[Gluster-users] Quorum in distributed-replicate volume

[Karthik Subrahmanya]

On Tue, Feb 27, 2018 at 1:40 PM, Dave Sherohman <dave at sherohman.org> wrote:

> On Tue, Feb 27, 2018 at 12:00:29PM +0530, Karthik Subrahmanya wrote:
> > I will try to explain how you can end up in split-brain even with cluster
> > wide quorum:
>
> Yep, the explanation made sense.  I hadn't considered the possibility of
> alternating outages.  Thanks!
>
> > > > It would be great if you can consider configuring an arbiter or
> > > > replica 3 volume.
> > >
> > > I can.  My bricks are 2x850G and 4x11T, so I can repurpose the small
> > > bricks as arbiters with minimal effect on capacity.  What would be the
> > > sequence of commands needed to:
> > >
> > > 1) Move all data off of bricks 1 & 2
> > > 2) Remove that replica from the cluster
> > > 3) Re-add those two bricks as arbiters
> > >
> > > (And did I miss any additional steps?)
> > >
> > > Unfortunately, I've been running a few months already with the current
> > > configuration and there are several virtual machines running off the
> > > existing volume, so I'll need to reconfigure it online if possible.
> > >
> > Without knowing the volume configuration it is difficult to suggest the
> > configuration change,
> > and since it is a live system you may end up in data unavailability or
> data
> > loss.
> > Can you give the output of "gluster volume info <volname>"
> > and which brick is of what size.
>
> Volume Name: palantir
> Type: Distributed-Replicate
> Volume ID: 48379a50-3210-41b4-9a77-ae143c8bcac0
> Status: Started
> Snapshot Count: 0
> Number of Bricks: 3 x 2 = 6
> Transport-type: tcp
> Bricks:
> Brick1: saruman:/var/local/brick0/data
> Brick2: gandalf:/var/local/brick0/data
> Brick3: azathoth:/var/local/brick0/data
> Brick4: yog-sothoth:/var/local/brick0/data
> Brick5: cthulhu:/var/local/brick0/data
> Brick6: mordiggian:/var/local/brick0/data
> Options Reconfigured:
> features.scrub: Inactive
> features.bitrot: off
> transport.address-family: inet
> performance.readdir-ahead: on
> nfs.disable: on
> network.ping-timeout: 1013
> performance.quick-read: off
> performance.read-ahead: off
> performance.io-cache: off
> performance.stat-prefetch: off
> cluster.eager-lock: enable
> network.remote-dio: enable
> cluster.quorum-type: auto
> cluster.server-quorum-type: server
> features.shard: on
> cluster.data-self-heal-algorithm: full
> storage.owner-uid: 64055
> storage.owner-gid: 64055
>
>
> For brick sizes, saruman/gandalf have
>
> $ df -h /var/local/brick0
> Filesystem                   Size  Used Avail Use% Mounted on
> /dev/mapper/gandalf-gluster  885G   55G  786G   7% /var/local/brick0
>
> and the other four have
>
> $ df -h /var/local/brick0
> Filesystem      Size  Used Avail Use% Mounted on
> /dev/sdb1        11T  254G   11T   3% /var/local/brick0
>

If you want to use the first two bricks as arbiter, then you need to be
aware of the following things:
- Your distribution count will be decreased to 2.
- Your data on the first subvol i.e., replica subvol - 1 will be
unavailable till it is copied to the other subvols
after removing the bricks from the cluster.

Since arbiter bricks need not be of same size as the data bricks, if you
can configure three more arbiter bricks
based on the guidelines in the doc [1], you can do it live and you will
have the distribution count also unchanged.

One more thing from the volume info; Only the options which are
reconfigured will appear in the volume info output.
The quorum-type is in the list which says it is manually reconfigured.

[1]
http://docs.gluster.org/en/latest/Administrator%20Guide/arbiter-volumes-and-quorum/#arbiter-bricks-sizing

Regards,
Karthik

>
>
> --
> Dave Sherohman
>
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