sheepdog: implement direct write semantics
Sheepdog supports both writeback/writethrough write but has not yet supported DIRECTIO semantics which bypass the cache completely even if Sheepdog daemon is set up with cache enabled. Suppose cache is enabled on Sheepdog daemon size, the new cache control is cache=writeback # enable the writeback semantics for write cache=writethrough # enable the emulated writethrough semantics for write cache=directsync # disable cache competely Guest WCE toggling on the run time to toggle writeback/writethrough is also supported. Cc: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp> Cc: Kevin Wolf <kwolf@redhat.com> Cc: Stefan Hajnoczi <stefanha@gmail.com> Signed-off-by: NLiu Yuan <tailai.ly@taobao.com> Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com> Reviewed-by: NMORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp> Signed-off-by: NKevin Wolf <kwolf@redhat.com>
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