On 05/11/2015 12:46 AM, Christopher Pereira wrote:
Hi,
Using a replica-2 gluster volume with oVirt is currently not supported
and causes split-brains, specially due to sanlock.
Replica-1 works fine (since split-brains are impossible here) and
replica-3 *should* work or at least reduces the probability of
suffering split-brains (depends on race-conditions?).
Replica 3 definitely reduces the probability of split-brains while
providing better availability than replica 2. The sanlock bug
(https://bugzilla.redhat.com/show_bug.cgi?id=1066996) was fixed quite
some time back.
Besides, geo-replication allows to replicate a replica-1 volume in
order to achieve similar results as replica-2.
But since geo-rep uses rsync I guess that it's less optimal than using
"replica-n" where I guess blocks are marked as dirty to be replicated.
Does geo-rep do the same?
How does replica-n and geo-rep compare in a continuous replication
scenario?
How safe is it to use replica-n or geo-rep for VM images? Will the
replicated VM images be mostly consistent compared to a bare-metal
sudden power-off?
My guess is that replica-n is safer than geo-rep since it replicates
writes synchronically in real-time, while geo-rep seems to do an
initial scan using rsync, but I'm not sure how it continues
replicating after that initial sync.
Anyway, I would like to ask, discuss or propose the following idea:
- Have an option to tell gluster to write to only one brick
(split-brains would be impossible) which will then replicate to other
bricks.
- A local brick (if exists) should be selected as the "write authority
brick".
If that one brick goes down, data previously written cannot be served
until the 'sync' has been completed to the other brick. Also, new writes
would not be possible until the brick comes up no?
This would increase the global write performance which is currently
constrained to the slowest node because writes are currently
replicated synchronically to all other replicas (=> writes are not
scalable for replica volumes).
There is an optimization that is on the cards where we write to all
bricks of the replica synchronously but we return success to the
application as soon as we get success from any one of the bricks.
(Currently we wait for replies from all bricks before returning
success/failure to the upper translator).
Basically, the idea here is to have an option to avoid split-brains
selecting an authority brick and to avoid sync writes.
The same goal could be achieved by forcing gluster to resolve *all*
split-brains by choosing the authority brick as the winner (?).
Do we currently have an option for doing something like this?
There is an arbiter feature for replica 3 volumes
(https://github.com/gluster/glusterfs/blob/master/doc/features/afr-arbiter-volumes.md)
being released in glusterfs 3.7 which would prevent files from going
into split-brains, you could try that out. If the writes can cause a
split-brain, it is failed with an ENOTCONN to the application.
Regards,
Ravi
Benefits for gluster:
- replica-n won't cause no split-brains
- scalability (write performance won't be limited to the slowed node)
Best regards,
Christopher Pereira
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