Re: [PATCH v2 03/11] pmem: enable REQ_FUA/REQ_FLUSH handling

From: Andreas Dilger
Date: Fri Nov 13 2015 - 19:43:39 EST

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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