virtio_blk: support barriers without FLUSH feature

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If we want to support barriers with the cache=writethrough mode in qemu
we need to tell the block layer that we only need queue drains to
implement a barrier.  Follow the model set by SCSI and IDE and assume
that there is no volatile write cache if the host doesn't advertize it.
While this might imply working barriers on old qemu versions or other
hypervisors that actually have a volatile write cache this is only a
cosmetic issue - these hypervisors don't guarantee any data integrity
with or without this patch, but with the patch we at least provide
data ordering.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

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Christoph Hellwig 2010-06-15 14:43:48 +02:00 • committed by Rusty Russell

parent 3cfc2c42c1

commit 10bc310c27

1 changed files with 23 additions and 3 deletions

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drivers/block/virtio_blk.c

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