Provided by: sbd_1.3.1-2_amd64 
NAME
sbd - STONITH Block Device daemon
SYNOPSIS
sbd <-d /dev/...> [options] "command"
SUMMARY
SBD provides a node fencing mechanism (Shoot the other node in the head, STONITH) for Pacemaker-based
clusters through the exchange of messages via shared block storage such as for example a SAN, iSCSI,
FCoE. This isolates the fencing mechanism from changes in firmware version or dependencies on specific
firmware controllers, and it can be used as a STONITH mechanism in all configurations that have reliable
shared storage.
SBD can also be used without any shared storage. In this mode, the watchdog device will be used to reset
the node if it loses quorum, if any monitored daemon is lost and not recovered or if Pacemaker decides
that the node requires fencing.
The sbd binary implements both the daemon that watches the message slots as well as the management tool
for interacting with the block storage device(s). This mode of operation is specified via the "command"
parameter; some of these modes take additional parameters.
To use SBD with shared storage, you must first "create" the messaging layout on one to three block
devices. Second, configure /etc/sysconfig/sbd to list those devices (and possibly adjust other options),
and restart the cluster stack on each node to ensure that "sbd" is started. Third, configure the
"external/sbd" fencing resource in the Pacemaker CIB.
Each of these steps is documented in more detail below the description of the command options.
"sbd" can only be used as root.
GENERAL OPTIONS
-d /dev/...
Specify the block device(s) to be used. If you have more than one, specify this option up to three
times. This parameter is mandatory for all modes, since SBD always needs a block device to interact
with.
This man page uses /dev/sda1, /dev/sdb1, and /dev/sdc1 as example device names for brevity. However,
in your production environment, you should instead always refer to them by using the long, stable
device name (e.g., /dev/disk/by-id/dm-uuid-part1-mpath-3600508b400105b5a0001500000250000).
-v Enable some verbose debug logging.
-h Display a concise summary of "sbd" options.
-n node
Set local node name; defaults to "uname -n". This should not need to be set.
-R Do not enable realtime priority. By default, "sbd" runs at realtime priority, locks itself into
memory, and also acquires highest IO priority to protect itself against interference from other
processes on the system. This is a debugging-only option.
-I N
Async IO timeout (defaults to 3 seconds, optional). You should not need to adjust this unless your IO
setup is really very slow.
(In daemon mode, the watchdog is refreshed when the majority of devices could be read within this
time.)
create
Example usage:
sbd -d /dev/sdc2 -d /dev/sdd3 create
If you specify the create command, sbd will write a metadata header to the device(s) specified and also
initialize the messaging slots for up to 255 nodes.
Warning: This command will not prompt for confirmation. Roughly the first megabyte of the specified block
device(s) will be overwritten immediately and without backup.
This command accepts a few options to adjust the default timings that are written to the metadata (to
ensure they are identical across all nodes accessing the device).
-1 N
Set watchdog timeout to N seconds. This depends mostly on your storage latency; the majority of
devices must be successfully read within this time, or else the node will self-fence.
If your sbd device(s) reside on a multipath setup or iSCSI, this should be the time required to
detect a path failure. You may be able to reduce this if your device outages are independent, or if
you are using the Pacemaker integration.
-2 N
Set slot allocation timeout to N seconds. You should not need to tune this.
-3 N
Set daemon loop timeout to N seconds. You should not need to tune this.
-4 N
Set msgwait timeout to N seconds. This should be twice the watchdog timeout. This is the time after
which a message written to a node's slot will be considered delivered. (Or long enough for the node
to detect that it needed to self-fence.)
This also affects the stonith-timeout in Pacemaker's CIB; see below.
list
Example usage:
# sbd -d /dev/sda1 list
0 hex-0 clear
1 hex-7 clear
2 hex-9 clear
List all allocated slots on device, and messages. You should see all cluster nodes that have ever been
started against this device. Nodes that are currently running should have a clear state; nodes that have
been fenced, but not yet restarted, will show the appropriate fencing message.
dump
Example usage:
# sbd -d /dev/sda1 dump
==Dumping header on disk /dev/sda1
Header version : 2
Number of slots : 255
Sector size : 512
Timeout (watchdog) : 15
Timeout (allocate) : 2
Timeout (loop) : 1
Timeout (msgwait) : 30
==Header on disk /dev/sda1 is dumped
Dump meta-data header from device.
watch
Example usage:
sbd -d /dev/sdc2 -d /dev/sdd3 -P watch
This command will make "sbd" start in daemon mode. It will constantly monitor the message slot of the
local node for incoming messages, reachability, and optionally take Pacemaker's state into account.
"sbd" must be started on boot before the cluster stack! See below for enabling this according to your
boot environment.
The options for this mode are rarely specified directly on the commandline directly, but most frequently
set via /etc/sysconfig/sbd.
It also constantly monitors connectivity to the storage device, and self-fences in case the partition
becomes unreachable, guaranteeing that it does not disconnect from fencing messages.
A node slot is automatically allocated on the device(s) the first time the daemon starts watching the
device; hence, manual allocation is not usually required.
If a watchdog is used together with the "sbd" as is strongly recommended, the watchdog is activated at
initial start of the sbd daemon. The watchdog is refreshed every time the majority of SBD devices has
been successfully read. Using a watchdog provides additional protection against "sbd" crashing.
If the Pacemaker integration is activated, "sbd" will not self-fence if device majority is lost, if:
1. The partition the node is in is still quorate according to the CIB;
2. it is still quorate according to Corosync's node count;
3. the node itself is considered online and healthy by Pacemaker.
This allows "sbd" to survive temporary outages of the majority of devices. However, while the cluster is
in such a degraded state, it can neither successfully fence nor be shutdown cleanly (as taking the
cluster below the quorum threshold will immediately cause all remaining nodes to self-fence). In short,
it will not tolerate any further faults. Please repair the system before continuing.
There is one "sbd" process that acts as a master to which all watchers report; one per device to monitor
the node's slot; and, optionally, one that handles the Pacemaker integration.
-W Enable or disable use of the system watchdog to protect against the sbd processes failing and the
node being left in an undefined state. Specify this once to enable, twice to disable.
Defaults to enabled.
-w /dev/watchdog
This can be used to override the default watchdog device used and should not usually be necessary.
-p /var/run/sbd.pid
This option can be used to specify a pidfile for the main sbd process.
-F N
Number of failures before a failing servant process will not be restarted immediately until the
dampening delay has expired. If set to zero, servants will be restarted immediately and indefinitely.
If set to one, a failed servant will be restarted once every -t seconds. If set to a different value,
the servant will be restarted that many times within the dampening period and then delay.
Defaults to 1.
-t N
Dampening delay before faulty servants are restarted. Combined with "-F 1", the most logical way to
tune the restart frequency of servant processes. Default is 5 seconds.
If set to zero, processes will be restarted indefinitely and immediately.
-P Enable Pacemaker integration which checks Pacemaker quorum and node health. Specify this once to
enable, twice to disable.
Defaults to enabled.
-S N
Set the start mode. (Defaults to 0.)
If this is set to zero, sbd will always start up unconditionally, regardless of whether the node was
previously fenced or not.
If set to one, sbd will only start if the node was previously shutdown cleanly (as indicated by an
exit request message in the slot), or if the slot is empty. A reset, crashdump, or power-off request
in any slot will halt the start up.
This is useful to prevent nodes from rejoining if they were faulty. The node must be manually
"unfenced" by sending an empty message to it:
sbd -d /dev/sda1 message node1 clear
-s N
Set the start-up wait time for devices. (Defaults to 120.)
Dynamic block devices such as iSCSI might not be fully initialized and present yet. This allows to
set a timeout for waiting for devices to appear on start-up. If set to 0, start-up will be aborted
immediately if no devices are available.
-Z Enable trace mode. Warning: this is unsafe for production, use at your own risk! Specifying this once
will turn all reboots or power-offs, be they caused by self-fence decisions or messages, into a
crashdump. Specifying this twice will just log them but not continue running.
-T By default, the daemon will set the watchdog timeout as specified in the device metadata. However,
this does not work for every watchdog device. In this case, you must manually ensure that the
watchdog timeout used by the system correctly matches the SBD settings, and then specify this option
to allow "sbd" to continue with start-up.
-5 N
Warn if the time interval for tickling the watchdog exceeds this many seconds. Since the node is
unable to log the watchdog expiry (it reboots immediately without a chance to write its logs to
disk), this is very useful for getting an indication that the watchdog timeout is too short for the
IO load of the system.
Default is 3 seconds, set to zero to disable.
-C N
Watchdog timeout to set before crashdumping. If SBD is set to crashdump instead of reboot - either
via the trace mode settings or the external/sbd fencing agent's parameter -, SBD will adjust the
watchdog timeout to this setting before triggering the dump. Otherwise, the watchdog might trigger
and prevent a successful crashdump from ever being written.
Defaults to 240 seconds. Set to zero to disable.
allocate
Example usage:
sbd -d /dev/sda1 allocate node1
Explicitly allocates a slot for the specified node name. This should rarely be necessary, as every node
will automatically allocate itself a slot the first time it starts up on watch mode.
message
Example usage:
sbd -d /dev/sda1 message node1 test
Writes the specified message to node's slot. This is rarely done directly, but rather abstracted via the
"external/sbd" fencing agent configured as a cluster resource.
Supported message types are:
test
This only generates a log message on the receiving node and can be used to check if SBD is seeing the
device. Note that this could overwrite a fencing request send by the cluster, so should not be used
during production.
reset
Reset the target upon receipt of this message.
off Power-off the target.
crashdump
Cause the target node to crashdump.
exit
This will make the "sbd" daemon exit cleanly on the target. You should not send this message
manually; this is handled properly during shutdown of the cluster stack. Manually stopping the daemon
means the node is unprotected!
clear
This message indicates that no real message has been sent to the node. You should not set this
manually; "sbd" will clear the message slot automatically during start-up, and setting this manually
could overwrite a fencing message by the cluster.
Base system configuration
Configure a watchdog
It is highly recommended that you configure your Linux system to load a watchdog driver with hardware
assistance (as is available on most modern systems), such as hpwdt, iTCO_wdt, or others. As a fall-back,
you can use the softdog module.
No other software must access the watchdog timer; it can only be accessed by one process at any given
time. Some hardware vendors ship systems management software that use the watchdog for system resets
(f.e. HP ASR daemon). Such software has to be disabled if the watchdog is to be used by SBD.
Choosing and initializing the block device(s)
First, you have to decide if you want to use one, two, or three devices.
If you are using multiple ones, they should reside on independent storage setups. Putting all three of
them on the same logical unit for example would not provide any additional redundancy.
The SBD device can be connected via Fibre Channel, Fibre Channel over Ethernet, or even iSCSI. Thus, an
iSCSI target can become a sort-of network-based quorum server; the advantage is that it does not require
a smart host at your third location, just block storage.
The SBD partitions themselves must not be mirrored (via MD, DRBD, or the storage layer itself), since
this could result in a split-mirror scenario. Nor can they reside on cLVM2 volume groups, since they must
be accessed by the cluster stack before it has started the cLVM2 daemons; hence, these should be either
raw partitions or logical units on (multipath) storage.
The block device(s) must be accessible from all nodes. (While it is not necessary that they share the
same path name on all nodes, this is considered a very good idea.)
SBD will only use about one megabyte per device, so you can easily create a small partition, or very
small logical units. (The size of the SBD device depends on the block size of the underlying device.
Thus, 1MB is fine on plain SCSI devices and SAN storage with 512 byte blocks. On the IBM s390x
architecture in particular, disks default to 4k blocks, and thus require roughly 4MB.)
The number of devices will affect the operation of SBD as follows:
One device
In its most simple implementation, you use one device only. This is appropriate for clusters where
all your data is on the same shared storage (with internal redundancy) anyway; the SBD device does
not introduce an additional single point of failure then.
If the SBD device is not accessible, the daemon will fail to start and inhibit openais startup.
Two devices
This configuration is a trade-off, primarily aimed at environments where host-based mirroring is
used, but no third storage device is available.
SBD will not commit suicide if it loses access to one mirror leg; this allows the cluster to continue
to function even in the face of one outage.
However, SBD will not fence the other side while only one mirror leg is available, since it does not
have enough knowledge to detect an asymmetric split of the storage. So it will not be able to
automatically tolerate a second failure while one of the storage arrays is down. (Though you can use
the appropriate crm command to acknowledge the fence manually.)
It will not start unless both devices are accessible on boot.
Three devices
In this most reliable and recommended configuration, SBD will only self-fence if more than one device
is lost; hence, this configuration is resilient against temporary single device outages (be it due to
failures or maintenance). Fencing messages can still be successfully relayed if at least two devices
remain accessible.
This configuration is appropriate for more complex scenarios where storage is not confined to a
single array. For example, host-based mirroring solutions could have one SBD per mirror leg (not
mirrored itself), and an additional tie-breaker on iSCSI.
It will only start if at least two devices are accessible on boot.
After you have chosen the devices and created the appropriate partitions and perhaps multipath alias
names to ease management, use the "sbd create" command described above to initialize the SBD metadata on
them.
Sharing the block device(s) between multiple clusters
It is possible to share the block devices between multiple clusters, provided the total number of nodes
accessing them does not exceed 255 nodes, and they all must share the same SBD timeouts (since these are
part of the metadata).
If you are using multiple devices this can reduce the setup overhead required. However, you should not
share devices between clusters in different security domains.
Configure SBD to start on boot
On systems using "sysvinit", the "openais" or "corosync" system start-up scripts must handle starting or
stopping "sbd" as required before starting the rest of the cluster stack.
For "systemd", sbd simply has to be enabled using
systemctl enable sbd.service
The daemon is brought online on each node before corosync and Pacemaker are started, and terminated only
after all other cluster components have been shut down - ensuring that cluster resources are never
activated without SBD supervision.
Configuration via sysconfig
The system instance of "sbd" is configured via /etc/sysconfig/sbd. In this file, you must specify the
device(s) used, as well as any options to pass to the daemon:
SBD_DEVICE="/dev/sda1;/dev/sdb1;/dev/sdc1"
SBD_PACEMAKER="true"
"sbd" will fail to start if no "SBD_DEVICE" is specified. See the installed template for more options
that can be configured here.
Testing the sbd installation
After a restart of the cluster stack on this node, you can now try sending a test message to it as root,
from this or any other node:
sbd -d /dev/sda1 message node1 test
The node will acknowledge the receipt of the message in the system logs:
Aug 29 14:10:00 node1 sbd: [13412]: info: Received command test from node2
This confirms that SBD is indeed up and running on the node, and that it is ready to receive messages.
Make sure that /etc/sysconfig/sbd is identical on all cluster nodes, and that all cluster nodes are
running the daemon.
Pacemaker CIB integration
Fencing resource
Pacemaker can only interact with SBD to issue a node fence if there is a configure fencing resource. This
should be a primitive, not a clone, as follows:
primitive fencing-sbd stonith:external/sbd \
params pcmk_delay_max=30
This will automatically use the same devices as configured in /etc/sysconfig/sbd.
While you should not configure this as a clone (as Pacemaker will register the fencing device on each
node automatically), the pcmk_delay_max setting enables random fencing delay which ensures, in a scenario
where a split-brain scenario did occur in a two node cluster, that one of the nodes has a better chance
to survive to avoid double fencing.
SBD also supports turning the reset request into a crash request, which may be helpful for debugging if
you have kernel crashdumping configured; then, every fence request will cause the node to dump core. You
can enable this via the "crashdump="true"" parameter on the fencing resource. This is not recommended for
production use, but only for debugging phases.
General cluster properties
You must also enable STONITH in general, and set the STONITH timeout to be at least twice the msgwait
timeout you have configured, to allow enough time for the fencing message to be delivered. If your
msgwait timeout is 60 seconds, this is a possible configuration:
property stonith-enabled="true"
property stonith-timeout="120s"
Caution: if stonith-timeout is too low for msgwait and the system overhead, sbd will never be able to
successfully complete a fence request. This will create a fencing loop.
Note that the sbd fencing agent will try to detect this and automatically extend the stonith-timeout
setting to a reasonable value, on the assumption that sbd modifying your configuration is preferable to
not fencing.
Management tasks
Recovering from temporary SBD device outage
If you have multiple devices, failure of a single device is not immediately fatal. "sbd" will retry to
restart the monitor for the device every 5 seconds by default. However, you can tune this via the options
to the watch command.
In case you wish the immediately force a restart of all currently disabled monitor processes, you can
send a SIGUSR1 to the SBD inquisitor process.
LICENSE
Copyright (C) 2008-2013 Lars Marowsky-Bree
This program is free software; you can redistribute it and/or modify it under the terms of the GNU
General Public License as published by the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This software is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even
the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
License for more details.
For details see the GNU General Public License at http://www.gnu.org/licenses/gpl-2.0.html (version 2)
and/or http://www.gnu.org/licenses/gpl.html (the newest as per "any later").
SBD 2017-11-25 SBD(8)