Re: vfs: Add MS_FLUSHONFSYNC mount flag

From: "Fernando Luis Vázquez Cao" <fernando@oss.ntt.co.jp>
To: Eric Sandeen <sandeen@redhat.com>
Cc: Jan Kara <jack@suse.cz>, Theodore Tso <tytso@MIT.EDU>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>, Pavel Machek <pavel@suse.cz>,
	kernel list <linux-kernel@vger.kernel.org>,
	Jens Axboe <jens.axboe@oracle.com>,
	fernando@kic.ac.jp, Ric Wheeler <rwheeler@redhat.com>
Subject: Re: vfs: Add MS_FLUSHONFSYNC mount flag
Date: Fri, 13 Feb 2009 10:14:39 +0900	[thread overview]
Message-ID: <1234487679.3795.15.camel@sebastian.kern.oss.ntt.co.jp> (raw)
In-Reply-To: <499458C1.90105@redhat.com>

On Thu, 2009-02-12 at 11:13 -0600, Eric Sandeen wrote:
> Fernando Luis Vázquez Cao wrote:
> > This mount flag will be used to determine whether the block device's write
> > cache should be flush or not on fsync()/fdatasync().
> > 
> > Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
> > ---
> 
> Again, apologies for chiming in late.
> 
> But wouldn't it be better to make this a block device property rather
> than a new filesystem mount option?
> 
> That way the filesystem can always do "the right thing" and call the
> blkdev flush on fsync.
> 
> The block device *could* choose to ignore this in hardware if it knows
> it's built with a nonvolatile write cache or if it has no write cache.
> 
> Somewhere in the middle, if an administrator knows they have a UPS they
> trust and hardware that stays connected to it, they could tune the bdev
> to ignore these flush requests.
> 
> Also that way if you have 8 partitions on a battery-backed blockdev, you
> can tune it once, instead of needing to mount all 8 filesystems with the
> new option.

The main reason I decided to go for the mount option approach is to be
consistent with what we do when it comes to write barriers. Treating one
as a mount option and the other as a (possibly) sysfs tunable property
seems a bit confusing to me.

Do you suggest using sysfs tunables instead?

- Fernando