[Gluster-users] design of gluster cluster

[Dave Sherohman]

On Wed, Jun 13, 2018 at 11:16:32AM +1200, Thing wrote:
> I am a bit lost here, why a replica 3 and arbiter 1?  ie not replica2
> arbiter1?

You'd have to ask the developers about that (I just use gluster, I'm not
a dev).  I agree that "replica 2 arbiter 1" seems more intuitive, but I
suppose "replica 3 arbiter 1" could be seen as more technically
accurate, if you look at it as three replicas, but one of the replicas
only stores metadata instead of a full copy.

> also no distributed part?  is the distributed flag automatically
> assumed?

When you create a replicated volume and specify more bricks than the
replica count, "distributed" is implied because that's the only way that
it would make sense for a replica-2 volume to have 6 bricks.  (See the
"To create a distributed replicated volume" section in the docs at
https://gluster.readthedocs.io/en/latest/Administrator%20Guide/Setting%20Up%20Volumes/
if you'd like to confirm that omitting "distributed" is correct.)

Come to think of it, that might also be why arbiter configurations are
considered to be "replica 3" instead of "replica 2" - you're specifying
bricks in groups of three, so it makes the syntax checking a little
easier if you can just say "number of bricks must be a multiple of
replica count" without adding a special case to increment the replica
count if there's an arbiter also specified.

> with a replica3 then there is a quorum (2 of 3) so no arbiter is
> needed?

Correct.  Arbiters are only needed (or allowed) in the specific case of
replica 2+arbiter.  Per the docs at
https://gluster.readthedocs.io/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/
"Note: Volumes using the arbiter feature can only be replica 3 arbiter 1"

> I have this running already like this so I am assuming its robust?

Yes, it definitely should be!  The only disadvantage of replica 3 vs.
replica 2+A is the lower storage capacity.

> So on each gluster node I have an-unused 120gb data1 partition which is
> left over from the OS install so the arbiter volume could go here?
> 
> in which case?
> 
> gluster volume create my-volume replica 2 arbiter 1 host1:/path/to/brick
> host2:/path/to/brick (arb-)host3:/path/to/brick2   host4:/path/to/brick
> host5:/path/to/brick (arb-)host6:/path/to/brick2 host3:/path/to/brick
> host6:/path/to/brick (arb-)host1:/path/to/brick2
> 
> is this a sane command?

Yep, looks reasonable to me aside from the "replica 2" needing to be
"replica 3".


-- 
Dave Sherohman