[Gluster-users] geo-replication vs replicated volumes

[M S Vishwanath Bhat]

On 10 June 2015 at 22:38, Aravinda <avishwan at redhat.com> wrote:

>
>
> 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.
>

Yes, I think you are confused between replication and geo-replication.

Just to add to what Aravinda mentioned, in AFR (Automatic File Replication,
that's what it's called in glusterfs) the replication happens between the
bricks of the same volume. The bricks are expected to be part of same
network. And the replication is synchronous. You can read more at
http://gluster.readthedocs.org/en/latest/Features/afr-v1/index.html?highlight=afr

And glusterfs geo-replication is between two (or more) glusterfs volumes.
These volumes in turn can have replicated setup internally. And volumes are
generally in different networks. And it's asynchronous one way replication.
It's mainly used for disaster recovery.

I found below link which is very old. But the content seems to be valid
still

*http://www.gluster.org/community/documentation/index.php/Gluster_3.2:_Replicated_Volumes_vs_Geo-replication
<http://www.gluster.org/community/documentation/index.php/Gluster_3.2:_Replicated_Volumes_vs_Geo-replication>*

HTH

Greetings,
Vishwanath


> >> 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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