On Tue, 30 Oct 2012 01:10:08 +0100 Jan Kara <jack@xxxxxxx> wrote: > On Tue 30-10-12 10:48:37, NeilBrown wrote: > > On Mon, 29 Oct 2012 19:30:51 +0100 Jan Kara <jack@xxxxxxx> wrote: > > > > > On Mon 29-10-12 19:13:58, Jan Kara wrote: > > > > On Fri 26-10-12 18:35:24, Darrick J. Wong wrote: > > > > > This creates BDI_CAP_STABLE_WRITES, which indicates that a device requires > > > > > stable page writes. It also plumbs in a sysfs attribute so that admins can > > > > > check the device status. > > > > > > > > > > Signed-off-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > > > > I guess Jens Axboe <axboe@xxxxxxxxx> would be the best target for this > > > > patch (so that he can merge it). The patch looks OK to me. You can add: > > > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > > One more thing popped up in my mind: What about NFS, Ceph or md RAID5? > > > These could (at least theoretically) care about stable writes as well. I'm > > > not sure if they really started to use them but it would be good to at > > > least let them know. > > > > > > > What exactly are the semantics of BDI_CAP_STABLE_WRITES ? > > > > If I set it for md/RAID5, do I get a cast-iron guarantee that no byte in any > > page submitted for write will ever change until after I call bio_endio()? > Yes. > > > If so, is this true for all filesystems? - I would expect a bigger patch would > > be needed for that. > Actually the code is in kernel for quite some time already. The problem > is it is always enabled causing unnecessary performance issues for some > workloads. So these patches try to be more selective in when the code gets > enabled. > > Regarding "all filesystems" question: If we update filemap_page_mkwrite() > to call wait_on_page_writeback() then it should be for all filesystems. Cool. I didn't realise it had progressed that far. I guess it is time to look at the possibility of removing the 'copy-into-cache' step for full-page, well-aligned bi_iovecs. I assume this applies to swap-out as well ?? It has been a minor source of frustration that when you swap-out to RAID1, you can occasionally get different data on the two devices because memory changed between the two DMA events. NeilBrown
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