Christoph Hellwig wrote:
Currently virtio-blk does support barriers for ordering requests which
is enough to guarantee filesystem metadata integrity with write back
caches, but it does not support any way to flush that writeback cache,
to guarantee that data is stable on disk on a fsync.
This patch implements a new VIRTIO_BLK_T_FLUSH command to flush the
cache and exposes the functionality to the block layer by implementing
a prepare_flush method.
What typically triggers a flush operation?
I would assume an fsync would, but would a flush happen after every
O_DIRECT write?
If the backend implementation of T_FLUSH is fsync, I would think that
this would result in rather poor performance for O_DIRECT operations in
the guest.
Regards,
Anthony Liguori
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