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Message-ID: <20121030104837.2e4b06fc@notabene.brown>
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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