On Tue, 30 Oct 2012 14:38:25 +0100 Jan Kara <jack@xxxxxxx> wrote: > On Tue 30-10-12 11:34:41, NeilBrown wrote: > > 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. > Really? I'm somewhat surprised. I was under the impression that when a > page is added to a swap cache it is unmapped so there should be no > modification to it possible while it is being swapped out. But maybe it > could get mapped back and modified after we unlock the page and submit the > bio. So mm/memory.c:do_swap_page() might need wait_on_page_writeback() as > well. But I'm not an expert on swap code. I guess I'll experiment with this > a bit. Thanks for a pointer. > > Honza Thanks for looking into it. I should mention that it was some years ago that this occasional inconsistency in RAID1 was reported and that I concluded that it as due to swap (though I don't recall how deeply I examined the code). It could well be different now. I never bothered pursuing it because I don't think that behaviour is really wrong, just mildly annoying. Thanks, NeilBrown
Attachment:
signature.asc
Description: PGP signature
