linux-ext4 - Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing device needs stable page writes

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Date:	Tue, 30 Oct 2012 11:34:41 +1100
From:	NeilBrown <neilb@...e.de>
To:	Jan Kara <jack@...e.cz>
Cc:	"Darrick J. Wong" <darrick.wong@...cle.com>,
	Theodore Ts'o <tytso@....edu>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] bdi: Create a flag to indicate that a backing
 device needs stable page writes

On Tue, 30 Oct 2012 01:10:08 +0100 Jan Kara <jack@...e.cz> wrote:

> On Tue 30-10-12 10:48:37, NeilBrown wrote:
> > On Mon, 29 Oct 2012 19:30:51 +0100 Jan Kara <jack@...e.cz> 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@...cle.com>
> > > >   I guess Jens Axboe <axboe@...nel.dk> 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@...e.cz>
> > >   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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