[sheepdog] questions about sheepdog write policy

Hitoshi Mitake mitake.hitoshi at gmail.com
Thu May 26 04:34:55 CEST 2016

On Tue, May 24, 2016 at 6:46 PM, Dong Wu <archer.wudong at gmail.com> wrote:

> hi,mitake
> I have questions about sheepdog write policy.
> for replication, sheepdog write default 3 copies, and is strong
> consistency.
> my doubt is
> 1) if some  replicas write successfully, others fail, then it will
> retry write anyway until all the 3 replicas success? but if there are
> only less than 3 nodes left, will it write only less than 3 replicas
> and return success to client?

In a case of disk and network I/O error, sheep returns an error to its
client immediately. In some case (e.g. epoch increasing caused by node
join/leave), it will retry.

> 2) if some replicas write success, others write fail, and return fail
> to client, how to deal with these replicas's data consistency(write
> success node has new data, but write fail node has old data)? if
> client read the same block, will it read new data  or old data?

In such a case, we need to repair consistency with "dog vdi check" command.
Note that in such a case the failed VDIs won't be accessed from VMs anymore
because they will be used in read-only mode.


> Thanks a lot.
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