On 18 Jul 2013 20:25, "Josh Durgin" <josh.durgin@xxxxxxxxxxx> wrote:
> Setting rbd_cache=true in ceph.conf will make librbd turn on the cache
> regardless of qemu. Setting qemu to cache=none tells qemu that it
> doesn't need to send flush requests to the underlying storage, so it
> does not do so. This means librbd is caching data, but qemu isn't
> telling it to persist that data when the guest requests it. This is
> the same as qemu's cache=unsafe mode, which makes it easy to get a
> corrupt fs if the guest isn't shut down cleanly.
>
> There's a ceph option to make this safer -
> rbd_cache_writethrough_until_flush. If this and rbd_cache are true,
> librbd will operate with the cache in writethrough mode until it is
> sure that the guest using it is capable of sending flushes (i.e. qemu
> has cache=writeback). Perhaps we should enable this by default so
> people are less likely to accidentally use an unsafe configuration.
Ok. Now it make sense. So the last question is how to make sure that qemu actually operates with cache=writeback with rbd?
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