On 10/14/2009 10:41 PM, Christoph Hellwig wrote:
But can't this be also implemented using QUEUE_ORDERED_DRAIN, and on the
host side disabling the backing device write cache? I'm talking about
cache=none, primarily.
Yes, it could. But as I found out in a long discussion with Stephen
it's not actually nessecary. All filesystems do the right thing for
a device not claiming to support barriers if it doesn't include write
caches, that is implement ordering internally. So there is no urge to
set QUEUE_ORDERED_DRAIN for the case without write cache.
Does virtio say it has a write cache or not (and how does one say it?)?
According to the report, a write+fdatasync completes too fast, at least
on Ubuntu's qemu. So perhaps somewhere this information is lost.
--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.
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