[Sheepdog] Drive snapshots and metadata

[Leo "costela" Antunes]

Hi,

On 04/02/11 18:37, MORITA Kazutaka wrote:
>> Finally, I see the rather intrusive qemu patch I contributed in the early
>> days of sheepdog to allow locking and live-migration to coexist has been
>> superseded by the total removal of the sheepdog locking requirement in
>> fe14318e31d8. This is a much nicer solution to the problem than mine! Out of
>> interest, what happens if several clients do access a vdi at the same time?
>> Is it identical behaviour to accessing (say) an iscsi block device from 2
>> hosts, e.g. cluster filesystems can be made to work, or are there weaker
>> ordering guarantees on the sequencing of writes and/or problems with
>> read-cache consistency that make it less useful?
> 
> Both write ordering problem and read-cache consistency problem would
> happen.  Sheepdog is not designed to support such situation; all
> objects must be one of the following:
>  - no writer and multiple reader
>  - one writer and one reader
> To use Sheepdog safely, something like a lock system is necessary.
> 
> But this assumption makes Sheepdog much simple and achieves low
> latency.

Quick follow-up newbie question: does this locking system exist in any
way? What _does_ happen if I start two kvm guests using the same image?


Cheers

-- 
Leo "costela" Antunes
[insert a witty retort here]