[Gluster-users] geo-replication vs replicated volumes

[Aravinda]

On 06/10/2015 09:43 PM, Gabriel Kuri wrote:
> > glusterfs doesn't support master-master yet. In your case, one of 
> the servers (A or B or C) should be a master and your client should 
> write to only that volume.
> > Other two volumes should be read-only till volume in server-A fails 
> for some reason.
>
> So the writes from the client will go directly to whichever server is 
> the master, even though the client has mounted the volume on one of 
> the slaves? What about the reads, do they still hit the server (ie 
> slave) the client is connected to or do the reads hit the master as well?
To be specific, Gluster Geo-rep doesn't support Master-Master. That 
means no automatic failover when master Volume goes down. Gluster 
replicated volumes support Master-Master within the Volume.

Replicated Volumes:
--------------------------
The replication is synchronous, all writes on the mount will be copied 
to multiple bricks(replica count) synchronously. If one node is down 
during the write, other nodes takes care of syncing data when node comes 
online. This is automatic using self-heal.

Replication is between bricks of single volume.

Geo-replicated Volumes:
--------------------------------
Asynchronous replication of whole Gluster Volume. That means their will 
be delay in syncing data from Master Volume to Slave Volume. Volume 
topology does not matter Geo-replication works even if Master and Slave 
Volume types are different.

Geo-replication is mainly used as disaster recovery mecanism like 
backups. Manual failover failback is also supported, if Master Volume 
goes down Slave Volume can become Master and can establish connection 
back. It is not supported to have Georep running in both ways at same time.

>
> In the case of geo-rep, how is split-brain handled? If the network is 
> down between server A (master) and server B (slave) and the client has 
> mounted to server B, I assume server B will then become the master and 
> writes will then be committed directly to server B, but if writes were 
> also committed to server A by other clients while the network was 
> down, what happens when the network is back up between server A and B, 
> does it just figure out which files had the most recent time stamp and 
> commit those changes across all the servers?
Since Master-Master is not supported in Geo-rep, Split brain is not 
handled.  I think there is some confusion between replicated volume and 
geo-rep. Replicated Volume replicates data within Volume. For example, 
Create a Gluster Volume with two bricks with replica count as 2. 
Bricks/Nodes cannot be across data centers. In case of Geo-rep, 
replication is between two Gluster Volumes.
>
> >> If it's not master-master, how does one get master-master 
> replication working over a WAN?
> > AFAIK, there is no work around as of now, at least I am not aware of it
>
> Does the basic replicated volume work in this fashion, reads and 
> writes to all servers? The only problem is it's meant for a low 
> latency network environment?
>
> Thanks ...
>
>
>
>
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-- 
regards
Aravinda

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