On Nov 13, 2015, at 5:20 PM, Dan Williams <dan.j.williams@xxxxxxxxx> wrote: > > On Fri, Nov 13, 2015 at 4:06 PM, Ross Zwisler > <ross.zwisler@xxxxxxxxxxxxxxx> wrote: >> Currently the PMEM driver doesn't accept REQ_FLUSH or REQ_FUA bios. These >> are sent down via blkdev_issue_flush() in response to a fsync() or msync() >> and are used by filesystems to order their metadata, among other things. >> >> When we get an msync() or fsync() it is the responsibility of the DAX code >> to flush all dirty pages to media. The PMEM driver then just has issue a >> wmb_pmem() in response to the REQ_FLUSH to ensure that before we return all >> the flushed data has been durably stored on the media. >> >> Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > Hmm, I'm not seeing why we need this patch. If the actual flushing of > the cache is done by the core why does the driver need support > REQ_FLUSH? Especially since it's just a couple instructions. REQ_FUA > only makes sense if individual writes can bypass the "drive" cache, > but no I/O submitted to the driver proper is ever cached we always > flush it through to media. If the upper level filesystem gets an error when submitting a flush request, then it assumes the underlying hardware is broken and cannot be as aggressive in IO submission, but instead has to wait for in-flight IO to complete. Since FUA/FLUSH is basically a no-op for pmem devices, it doesn't make sense _not_ to support this functionality. Cheers, Andreas
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