Provided by: drbd-utils_8.9.6-1_amd64 
NAME
drbd.conf - DRBD Configuration Files
INTRODUCTION
DRBD implements block devices which replicate their data to all nodes of a cluster. The
actual data and associated metadata are usually stored redundantly on "ordinary" block
devices on each cluster node.
Replicated block devices are called /dev/drbdminor by default. They are grouped into
resources, with one or more devices per resource. Replication among the devices in a
resource takes place in chronological order. With DRBD, we refer to the devices inside a
resource as volumes.
In DRBD 9, a resource can be replicated between two or more cluster nodes. The connections
between cluster nodes are point-to-point links, and use TCP or a TCP-like protocol. All
nodes must be directly connected.
DRBD consists of low-level user-space components which interact with the kernel and
perform basic operations (drbdsetup, drbdmeta), a high-level user-space component which
understands and processes the DRBD configuration and translates it into basic operations
of the low-level components (drbdadm), and a kernel component.
The default DRBD configuration consists of /etc/drbd.conf and of additional files included
from there, usually global_common.conf and all *.res files inside /etc/drbd.d/. It has
turned out to be useful to define each resource in a separate *.res file.
The configuration files are designed so that each cluster node can contain an identical
copy of the entire cluster configuration. The host name of each node determines which
parts of the configuration apply (uname -n). It is highly recommended to keep the cluster
configuration on all nodes in sync by manually copying it to all nodes, or by automating
the process with csync2 or a similar tool.
EXAMPLE CONFIGURATION FILE
resource r0 {
net {
cram-hmac-alg sha1;
shared-secret "FooFunFactory";
}
volume 0 {
device /dev/drbd1;
disk /dev/sda7;
meta-disk internal;
}
on alice {
node-id 0;
address 10.1.1.31:7000;
}
on bob {
node-id 1;
address 10.1.1.32:7000;
}
connection {
host alice port 7000;
host bob port 7000;
net {
protocol C;
}
}
}
This example defines a resource r0 which contains a single replicated device with volume
number 0. The resource is replicated among hosts alice and bob, which have the IPv4
addresses 10.1.1.31 and 10.1.1.32 and the node identifiers 0 and 1, respectively. On both
hosts, the replicated device is called /dev/drbd1, and the actual data and metadata are
stored on the lower-level device /dev/sda7. The connection between the hosts uses protocol
C.
Please refer to the DRBD User's Guide[1] for more examples.
FILE FORMAT
DRBD configuration files consist of sections, which contain other sections and parameters
depending on the section types. Each section consists of one or more keywords, sometimes a
section name, an opening brace (“{”), the section's contents, and a closing brace (“}”).
Parameters inside a section consist of a keyword, followed by one or more keywords or
values, and a semicolon (“;”).
Some parameter values have a default scale which applies when a plain number is specified
(for example Kilo, or 1024 times the numeric value). Such default scales can be overridden
by using a suffix (for example, M for Mega). The common suffixes K = 2^10 = 1024, M = 1024
K, and G = 1024 M are supported.
Comments start with a hash sign (“#”) and extend to the end of the line. In addition, any
section can be prefixed with the keyword skip, which causes the section and any
sub-sections to be ignored.
Additional files can be included with the include file-pattern statement (see glob(7) for
the expressions supported in file-pattern). Include statements are only allowed outside of
sections.
The following sections are defined (indentation indicates in which context):
common
[disk]
[handlers]
[net]
[options]
[startup]
global
resource
connection
path
net
connection-mesh
net
[disk]
floating
handlers
[net]
on
volume
disk
[disk]
options
stacked-on-top-of
startup
Sections in brackets affect other parts of the configuration: inside the common section,
they apply to all resources. A disk section inside a resource or on section applies to all
volumes of that resource, and a net section inside a resource section applies to all
connections of that resource. This allows to avoid repeating identical options for each
resource, connection, or volume. Options can be overridden in a more specific resource,
connection, on, or volume section.
Sections
common
This section can contain each a disk, handlers, net, options, and startup section. All
resources inherit the parameters in these sections as their default values.
connection [name]
Define a connection between two hosts. This section must contain two host parameters
or multiple path sections. The optional name is used to refer to the connection in the
system log and in other messages. If no name is specified, the peer's host name is
used instead.
path
Define a path between two hosts. This section must contain two host parameters.
connection-mesh
Define a connection mesh between multiple hosts. This section must contain a hosts
parameter, which has the host names as arguments. This section is a shortcut to define
many connections which share the same network options.
disk
Define parameters for a volume. All parameters in this section are optional.
floating [address-family] addr:port
Like the on section, except that instead of the host name a network address is used to
determine if it matches a floating section.
The node-id parameter in this section is required. If the address parameter is not
provided, no connections to peers will be created by default. The device, disk, and
meta-disk parameters must be defined in, or inherited by, this section.
global
Define some global parameters. All parameters in this section are optional. Only one
global section is allowed in the configuration.
handlers
Define handlers to be invoked when certain events occur. The kernel passes the
resource name in the first command-line argument and sets the following environment
variables depending on the event's context:
• For events related to a particular device: the device's minor number in
DRBD_MINOR, the device's volume number in DRBD_VOLUME.
• For events related to a particular device on a particular peer: the connection
endpoints in DRBD_MY_ADDRESS, DRBD_MY_AF, DRBD_PEER_ADDRESS, and DRBD_PEER_AF; the
device's local minor number in DRBD_MINOR, and the device's volume number in
DRBD_VOLUME.
• For events related to a particular connection: the connection endpoints in
DRBD_MY_ADDRESS, DRBD_MY_AF, DRBD_PEER_ADDRESS, and DRBD_PEER_AF; and, for each
device defined for that connection: the device's minor number in
DRBD_MINOR_volume-number.
• For events that identify a device, if a lower-level device is attached, the
lower-level device's device name is passed in DRBD_BACKING_DEV (or
DRBD_BACKING_DEV_volume-number).
All parameters in this section are optional. Only a single handler can be defined for
each event; if no handler is defined, nothing will happen.
net
Define parameters for a connection. All parameters in this section are optional.
on host-name [...]
Define the properties of a resource on a particular host or set of hosts. Specifying
more than one host name can make sense in a setup with IP address failover, for
example. The host-name argument must match the Linux host name (uname -n).
Usually contains or inherits at least one volume section. The node-id and address
parameters must be defined in this section. The device, disk, and meta-disk parameters
must be defined in, or inherited by, this section.
A normal configuration file contains two or more on sections for each resource. Also
see the floating section.
options
Define parameters for a resource. All parameters in this section are optional.
resource name
Define a resource. Usually contains at least two on sections and at least one
connection section.
stacked-on-top-of resource
Used instead of an on section for configuring a stacked resource with three to four
nodes.
Starting with DRBD 9, stacking is deprecated. It is advised to use resources which are
replicated among more than two nodes instead.
startup
The parameters in this section determine the behavior of a resource at startup time.
volume volume-number
Define a volume within a resource. The volume numbers in the various volume sections
of a resource define which devices on which hosts form a replicated device.
Section connection Parameters
host name [address [address-family] address] [port port-number]
Defines an endpoint for a connection. Each host statement refers to an on section in a
resource. If a port number is defined, this endpoint will use the specified port
instead of the port defined in the on section. Each connection section must contain
exactly two host parameters. Instead of two host parameters the connection may contain
multiple path sections.
Section path Parameters
host name [address [address-family] address] [port port-number]
Defines an endpoint for a connection. Each host statement refers to an on section in a
resource. If a port number is defined, this endpoint will use the specified port
instead of the port defined in the on section. Each path section must contain exactly
two host parameters.
Section connection-mesh Parameters
hosts name...
Defines all nodes of a mesh. Each name refers to an on section in a resource. The port
that is defined in the on section will be used.
Section disk Parameters
al-extents extents
DRBD automatically maintains a "hot" or "active" disk area likely to be written to
again soon based on the recent write activity. The "active" disk area can be written
to immediately, while "inactive" disk areas must be "activated" first, which requires
a meta-data write. We also refer to this active disk area as the "activity log".
The activity log saves meta-data writes, but the whole log must be resynced upon
recovery of a failed node. The size of the activity log is a major factor of how long
a resync will take and how fast a replicated disk will become consistent after a
crash.
The activity log consists of a number of 4-Megabyte segments; the al-extents parameter
determines how many of those segments can be active at the same time. The default
value for al-extents is 1237, with a minimum of 7 and a maximum of 65536.
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}
With this parameter, the activity log can be turned off entirely (see the al-extents
parameter). This will speed up writes because fewer meta-data writes will be
necessary, but the entire device needs to be resynchronized opon recovery of a failed
primary node. The default value for al-updates is yes.
c-delay-target delay_target,
c-fill-target fill_target,
c-max-rate max_rate,
c-plan-ahead plan_time
Dynamically control the resync speed. This mechanism is enabled by setting the
c-plan-ahead parameter to a positive value. The goal is to either fill the buffers
along the data path with a defined amount of data if c-fill-target is defined, or to
have a defined delay along the path if c-delay-target is defined. The maximum
bandwidth is limited by the c-max-rate parameter.
The c-plan-ahead parameter defines how fast drbd adapts to changes in the resync
speed. It should be set to five times the network round-trip time or more. Common
values for c-fill-target for "normal" data paths range from 4K to 100K. If drbd-proxy
is used, it is advised to use c-delay-target instead of c-fill-target. The
c-delay-target parameter is used if the c-fill-target parameter is undefined or set to
0. The c-delay-target parameter should be set to five times the network round-trip
time or more. The c-max-rate option should be set to either the bandwidth available
between the DRBD-hosts and the machines hosting DRBD-proxy, or to the available disk
bandwidth.
The default values of these parameters are: c-plan-ahead = 20 (in units of 0.1
seconds), c-fill-target = 0 (in units of sectors), c-delay-target = 1 (in units of 0.1
seconds), and c-max-rate = 102400 (in units of KiB/s).
Dynamic resync speed control is available since DRBD 8.3.9.
c-min-rate min_rate
A node which is primary and sync-source has to schedule application I/O requests and
resync I/O requests. The c-min-rate parameter limits how much bandwidth is available
for resync I/O; the remaining bandwidth is used for application I/O.
A c-min-rate value of 0 means that there is no limit on the resync I/O bandwidth. This
can slow down application I/O significantly. Use a value of 1 (1 KiB/s) for the lowest
possible resync rate.
The default value of c-min-rate is 4096, in units of KiB/s.
disk-barrier,
disk-flushes,
disk-drain
DRBD has three methods of handling the ordering of dependent write requests:
disk-barrier
Use disk barriers to make sure that requests are written to disk in the right
order. Barriers ensure that all requests submitted before a barrier make it to the
disk before any requests submitted after the barrier. This is implemented using
'tagged command queuing' on SCSI devices and 'native command queuing' on SATA
devices. Only some devices and device stacks support this method. The device
mapper (LVM) only supports barriers in some configurations.
Note that on systems which do not support disk barriers, enabling this option can
lead to data loss or corruption. Until DRBD 8.4.1, disk-barrier was turned on if
the I/O stack below DRBD did support barriers. Kernels since linux-2.6.36 (or
2.6.32 RHEL6) no longer allow to detect if barriers are supported. Since
drbd-8.4.2, this option is off by default and needs to be enabled explicitly.
disk-flushes
Use disk flushes between dependent write requests, also referred to as 'force unit
access' by drive vendors. This forces all data to disk. This option is enabled by
default.
disk-drain
Wait for the request queue to "drain" (that is, wait for the requests to finish)
before submitting a dependent write request. This method requires that requests
are stable on disk when they finish. Before DRBD 8.0.9, this was the only method
implemented. This option is enabled by default. Do not disable in production
environments.
From these three methods, drbd will use the first that is enabled and supported by the
backing storage device. If all three of these options are turned off, DRBD will submit
write requests without bothering about dependencies. Depending on the I/O stack, write
requests can be reordered, and they can be submitted in a different order on different
cluster nodes. This can result in data loss or corruption. Therefore, turning off all
three methods of controlling write ordering is strongly discouraged.
A general guideline for configuring write ordering is to use disk barriers or disk
flushes when using ordinary disks (or an ordinary disk array) with a volatile write
cache. On storage without cache or with a battery backed write cache, disk draining
can be a reasonable choice.
disk-timeout
If the lower-level device on which a DRBD device stores its data does not finish an
I/O request within the defined disk-timeout, DRBD treats this as a failure. The
lower-level device is detached, and the device's disk state advances to Diskless. If
DRBD is connected to one or more peers, the failed request is passed on to one of
them.
This option is dangerous and may lead to kernel panic!
"Aborting" requests, or force-detaching the disk, is intended for completely
blocked/hung local backing devices which do no longer complete requests at all, not
even do error completions. In this situation, usually a hard-reset and failover is the
only way out.
By "aborting", basically faking a local error-completion, we allow for a more graceful
swichover by cleanly migrating services. Still the affected node has to be rebooted
"soon".
By completing these requests, we allow the upper layers to re-use the associated data
pages.
If later the local backing device "recovers", and now DMAs some data from disk into
the original request pages, in the best case it will just put random data into unused
pages; but typically it will corrupt meanwhile completely unrelated data, causing all
sorts of damage.
Which means delayed successful completion, especially for READ requests, is a reason
to panic(). We assume that a delayed *error* completion is OK, though we still will
complain noisily about it.
The default value of disk-timeout is 0, which stands for an infinite timeout. Timeouts
are specified in units of 0.1 seconds. This option is available since DRBD 8.3.12.
fencing fencing_policy
Fencing is a preventive measure to avoid situations where both nodes are primary and
disconnected. This is also known as a split-brain situation. DRBD supports the
following fencing policies:
dont-care
No fencing actions are taken. This is the default policy.
resource-only
If a node becomes a disconnected primary, it tries to fence the peer. This is done
by calling the fence-peer handler. The handler is supposed to reach the peer over
an alternative communication path and call 'drbdadm outdate minor' 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 an alternative communication path and call 'drbdadm outdate minor' there. In
case it cannot do that, it should stonith the peer. IO is resumed as soon as the
situation is resolved. In case the fence-peer handler fails, I/O can be resumed
manually with 'drbdadm resume-io'.
md-flushes
Enable disk flushes and disk barriers on the meta-data device. This option is enabled
by default. See the disk-flushes parameter.
on-io-error handler
Configure how DRBD reacts to I/O errors on a lower-level device. The following
policies are defined:
pass_on
Change the disk status to Inconsistent, mark the failed block as inconsistent in
the bitmap, and retry the I/O operation on a remote cluster node.
call-local-io-error
Call the local-io-error handler (see the handlers section).
detach
Detach the lower-level device and continue in diskless mode.
read-balancing policy
Distribute read requests among cluster nodes as defined by policy. The supported
policies are prefer-local (the default), prefer-remote, round-robin, least-pending,
when-congested-remote, 32K-striping, 64K-striping, 128K-striping, 256K-striping,
512K-striping and 1M-striping.
This option is available since DRBD 8.4.1.
resync-after res-name/volume
Define that a device should only resynchronize after the specified other device. By
default, no order between devices is defined, and all devices will resynchronize in
parallel. Depending on the configuration of the lower-level devices, and the available
network and disk bandwidth, this can slow down the overall resync process. This option
can be used to form a chain or tree of dependencies among devices.
resync-rate rate
Define how much bandwidth DRBD may use for resynchronizing. DRBD allows "normal"
application I/O even during a resync. If the resync takes up too much bandwidth,
application I/O can become very slow. This parameter allows to avoid that. Please note
this is option only works when the dynamic resync controller is disabled.
rs-discard-granularity byte
When rs-discard-granularity is set to a non zero, positive value then DRBD tries to do
a resync operation in requests of this size. In case such a block contains only zero
bytes on the sync source node, the sync target node will issue a discard/trim/unmap
command for the area.
The value is constrained by the discard granularity of the backing block device. In
case rs-discard-granularity is not a multiplier of the discard granularity of the
backing block device DRBD rounds it up. The feature only gets active if the backing
block device reads back zeroes after a discard command.
The default value of is 0. This option is available since 8.4.7.
discard-zeroes-if-aligned {yes | no}
There are several aspects to discard/trim/unmap support on linux block devices. Even
if discard is supported in general, it may fail silently, or may partially ignore
discard requests. Devices also announce whether reading from unmapped blocks returns
defined data (usually zeroes), or undefined data (possibly old data, possibly
garbage).
If on different nodes, DRBD is backed by devices with differing discard
characteristics, discards may lead to data divergence (old data or garbage left over
on one backend, zeroes due to unmapped areas on the other backend). Online verify
would now potentially report tons of spurious differences. While probably harmless for
most use cases (fstrim on a file system), DRBD cannot have that.
To play safe, we have to disable discard support, if our local backend (on a Primary)
does not support "discard_zeroes_data=true". We also have to translate discards to
explicit zero-out on the receiving side, unless the receiving side (Secondary)
supports "discard_zeroes_data=true", thereby allocating areas what were supposed to be
unmapped.
There are some devices (notably the LVM/DM thin provisioning) that are capable of
discard, but announce discard_zeroes_data=false. In the case of DM-thin, discards
aligned to the chunk size will be unmapped, and reading from unmapped sectors will
return zeroes. However, unaligned partial head or tail areas of discard requests will
be silently ignored.
If we now add a helper to explicitly zero-out these unaligned partial areas, while
passing on the discard of the aligned full chunks, we effectively achieve
discard_zeroes_data=true on such devices.
Setting discard-zeroes-if-aligned to yes will allow DRBD to use discards, and to
announce discard_zeroes_data=true, even on backends that announce
discard_zeroes_data=false.
Setting discard-zeroes-if-aligned to no will cause DRBD to always fall-back to
zero-out on the receiving side, and to not even announce discard capabilities on the
Primary, if the respective backend announces discard_zeroes_data=false.
We used to ignore the discard_zeroes_data setting completely. To not break established
and expected behaviour, and suddenly cause fstrim on thin-provisioned LVs to run
out-of-space instead of freeing up space, the default value is yes.
This option is available since 8.4.7.
Section global Parameters
dialog-refresh time
The DRBD init script can be used to configure and start DRBD devices, which can
involve waiting for other cluster nodes. While waiting, the init script shows the
remaining waiting time. The dialog-refresh defines the number of seconds between
updates of that countdown. The default value is 1; a value of 0 turns off the
countdown.
disable-ip-verification
Normally, DRBD verifies that the IP addresses in the configuration match the host
names. Use the disable-ip-verification parameter to disable these checks.
usage-count {yes | no | ask}
A explained on DRBD's Online Usage Counter[2] web page, DRBD includes a mechanism for
anonymously counting how many installations are using which versions of DRBD. The
results are available on the web page for anyone to see.
This parameter defines if a cluster node participates in the usage counter; the
supported values are yes, no, and ask (ask the user, the default).
We would like to ask users to participate in the online usage counter as this provides
us valuable feedback for steering the development of DRBD.
Section handlers Parameters
after-resync-target cmd
Called on a resync target when a node state changes from Inconsistent to Consistent
when a resync finishes. This handler can be used for removing the snapshot created in
the before-resync-target handler.
before-resync-target cmd
Called on a resync target before a resync begins. This handler can be used for
creating a snapshot of the lower-level device for the duration of the resync: if the
resync source becomes unavailable during a resync, reverting to the snapshot can
restore a consistent state.
fence-peer cmd
Called when a node should fence a resource on a particular peer. The handler should
not use the same communication path that DRBD uses for talking to the peer.
unfence-peer cmd
Called when a node should remove fencing constraints from other nodes.
initial-split-brain cmd
Called when DRBD connects to a peer and detects that the peer is in a split-brain
state with the local node. This handler is also called for split-brain scenarios which
will be resolved automatically.
local-io-error cmd
Called when an I/O error occurs on a lower-level device.
pri-lost cmd
The local node is currently primary, but DRBD believes that it should become a sync
target. The node should give up its primary role.
pri-lost-after-sb cmd
The local node is currently primary, but it has lost the after-split-brain auto
recovery procedure. The node should be abandoned.
pri-on-incon-degr cmd
The local node is primary, and neither the local lower-level device nor a lower-level
device on a peer is up to date. (The primary has no device to read from or to write
to.)
split-brain cmd
DRBD has detected a split-brain situation which could not be resolved automatically.
Manual recovery is necessary. This handler can be used to call for administrator
attention.
Section net Parameters
after-sb-0pri policy
Define how to react if a split-brain scenario is detected and none of the two nodes is
in primary role. (We detect split-brain scenarios when two nodes connect; split-brain
decisions are always between two nodes.) The defined policies are:
disconnect
No automatic resynchronization; simply disconnect.
discard-younger-primary,
discard-older-primary
Resynchronize from the node which became primary first (discard-younger-primary)
or last (discard-older-primary). If both nodes became primary independently, the
discard-least-changes policy is used.
discard-zero-changes
If only one of the nodes wrote data since the split brain situation was detected,
resynchronize from this node to the other. If both nodes wrote data, disconnect.
discard-least-changes
Resynchronize from the node with more modified blocks.
discard-node-nodename
Always resynchronize to the named node.
after-sb-1pri policy
Define how to react if a split-brain scenario is detected, with one node in primary
role and one node in secondary role. (We detect split-brain scenarios when two nodes
connect, so split-brain decisions are always among two nodes.) The defined policies
are:
disconnect
No automatic resynchronization, simply disconnect.
consensus
Discard the data on the secondary node if the after-sb-0pri algorithm would also
discard the data on the secondary node. Otherwise, disconnect.
violently-as0p
Always take the decision of the after-sb-0pri algorithm, even if it causes an
erratic change of the primary's view of the data. This is only useful if a
single-node file system (i.e., not OCFS2 or GFS) with the allow-two-primaries flag
is used. This option can cause the primary node to crash, and should not be used.
discard-secondary
Discard the data on the secondary node.
call-pri-lost-after-sb
Always take the decision of the after-sb-0pri algorithm. If the decision is to
discard the data on the primary node, call the pri-lost-after-sb handler on the
primary node.
after-sb-2pri policy
Define how to react if a split-brain scenario is detected and both nodes are in
primary role. (We detect split-brain scenarios when two nodes connect, so split-brain
decisions are always among two nodes.) The defined policies are:
disconnect
No automatic resynchronization, simply disconnect.
violently-as0p
See the violently-as0p policy for after-sb-1pri.
call-pri-lost-after-sb
Call the pri-lost-after-sb helper program on one of the machines unless that
machine can demote to secondary. The helper program is expected to reboot the
machine, which brings the node into a secondary role. Which machine runs the
helper program is determined by the after-sb-0pri strategy.
allow-two-primaries
The most common way to configure DRBD devices is to allow only one node to be primary
(and thus writable) at a time.
In some scenarios it is preferable to allow two nodes to be primary at once; a
mechanism outside of DRBD then must make sure that writes to the shared, replicated
device happen in a coordinated way. This can be done with a shared-storage cluster
file system like OCFS2 and GFS, or with virtual machine images and a virtual machine
manager that can migrate virtual machines between physical machines.
The allow-two-primaries parameter tells DRBD to allow two nodes to be primary at the
same time. Never enable this option when using a non-distributed file system;
otherwise, data corruption and node crashes will result!
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.)
connect-int time
As soon as a connection between two nodes is configured with drbdsetup connect, DRBD
immediately tries to establish the connection. If this fails, DRBD waits for
connect-int seconds and then repeats. The default value of connect-int is 10 seconds.
cram-hmac-alg hash-algorithm
Configure the hash-based message authentication code (HMAC) or secure hash algorithm
to use for peer authentication. The kernel supports a number of different algorithms,
some of which may be loadable as kernel modules. See the shash algorithms listed in
/proc/crypto. By default, cram-hmac-alg is unset. Peer authentication also requires a
shared-secret to be configured.
csums-alg hash-algorithm
Normally, when two nodes resynchronize, the sync target requests a piece of
out-of-sync data from the sync source, and the sync source sends the data. With many
usage patterns, a significant number of those blocks will actually be identical.
When a csums-alg algorithm is specified, when requesting a piece of out-of-sync data,
the sync target also sends along a hash of the data it currently has. The sync source
compares this hash with its own version of the data. It sends the sync target the new
data if the hashes differ, and tells it that the data are the same otherwise. This
reduces the network bandwidth required, at the cost of higher cpu utilization and
possibly increased I/O on the sync target.
The csums-alg can be set to one of the secure hash algorithms supported by the kernel;
see the shash algorithms listed in /proc/crypto. By default, csums-alg is unset.
csums-after-crash-only
Enabling this option (and csums-alg, above) makes it possible to use the checksum
based resync only for the first resync after primary crash, but not for later "network
hickups".
In most cases, block that are marked as need-to-be-resynced are in fact changed, so
calculating checksums, and both reading and writing the blocks on the resync target is
all effective overhead.
The advantage of checksum based resync is mostly after primary crash recovery, where
the recovery marked larger areas (those covered by the activity log) as
need-to-be-resynced, just in case. Introduced in 8.4.5.
data-integrity-alg alg
DRBD normally relies on the data integrity checks built into the TCP/IP protocol, but
if a data integrity algorithm is configured, it will additionally use this algorithm
to make sure that the data received over the network match what the sender has sent.
If a data integrity error is detected, DRBD will close the network connection and
reconnect, which will trigger a resync.
The data-integrity-alg can be set to one of the secure hash algorithms supported by
the kernel; see the shash algorithms listed in /proc/crypto. By default, this
mechanism is turned off.
Because of the CPU overhead involved, we recommend not to use this option in
production environments. Also see the notes on data integrity below.
ko-count number
If a secondary node fails to complete a write request in ko-count times the timeout
parameter, it is excluded from the cluster. The primary node then sets the connection
to this secondary node to Standalone. To disable this feature, you should explicitly
set it to 0; defaults may change between versions.
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.
max-epoch-size number
Define the maximum number of write requests DRBD may issue before issuing a write
barrier. The default value is 2048, with a minimum of 1 and a maximum of 20000.
Setting this parameter to a value below 10 is likely to decrease performance.
on-congestion policy,
congestion-fill threshold,
congestion-extents threshold
By default, DRBD blocks when the TCP send queue is full. This prevents applications
from generating further write requests until more buffer space becomes available
again.
When DRBD is used together with DRBD-proxy, it can be better to use the pull-ahead
on-congestion policy, which can switch DRBD into ahead/behind mode before the send
queue is full. DRBD then records the differences between itself and the peer in its
bitmap, but it no longer replicates them to the peer. When enough buffer space becomes
available again, the node resynchronizes with the peer and switches back to normal
replication.
This has the advantage of not blocking application I/O even when the queues fill up,
and the disadvantage that peer nodes can fall behind much further. Also, while
resynchronizing, peer nodes will become inconsistent.
The available congestion policies are block (the default) and pull-ahead. The
congestion-fill parameter defines how much data is allowed to be "in flight" in this
connection. The default value is 0, which disables this mechanism of congestion
control, with a maximum of 10 GiBytes. The congestion-extents parameter defines how
many bitmap extents may be active before switching into ahead/behind mode, with the
same default and limits as the al-extents parameter. The congestion-extents parameter
is effective only when set to a value smaller than al-extents.
Ahead/behind mode is available since DRBD 8.3.10.
ping-int interval
When the TCP/IP connection to a peer is idle for more than ping-int seconds, DRBD will
send a keep-alive packet to make sure that a failed peer or network connection is
detected reasonably soon. The default value is 10 seconds, with a minimum of 1 and a
maximum of 120 seconds. The unit is seconds.
ping-timeout timeout
Define the timeout for replies to keep-alive packets. If the peer does not reply
within ping-timeout, DRBD will close and try to reestablish the connection. The
default value is 0.5 seconds, with a minimum of 0.1 seconds and a maximum of 3
seconds. The unit is tenths of a second.
socket-check-timeout timeout
In setups involving a DRBD-proxy and connections that experience a lot of buffer-bloat
it might be necessary to set ping-timeout to an unusual high value. By default DRBD
uses the same value to wait if a newly established TCP-connection is stable. Since the
DRBD-proxy is usually located in the same data center such a long wait time may hinder
DRBD's connect process.
In such setups socket-check-timeout should be set to at least to the round trip time
between DRBD and DRBD-proxy. I.e. in most cases to 1.
The default unit is tenths of a second, the default value is 0 (which causes DRBD to
use the value of ping-timeout instead). Introduced in 8.4.5.
protocol name
Use the specified protocol on this connection. The supported protocols are:
A
Writes to the DRBD device complete as soon as they have reached the local disk and
the TCP/IP send buffer.
B
Writes to the DRBD device complete as soon as they have reached the local disk,
and all peers have acknowledged the receipt of the write requests.
C
Writes to the DRBD device complete as soon as they have reached the local and all
remote disks.
rcvbuf-size size
Configure the size of the TCP/IP receive buffer. A value of 0 (the default) causes the
buffer size to adjust dynamically. This parameter usually does not need to be set, but
it can be set to a value up to 10 MiB. The default unit is bytes.
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. The defined policies
are:
disconnect
No automatic resynchronization, simply disconnect.
violently
Resync to the primary node is allowed, violating the assumption that data on a
block device are stable for one of the nodes. Do not use this option, it is
dangerous.
call-pri-lost
Call the pri-lost handler on one of the machines. The handler is expected to
reboot the machine, which puts it into secondary role.
shared-secret secret
Configure the shared secret used for peer authentication. The secret is a string of up
to 64 characters. Peer authentication also requires the cram-hmac-alg parameter to be
set.
sndbuf-size size
Configure the size of the TCP/IP send buffer. Since DRBD 8.0.13 / 8.2.7, a value of 0
(the default) causes the buffer size to adjust dynamically. Values below 32 KiB are
harmful to the throughput on this connection. Large buffer sizes can be useful
especially when protocol A is used over high-latency networks; the maximum value
supported is 10 MiB.
tcp-cork
By default, DRBD uses the TCP_CORK socket option to prevent the kernel from sending
partial messages; this results in fewer and bigger packets on the network. Some
network stacks can perform worse with this optimization. On these, the tcp-cork
parameter can be used to turn this optimization off.
timeout time
Define the timeout for replies over the network: if a peer node does not send an
expected reply within the specified timeout, it is considered dead and the TCP/IP
connection is closed. The timeout value must be lower than connect-int and lower than
ping-int. The default is 6 seconds; the value is specified in tenths of a second.
unplug-watermark number
Mainline kernels before version 2.6.39-rc1 use an explicit plug / unplug mechanism to
control when a block device starts processing queued requests. On those kernels, the
unplug-watermark parameter defines how many requests must be queued until a secondary
node starts processing them. Some storage controllers perform best when
unplug-watermark is set to the same value as max-buffers; others are more efficient
with smaller values. The default value for unplug-watermark is 128, with a minimum of
16 and a maximum of 131072.
More recent kernels handle plugging and unplugging implicitly; on those kernels, this
parameter has no effect. Note that some distributions have backported this feature to
older kernel versions.
use-rle
Each replicated device on a cluster node has a separate bitmap for each of its peer
devices. The bitmaps are used for tracking the differences between the local and peer
device: depending on the cluster state, a disk range can be marked as different from
the peer in the device's bitmap, in the peer device's bitmap, or in both bitmaps. When
two cluster nodes connect, they exchange each other's bitmaps, and they each compute
the union of the local and peer bitmap to determine the overall differences.
Bitmaps of very large devices are also relatively large, but they usually compress
very well using run-length encoding. This can save time and bandwidth for the bitmap
transfers.
The use-rle parameter determines if run-length encoding should be used. It is on by
default since DRBD 8.4.0.
verify-alg hash-algorithm
Online verification (drbdadm verify) computes and compares checksums of disk blocks
(i.e., hash values) in order to detect if they differ. The verify-alg parameter
determines which algorithm to use for these checksums. It must be set to one of the
secure hash algorithms supported by the kernel before online verify can be used; see
the shash algorithms listed in /proc/crypto.
We recommend to schedule online verifications regularly during low-load periods, for
example once a month. Also see the notes on data integrity below.
Section on Parameters
address [address-family] address:port
Defines the address family, address, and port of a connection endpoint.
The address families ipv4, ipv6, ssocks (Dolphin Interconnect Solutions' "super
sockets"), sdp (Infiniband Sockets Direct Protocol), and sci are supported (sci is an
alias for ssocks). If no address family is specified, ipv4 is assumed. For all address
families except ipv6, the address is specified in IPV4 address notation (for example,
1.2.3.4). For ipv6, the address is enclosed in brackets and uses IPv6 address notation
(for example, [fd01:2345:6789:abcd::1]). The port is always specified as a decimal
number from 1 to 65535.
On each host, the port numbers must be unique for each address; ports cannot be
shared.
node-id value
Defines the unique node identifier for a node in the cluster. Node identifiers are
used to identify individual nodes in the network protocol, and to assign bitmap slots
to nodes in the metadata.
Node identifiers can only be reasssigned in a cluster when the cluster is down. It is
essential that the node identifiers in the configuration and in the device metadata
are changed consistently on all hosts. To change the metadata, dump the current state
with drbdmeta dump-md, adjust the bitmap slot assignment, and update the metadata with
drbdmeta restore-md.
The node-id parameter exists since DRBD 9. Its value ranges from 0 to 16; there is no
default.
Section options Parameters (Resource Options)
auto-promote bool-value
A resource must be promoted to primary role before any of its devices can be mounted
or opened for writing.
Before DRBD 9, this could only be done explicitly ("drbdadm primary"). Since DRBD 9,
the auto-promote parameter allows to automatically promote a resource to primary role
when one of its devices is mounted or opened for writing. As soon as all devices are
unmounted or closed with no more remaining users, the role of the resource changes
back to secondary.
Automatic promotion only succeeds if the cluster state allows it (that is, if an
explicit drbdadm primary command would succeed). Otherwise, mounting or opening the
device fails as it already did before DRBD 9: the mount(2) system call fails with
errno set to EROFS (Read-only file system); the open(2) system call fails with errno
set to EMEDIUMTYPE (wrong medium type).
Irrespective of the auto-promote parameter, if a device is promoted explicitly
(drbdadm primary), it also needs to be demoted explicitly (drbdadm secondary).
The auto-promote parameter is available since DRBD 9.0.0, and defaults to yes.
cpu-mask cpu-mask
Set the cpu affinity mask for DRBD kernel threads. The cpu mask is specified as a
hexadecimal number. The default value is 0, which lets the scheduler decide which
kernel threads run on which CPUs. CPU numbers in cpu-mask which do not exist in the
system are ignored.
on-no-data-accessible policy
Determine how to deal with I/O requests when the requested data is not available
locally or remotely (for example, when all disks have failed). The defined policies
are:
io-error
System calls fail with errno set to EIO.
suspend-io
The resource suspends I/O. I/O can be resumed by (re)attaching the lower-level
device, by connecting to a peer which has access to the data, or by forcing DRBD
to resume I/O with drbdadm resume-io res. When no data is available, forcing I/O
to resume will result in the same behavior as the io-error policy.
This setting is available since DRBD 8.3.9; the default policy is io-error.
peer-ack-window value
On each node and for each device, DRBD maintains a bitmap of the differences between
the local and remote data for each peer device. For example, in a three-node setup
(nodes A, B, C) each with a single device, every node maintains one bitmap for each of
its peers.
When nodes receive write requests, they know how to update the bitmaps for the writing
node, but not how to update the bitmaps between themselves. In this example, when a
write request propagates from node A to B and C, nodes B and C know that they have the
same data as node A, but not whether or not they both have the same data.
As a remedy, the writing node occasionally sends peer-ack packets to its peers which
tell them which state they are in relative to each other.
The peer-ack-window parameter specifies how much data a primary node may send before
sending a peer-ack packet. A low value causes increased network traffic; a high value
causes less network traffic but higher memory consumption on secondary nodes and
higher resync times between the secondary nodes after primary node failures. (Note:
peer-ack packets may be sent due to other reasons as well, e.g. membership changes or
expiry of the peer-ack-delay timer.)
The default value for peer-ack-window is 2 MiB, the default unit is sectors. This
option is available since 9.0.0.
peer-ack-delay expiry-time
If after the last finished write request no new write request gets issued for
expiry-time, then a peer-ack packet is sent. If a new write request is issued before
the timer expires, the timer gets reset to expiry-time. (Note: peer-ack packets may be
sent due to other reasons as well, e.g. membership changes or the peer-ack-window
option.)
This parameter may influence resync behavior on remote nodes. Peer nodes need to wait
until they receive an peer-ack for releasing a lock on an AL-extent. Resync operations
between peers may need to wait for for these locks.
The default value for peer-ack-delay is 100 milliseconds, the default unit is
milliseconds. This option is available since 9.0.0.
Section startup Parameters
The parameters in this section define the behavior of DRBD at system startup time, in the
DRBD init script. They have no effect once the system is up and running.
degr-wfc-timeout timeout
Define how long to wait until all peers are connected in case the cluster consisted of
a single node only when the system went down. This parameter is usually set to a value
smaller than wfc-timeout. The assumption here is that peers which were unreachable
before a reboot are less likely to be be reachable after the reboot, so waiting is
less likely to help.
The timeout is specified in seconds. The default value is 0, which stands for an
infinite timeout. Also see the wfc-timeout parameter.
outdated-wfc-timeout timeout
Define how long to wait until all peers are connected if all peers were outdated when
the system went down. This parameter is usually set to a value smaller than
wfc-timeout. The assumption here is that an outdated peer cannot have become primary
in the meantime, so we don't need to wait for it as long as for a node which was alive
before.
The timeout is specified in seconds. The default value is 0, which stands for an
infinite timeout. Also see the wfc-timeout parameter.
stacked-timeouts
On stacked devices, the wfc-timeout and degr-wfc-timeout parameters in the
configuration are usually ignored, and both timeouts are set to twice the connect-int
timeout. The stacked-timeouts parameter tells DRBD to use the wfc-timeout and
degr-wfc-timeout parameters as defined in the configuration, even on stacked devices.
Only use this parameter if the peer of the stacked resource is usually not available,
or will not become primary. Incorrect use of this parameter can lead to unexpected
split-brain scenarios.
wait-after-sb
This parameter causes DRBD to continue waiting in the init script even when a
split-brain situation has been detected, and the nodes therefore refuse to connect to
each other.
wfc-timeout timeout
Define how long the init script waits until all peers are connected. This can be
useful in combination with a cluster manager which cannot manage DRBD resources: when
the cluster manager starts, the DRBD resources will already be up and running. With a
more capable cluster manager such as Pacemaker, it makes more sense to let the cluster
manager control DRBD resources. The timeout is specified in seconds. The default value
is 0, which stands for an infinite timeout. Also see the degr-wfc-timeout parameter.
Section volume Parameters
device /dev/drbdminor-number
Define the device name and minor number of a replicated block device. This is the
device that applications are supposed to access; in most cases, the device is not used
directly, but as a file system. This parameter is required and the standard device
naming convention is assumed.
In addition to this device, udev will create /dev/drbd/by-res/resource/volume and
/dev/drbd/by-disk/lower-level-device symlinks to the device.
disk {[disk] | none}
Define the lower-level block device that DRBD will use for storing the actual data.
While the replicated drbd device is configured, the lower-level device must not be
used directly. Even read-only access with tools like dumpe2fs(8) and similar is not
allowed. The keyword none specifies that no lower-level block device is configured;
this also overrides inheritance of the lower-level device.
meta-disk internal,
meta-disk device,
meta-disk device [index]
Define where the metadata of a replicated block device resides: it can be internal,
meaning that the lower-level device contains both the data and the metadata, or on a
separate device.
When the index form of this parameter is used, multiple replicated devices can share
the same metadata device, each using a separate index. Each index occupies 128 MiB of
data, which corresponds to a replicated device size of at most 4 TiB with two cluster
nodes. We recommend not to share metadata devices anymore, and to instead use the lvm
volume manager for creating metadata devices as needed.
When the index form of this parameter is not used, the size of the lower-level device
determines the size of the metadata. The size needed is 36 KiB + (size of lower-level
device) / 32K * (number of nodes - 1). If the metadata device is bigger than that, the
extra space is not used.
This parameter is required if a disk other than none is specified, and ignored if disk
is set to none. A meta-disk parameter without a disk parameter is not allowed.
NOTES ON DATA INTEGRITY
DRBD supports two different mechanisms for data integrity checking: first, the
data-integrity-alg network parameter allows to add a checksum to the data sent over the
network. Second, the online verification mechanism (drbdadm verify and the verify-alg
parameter) allows to check for differences in the on-disk data.
Both mechanisms can produce false positives if the data is modified during I/O (i.e.,
while it is being sent over the network or written to disk). This does not always indicate
a problem: for example, some file systems and applications do modify data under I/O for
certain operations. Swap space can also undergo changes while under I/O.
Network data integrity checking tries to identify data modification during I/O by
verifying the checksums on the sender side after sending the data. If it detects a
mismatch, it logs an error. The receiver also logs an error when it detects a mismatch.
Thus, an error logged only on the receiver side indicates an error on the network, and an
error logged on both sides indicates data modification under I/O.
The most recent example of systematic data corruption was identified as a bug in the TCP
offloading engine and driver of a certain type of GBit NIC in 2007: the data corruption
happened on the DMA transfer from core memory to the card. Because the TCP checksum were
calculated on the card, the TCP/IP protocol checksums did not reveal this problem.
VERSION
This document was revised for version 9.0.0 of the DRBD distribution.
AUTHOR
Written by Philipp Reisner <philipp.reisner@linbit.com> and Lars Ellenberg
<lars.ellenberg@linbit.com>.
REPORTING BUGS
Report bugs to <drbd-user@lists.linbit.com>.
COPYRIGHT
Copyright 2001-2012 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
http://www.drbd.org/users-guide/
2.
Online Usage Counter
http://usage.drbd.org
3. DRBD Web Site
http://www.drbd.org/