Messages in this thread | | | Date | Thu, 19 May 2011 16:06:27 +0100 | From | Alex Bligh <> | Subject | REQ_FLUSH, REQ_FUA and open/close of block devices |
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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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