[Gluster-users] Remove an artificial limitation of disperse volume

[Olivier Lambert]

Sure, RAID level were only here to have a approximative comparison.
I've read about reed-solomon, that's a very neat algorithm.

In my 4x nodes example, 3x replication won't be possible (not a
multiple). I don't see any alternative to avoid the "shortcoming" of
replication, except using disperse with redundancy 2. Which is sadly
not possible for a unknown reason (because reed-solomon is totally
flexible in theory, therefore I don't understand why it's not
authorized in Gluster!)

Any thoughts?

On Tue, Feb 7, 2017 at 2:51 PM, Nag Pavan Chilakam <nchilaka at redhat.com> wrote:
> You can always go for x3(3 replica copies), to address your need which you have asked
> EC volumes can be seen as raid for understanding purpose, but don't see it as an apple-to-apple comparison.
> Raid4/6(mostly) relies on XOR'ing bits(so basic addition and subtraction), but EC involves a more complex algorithm(reed-solomon)
>
>
> ----- Original Message -----
> From: "Olivier Lambert" <lambert.olivier at gmail.com>
> To: "gluster-users" <gluster-users at gluster.org>
> Sent: Tuesday, 7 February, 2017 6:46:37 PM
> Subject: [Gluster-users] Remove an artificial limitation of disperse volume
>
> Hi everyone!
>
> I'm currently working on implementing Gluster on XenServer/Xen Orchestra.
>
> I want to expose some Gluster features (in the easiest possible way to
> the user).
>
> Therefore, I want to expose only "distributed/replicated" and
> "disperse" mode. From what I understand, they are working differently.
> Let's take a simple example.
>
> Setup: 6x nodes with 1x 200GB disk each.
>
> * Disperse with redundancy 2 (4+2): I can lose **any 2 of all my
> disks**. Total usable space is 800GB. It's a kind of RAID6 (or RAIDZ2)
> * Distributed/replicated with replica 2: I can lose 2 disks **BUT**
> not on the same "mirror". Total usable space is 600GB. It's a kind of
> RAID10
>
> So far, is it correct?
>
> My main point is that behavior is very different (pairing disks in
> distributed/replicated and "shared" parity in disperse).
>
> Now, let's imagine something else. 4x nodes with 1x 200GB disk each.
>
> Why not having disperse with redundancy 2? It will be the same in
> terms of storage space than distributed/replicated, **BUT** in
> disperse I can lose any of 2 disks. In dist/rep, only if they are not
> on the same "mirror".
>
> So far, I can't create a disperse volume if the redundancy level is
> 50% or more the number of bricks. I know that perfs would be better in
> dist/rep, but what if I prefer anyway to have disperse?
>
> Conclusion: would it be possible to have a "force" flag during
> disperse volume creation even if redundancy is higher that 50%?
>
>
>
> Thanks!
>
>
>
> Olivier.
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