[sheepdog] [PATCH 1/3] sheep: use qemu cache option to use writeback/writethrough mode

[Liu Yuan]

On 08/30/2012 10:42 PM, MORITA Kazutaka wrote:
> I wonder if there is a situation where some users want to enable a
> disk cache of Sheepdog, but the others don't want to.

The biggest problem is we don't have a sane disk cache implementation for backend (seems impossible to me).
Current proposed approach doesn't scale. Suppose we have 1000 nodes, and sync request will be issued in
30 second. Because we have to broadcast sync request to all the nodes, then we'll end up with

 1000^2 / 30 = 33333

for every host every second. This will definitely degrade the performance. That is, O(n^2) will only apply
to very small sized cluster. If we have an O(n) disk cache for backend, then I think we don't need to disable
it.

Thanks,
Yuan