Add support for Thin Arbiter volumes

[sammcj]

Very useful recent feature, should be easy to add - I'm about to go overseas though so won't get a chance to do a MR for it for a few weeks at least.

https://gluster.readthedocs.io/en/latest/Administrator%20Guide/Thin-Arbiter-Volumes/

Thin Arbiter volumes in Gluster

Thin Arbiter is a new type of quorum node where granularity of what is good and what is bad data is less compared to the traditional arbiter brick. In this type of volume, quorum is taken into account at a brick level rather than per file basis. If there is even one file that is marked bad (i.e. needs healing) on a data brick, that brick is considered bad for all files as a whole. So, even different file, if the write fails on the other data brick but succeeds on this 'bad' brick we will return failure for the write.

Why Thin Arbiter?

This is a solution for handling stretch cluster kind of workload, but it can be used for regular workloads as well in case users are satisfied with this kind of quorum in comparison to arbiter/3-way-replication. Thin arbiter node can be placed outside of trusted storage pool i.e, thin arbiter is the "stretched" node in the cluster. This node can be placed on cloud or anywhere even if that connection has high latency. As this node will take part only in case of failure (or a brick is down) and to decide the quorum, it will not impact the performance in normal cases. Cost to perform any file operation would be lesser than arbiter if everything is fine. I/O will only go to the data bricks and goes to thin-arbiter only in the case of first failure until heal completes.

Setting UP Thin Arbiter Volume

The command to run thin-arbiter process on node:

/usr/local/sbin/glusterfsd -N --volfile-id ta-vol -f /var/lib/glusterd/vols/thin-arbiter.vol --brick-port 24007 --xlator-option ta-vol-server.transport.socket.listen-port=24007

Creating a thin arbiter replica 2 volume:

glustercli volume create <volname> --replica 2 <host1>:<brick1> <host2>:<brick2> --thin-arbiter <quorum-host>:<path-to-store-replica-id-file>`

For example:

glustercli volume create testvol --replica 2 server{1..2}:/bricks/brick-{1..2} --thin-arbiter server-3:/bricks/brick_ta --force
volume create: testvol: success: please start the volume to access data