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 10:48:37 +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 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()?

If so, is this true for all filesystems? - I would expect a bigger patch would
be needed for that.

If not, what exactly are the semantics?

Thanks,
NeilBrown

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