Gluster users - Feb 2017 - Remove an artificial limitation of disperse volume

Okay so the 4 nodes thing is a kind of exception? What about 8 nodes
with redundancy 4?

I made a table to recap possible configurations, can you take a quick
look and tell me if it's OK?

Here: https://gist.github.com/olivierlambert/8d530ac11b10dd8aac95749681f19d2c



On Tue, Feb 7, 2017 at 4:18 PM, Jeff Darcy <jdarcy at redhat.com>
wrote:
>> 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.

----- Original Message -----
> Okay so the 4 nodes thing is a kind of exception? What about 8 nodes
> with redundancy 4?
> 
> I made a table to recap possible configurations, can you take a quick
> look and tell me if it's OK?
> 
> Here:
https://gist.github.com/olivierlambert/8d530ac11b10dd8aac95749681f19d2c
As I understand it, the "power of two" thing is only about maximum
efficiency, and other values can work without wasting space (they'll
just be a bit slower).  So, for example, with 12 disks you would be
able to do 10+2 and get 83% space efficiency.  Xavier's the expert,
though, so it's probably best to let him clarify.