Gluster users - Feb 2014 - number of bricks given doesn't match required count

Hi all,

I would like to try GlusterFS but I really don't understand how is
handled stripping and replication.

I have 3 nodes, with 1 brick each.

What can I do with that ?

I would like files stripped across the 3 servers and also files
replicated on the 3 servers. Why not can I do that ?

Then, why when adding new nodes, do we have to set multiple of {I don't
know which count} ? I mean, I want 3 replicas of every files, I have 4
servers, what is the problem ? Why GlusterFS can't balance replicas
across the available nodes ?

I could understand that putting 4 replicas into 3 nodes is problematic,
but 3 in 4 I don't understand.

I didn't find any documentation about that, except that story of
"multiple" without explaining why.

Thank you for your help :)

Florent

On 02/14/2014 10:44 AM, Florent Bautista wrote:
> Hi all,
>
> I would like to try GlusterFS but I really don't understand how is
> handled stripping and replication.
>
> I have 3 nodes, with 1 brick each.
>
> What can I do with that ?
>
> I would like files stripped across the 3 servers and also files
> replicated on the 3 servers. Why not can I do that ?
>
> Then, why when adding new nodes, do we have to set multiple of {I don't
> know which count} ? I mean, I want 3 replicas of every files, I have 4
> servers, what is the problem ? Why GlusterFS can't balance replicas
> across the available nodes ?
>
> I could understand that putting 4 replicas into 3 nodes is problematic,
> but 3 in 4 I don't understand.
With 4 bricks (brick = node + volume) and "replica 3" you'd only
have a
replica of one of your bricks, the other two would not be protected.

But let's imagine that your first three bricks are 1TB disks, and the 
fourth is a 3TB disk. You could split the 3TB disk, e.g. with LVM, into 
three volumes to use as bricks to give you the full ?3 replication 
required by gluster replication.

Also Gluster's stripe is not the same as RAID-0. While similar, it 
doesn't give the performance improvement that real RAID-0 striping 
gives. I'd use gluster's stripe only if I have files that are so large 
they won't fit on a single brick. If you just want greater capacity for 
regular files you should use distribute (dht).

-- 

Kaleb