[sheepdog-users] [Sheepdog Announcement] Erasure coding is fully functional with Sheepdog now

Liu Yuan namei.unix at gmail.com
Tue Oct 22 16:09:50 CEST 2013

On Tue, Oct 22, 2013 at 02:00:19PM +0000, Andrew J. Hobbs wrote:
> I love the work you've been doing for this but I do have some questions 
> regarding erasure.
> In your notes, you mention specifying the policy such as 4:2, how does 
> this map to physical nodes?  Would that mean you could use that 4:2 for 
> 6 or more nodes?  How does that work exactly?  Would that mean the 16:15 
> configuration requires 31 nodes or more?

Yes. 16:15 means we need at least 31 nodes to get it work.

Suppose you have specified x:y scheme, then (x + y) strips that contains the
data and parity will spread on (x + y) nodes as (x + y) objects in the cluster
by hashing algorithm the same as for replicated objects.

> If your node count exceeds the stripe/parity setting does the data get 
> rotated across nodes so all nodes contribute to throughput?

Yes. All the earsured objects will be spread evently in the consistent hash ring
that maps to virtual nodes of weighted physical nodes.

The erarsured object placement policy goes the same as replicated objects.


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