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[Xen-devel] What are the semantics of BLKIF_OP_WRITE_BARRIER?
| To: | Daniel Stodden <daniel.stodden@xxxxxxxxxx> |
|---|---|
| Subject: | [Xen-devel] What are the semantics of BLKIF_OP_WRITE_BARRIER? |
| From: | Jeremy Fitzhardinge <jeremy@xxxxxxxx> |
| Date: | Thu, 22 Jul 2010 13:41:25 -0700 |
| Cc: | "Xen-devel@xxxxxxxxxxxxxxxxxxx" <Xen-devel@xxxxxxxxxxxxxxxxxxx>, Jake Wires <Jake.Wires@xxxxxxxxxx> |
| Delivery-date: | Thu, 22 Jul 2010 13:42:19 -0700 |
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What does BLKIF_OP_WRITE_BARRIER mean precisely? Presumably it prevents
writes from being reordered past it. But does it also imply a flush, in
that all previous writes are on stable storage? If not, is there
another operation which does guarantee that? If the backend doesn't
support barriers, is there any other way for a guest to force writes to
stable storage?
How does blktap(2) deal with devices with volatile write caches? Does
it generate its own barriers/flushes/FUA requests to push things out to
stable storage?
Thanks,
J
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