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authorRavishankar N <ravishankar@redhat.com>2015-05-05 10:07:13 +0530
committerHumble Devassy Chirammal <humble.devassy@gmail.com>2015-05-05 02:27:57 -0700
commitb7da3d569dfa6d6c27480a74a345cf0e166e3fef (patch)
tree7a377326a10e87bc8cacf751f552f103dc1c2d39 /doc
parent9ddea81a3b6f557c899c90ec84ac8463616e0bc5 (diff)
doc: AFR arbiter volume usage
Contains information on creation and behaviour of replica 3 arbiter volumes. Signed-off-by: Ravishankar N <ravishankar@redhat.com> Change-Id: I6af4aa3488649686fdb9b839c733046160e0785b BUG: 1199985 Reviewed-on: http://review.gluster.org/10541 Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com> Tested-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
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@@ -266,6 +266,13 @@ high-availability and high-reliability are critical.
> Use the `force` option at the end of command if you want to create the volume in this case.
+###Arbiter configuration for replica volumes
+Arbiter volumes are replica 3 volumes where the 3rd brick acts as the arbiter brick. This configuration has mechanisms that prevent occurrence of split-brains.
+It can be created with the following command:
+`# gluster volume create <VOLNAME> replica 3 arbiter 1 host1:brick1 host2:brick2 host3:brick3`
+More information about this configuration can be found at `doc/features/afr-arbiter-volumes.md`
+Note that the arbiter configuration for replica 3 can be used to create distributed-replicate volumes as well.
+
##Creating Striped Volumes
Striped volumes stripes data across bricks in the volume. For best
diff --git a/doc/features/afr-arbiter-volumes.md b/doc/features/afr-arbiter-volumes.md
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+Usage guide: Replicate volumes with arbiter configuration
+==========================================================
+Arbiter volumes are replica 3 volumes where the 3rd brick of the replica is
+automatically configured as an arbiter node. What this means is that the 3rd
+brick will store only the file name and metadata, but does not contain any data.
+This configuration is helpful in avoiding split-brains while providing the same
+level of consistency as a normal replica 3 volume.
+
+The arbiter volume can be created with the following command:
+`gluster volume create <VOLNAME> replica 3 arbiter 1 host1:brick1 host2:brick2 host3:brick3`
+
+Note that the syntax is similar to creating a normal replica 3 volume with the
+exception of the `arbiter 1` keyword. As seen in the command above, the only
+permissible values for the replica count and arbiter count are 3 and 1
+respectively. Also, the 3rd brick is always chosen as the arbiter brick and it
+is not configurable to have any other brick as the arbiter.
+
+Client/ Mount behaviour:
+========================
+By default, client quorum (`cluster.quorum-type`) is set to `auto` for a replica
+3 volume when it is created; i.e. at least 2 bricks need to be up to satisfy
+quorum and to allow writes. This setting is not to be changed for arbiter
+volumes also. Additionally, the arbiter volume has additional some checks to
+prevent files from ending up in split-brain:
+
+* Clients take full file locks when writing to a file as opposed to range locks
+ in a normal replica 3 volume.
+
+* If 2 bricks are up and if one of them is the arbiter (i.e. the 3rd brick) *and*
+ it blames the other up brick, then all FOPS will fail with ENOTCONN (Transport
+ endpoint is not connected). IF the arbiter doesn't blame the other brick,
+ FOPS will be allowed to proceed. 'Blaming' here is w.r.t the values of AFR
+ changelog extended attributes.
+
+* If 2 bricks are up and the arbiter is down, then FOPS will be allowed.
+
+* In all cases, if there is only one source before the FOP is initiated and if
+ the FOP fails on that source, the application will receive ENOTCONN.
+
+Note: It is possible to see if a replica 3 volume has arbiter configuration from
+the mount point. If
+`$mount_point/.meta/graphs/active/$V0-replicate-0/options/arbiter-count` exists
+and its value is 1, then it is an arbiter volume. Also the client volume graph
+will have arbiter-count as a xlator option for AFR translators.
+
+Self-heal daemon behaviour:
+===========================
+Since the arbiter brick does not store any data for the files, data-self-heal
+from the arbiter brick will not take place. For example if there are 2 source
+bricks B2 and B3 (B3 being arbiter brick) and B2 is down, then data-self-heal
+will *not* happen from B3 to sink brick B1, and will be pending until B2 comes
+up and heal can happen from it. Note that metadata and entry self-heals can
+still happen from B3 if it is one of the sources.