REQ_FLUSH, REQ_FUA and open/close of block devices

From: Alex Bligh
Date: Thu May 19 2011 - 11:06:34 EST

I am doing some work on making REQ_FLUSH and REQ_FUA work with block devices
and have some patches that make them perform as expected with nbd if
an nbd device is mounted (e.g. -t ext3, -odata-journal,barriers=1), and
I see the relevant REQ_FLUSH and REQ_FUA appearing much as expected.

However, if I do a straight dd to the device (which generates an open()
and a close()), I see no barrier activity at all (i.e. no REQ_FLUSH and
no REQ_FUA). It is surprising to me that a close() on a raw device does
not generate a REQ_FLUSH. I cannot imagine it is a performance overhead.

I would have thought this would useful anyway (if I've written
to a raw device I'd rather expect it to hit it when I do the close()),
but my specific application is ensuring cache coherency on live migrate
of virtual servers: if migrating from node A to node B, then when the
hypervisor closes the block device on node A, I want to be sure that any
locally cached write data is written to the remote disk before it
unfreezes node B.

Should a close() of a dirty block device result in a REQ_FLUSH?

Alex Bligh
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