go unresolved.
split-brain cmd
DRBD detected a split brain situation but
remains unresolved. Manual recovery is necessary. This handler should alert
someone on duty.
before-resync-target cmd
DRBD calls this handler just before a resync
begins on the node that becomes resync target. It might be used to take a
snapshot of the backing block device.
after-resync-target cmd
DRBD calls this handler just after a resync
operation finished on the node whose disk just became consistent after being
inconsistent for the duration of the resync. It might be used to remove a
snapshot of the backing device that was created by the
before-resync-target handler.
Other Keywords¶
include file-patternInclude 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 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 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 8.3.2 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.SEE ALSO¶
drbd(8), drbddisk(8), drbdsetup(8), drbdadm(8), DRBD User's Guide[1], DRBD web site[3]NOTES¶
- 1.
- DRBD User's Guide
- 2.
- DRBD's online usage counter
- 3.
- DRBD web site
5 Dec 2008 | DRBD 8.3.2 |