On Mon, Nov 16, 2015 at 03:05:26PM +0100, Jan Kara wrote: > On Mon 16-11-15 14:37:14, Jan Kara wrote: > > On Fri 13-11-15 18:32:40, Dan Williams wrote: > > > On Fri, Nov 13, 2015 at 4:43 PM, Andreas Dilger <adilger@xxxxxxxxx> wrote: > > > > 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. > > > > > > Upper level filesystems won't get errors when the driver does not > > > support flush. Those requests are ended cleanly in > > > generic_make_request_checks(). Yes, the fs still needs to wait for > > > outstanding I/O to complete but in the case of pmem all I/O is > > > synchronous. There's never anything to await when flushing at the > > > pmem driver level. > > > > > > > Since FUA/FLUSH is basically a no-op for pmem devices, > > > > it doesn't make sense _not_ to support this functionality. > > > > > > Seems to be a nop either way. Given that DAX may lead to dirty data > > > pending to the device in the cpu cache that a REQ_FLUSH request will > > > not touch, its better to leave it all to the mm core to handle. I.e. > > > it doesn't make sense to call the driver just for two instructions > > > (sfence + pcommit) when the mm core is taking on the cache flushing. > > > Either handle it all in the mm or the driver, not a mixture. > > > > So I think REQ_FLUSH requests *must* end up doing sfence + pcommit because > > e.g. journal writes going through block layer or writes done through > > dax_do_io() must be on permanent storage once REQ_FLUSH request finishes > > and the way driver does IO doesn't guarantee this, does it? > > Hum, and looking into how dax_do_io() works and what drivers/nvdimm/pmem.c > does, I'm indeed wrong because they both do wmb_pmem() after each write > which seems to include sfence + pcommit. Sorry for confusion. Which I want to remove, because it makes DAX IO 3x slower than buffered IO on ramdisk based testing. > But a question: Won't it be better to do sfence + pcommit only in response > to REQ_FLUSH request and don't do it after each write? I'm not sure how > expensive these instructions are but in theory it could be a performance > win, couldn't it? For filesystems this is enough wrt persistency > guarantees... I'm pretty sure it would be, because all of the overhead (and therefore latency) I measured is in the cache flushing instructions. But before we can remove the wmb_pmem() from dax_do_io(), we need the underlying device to support REQ_FLUSH correctly... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
