Provided by: drbd8-utils_8.3.11-0ubuntu1_i386 bug


       drbd.conf - Configuration file for DRBD's devices .


       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].


       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

       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.

           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”'.

           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

           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.

                    resource r1 {
                         protocol C;
                         device minor 1;
                         meta-disk internal;

                         on alice bob {
                              disk /dev/mapper/some-san;
                         on charlie {
                              disk /dev/mapper/other-san;
                         on daisy {
                              disk /dev/mapper/other-san-as-seen-from-daisy;

           See also the floating section keyword. Required parameters in this
           section: device, disk, address, meta-disk, flexible-meta-disk.

       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, 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.

                    resource r2 {
                         protocol C;
                         device minor 2;
                         disk      /dev/sda7;
                         meta-disk internal;

                         # short form, device, disk and meta-disk inherited

                         # longer form, only device inherited
                         floating {
                              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.

           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

           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

           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

           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,

           The interface is done via environment variables:

               is the name of the resource

               is the minor number of the DRBD device, in decimal.

               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.

               are the address family (e.g.  ipv6), the peer's address and

           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

       minor-count count
           may be a number from 1 to 255.

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

       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.

           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/drbdminor will be used.

           Udev will create additional symlinks in /dev/drbd/by-res and

       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

           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

           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:

               This is the default policy. No fencing actions are taken.

               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'

               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 or
           4. Option 4 will deliver the best performance on such devices.

           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 disabled by the no-disk-barrier option.

               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.

               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. You can disable this
               method by using the no-disk-drain option.

               The fourth method is to not express write-after-write
               dependencies to the backing store at all.

           Disables the use of disk flushes and barrier BIOs when accessing
           the meta data device. See the notes on no-disk-flushes.

           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.

       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
           Maximum 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 number
           The highest number of data blocks between two write barriers. If
           you set this smaller than 10, you might decrease your performance.

           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
           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. Minimum 16, default 128, maximum 131072.

           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.

           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:

               No automatic resynchronization, simply disconnect.

               Auto sync from the node that was primary before the split-brain
               situation happened.

               Auto sync from the node that became primary as second during
               the split-brain situation.

               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.

               Auto sync from the node that touched more blocks during the
               split brain situation.

               Auto sync to the named node.

       after-sb-1pri  policy
           possible policies are:

               No automatic resynchronization, simply disconnect.

               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.

               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 the secondary's version.

               Always honor the outcome of the after-sb-0pri algorithm. In
               case it decides the current secondary has the right data, it
               calls the "pri-lost-after-sb" handler on the current primary.

       after-sb-2pri  policy
           possible policies are:

               No automatic resynchronization, simply disconnect.

               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 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

           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

       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

               No automatic resynchronization, simply disconnect.

               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 the "pri-lost" helper program on one of the machines. This
               program is expected to reboot the machine, i.e. make it

       data-integrity-alg  alg
           DRBD can ensure the data integrity of the user's data on the
           network by comparing hash values. Normally this is ensured by the
           16 bit checksums in the headers of TCP/IP packets.

           This option can be set to any of the kernel's data digest
           algorithms. In a typical kernel configuration you should have at
           least one of md5, sha1, and crc32c available. By default this is
           not enabled.

           See also the notes on data integrity.

           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

       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

           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

           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

       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.

           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).

           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.

           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

           The default is io-error. This setting is available since DRBD

       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

       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-pattern
           Include all files matching the wildcard pattern file-pattern. The
           include statement is only allowed on the top level, i.e. it is not
           allowed inside any section.


       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.


       This document was revised for version 8.3.2 of the DRBD distribution.


       Written by Philipp Reisner and Lars


       Report bugs to


       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


       drbd(8), drbddisk(8), drbdsetup(8), drbdadm(8), DRBD User's Guide[1],
       DRBD web site[3]


        1. DRBD User's Guide

        2. DRBD's online usage counter

        3. DRBD web site