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

[Nag Pavan Chilakam]

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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