[Gluster-users] Question on replicated volumes with bricks on the same server

[Vijay Bellur]

On 02/11/2014 03:21 PM, Antonio Messina wrote:
> Hi all,
>
> I would like to know how gluster distribute the data when two bricks
> of the same volumes are on the same server. Specifically, I would like
> to know if there is any way to spread the replicas on different nodes
> whenever possible, in order not to lose any data if the node goes
> down.
>
> I did a simple test and it seems that the way replicas are spread over
> the bricks is related to the way the volume is created, that is if I
> create a volume with:
>
>      gluster volume create vol1 replica 2\
>          gluster-data001:/srv/gluster/vol1.1 \
>          gluster-data001:/srv/gluster/vol1.2 \
>          gluster-data002:/srv/gluster/vol1.1 \
>          gluster-data002:/srv/gluster/vol1.2
>
> replicas of a file will be stored on the two bricks of the same
> server, while if I create the volume with
>
>      gluster volume create vol1 replica 2\
>          gluster-data001:/srv/gluster/vol1.1 \
>          gluster-data002:/srv/gluster/vol1.1 \
>          gluster-data001:/srv/gluster/vol1.2 \
>          gluster-data002:/srv/gluster/vol1.2
>
> replicas will be saved on two bricks of different servers.
>
> So, my guess is that if I create a "replica N" replicated+distributed
> volumes using the bricks:
>
>    gluster-1:/srv/gluster
>    ...
>    gluster-[N*M]:/srv/gluster
>
> gluster internally creates a distributed volumes made of the following
> replicated "volumes":
>
>    replicated volume 1: gluster-[1..N]:/srv/gluster
>    replicated volume 2: gluster-[N+1..2N]:/srv/gluster
>    ...
>    replicated volume M: gluster-[N*(M-1)+1..N*M]:/srv/gluster
>
> Is that correct or there is a more complex algorithm involved?
>

The interpretation is correct. The way replica sets are chosen is 
related to the order in which bricks are defined at the time of volume 
creation.

-Vijay