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

Jeff Darcy jdarcy at redhat.com
Tue Feb 7 15:18:48 UTC 2017

> 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%?

The problem is that the math behind erasure coding doesn't work for all
fragment counts and redundancy levels.  To get two-failure protection
you need more than four bricks.  If you had multiple disks in each
server you could get protection against multiple disk failures, but you
still wouldn't have protection against multiple server failures.  The
only thing your "force" flag could do is allow placement of multiple
fragments on a single physical disk, but then you wouldn't even have
protection against two disk failures.  If you want higher levels of
protection you need more disks, either to satisfy the mathematical
requirements of EC or to overcome the space inefficiency of replication.

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