linux-kernel - Re: vfs: Add MS_FLUSHONFSYNC mount flag

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Message-ID: <20090212212304.GA7935@duck.suse.cz>
Date:	Thu, 12 Feb 2009 22:23:04 +0100
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>, Theodore Tso <tytso@....EDU>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Pavel Machek <pavel@...e.cz>,
	kernel list <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>, fernando@....ac.jp,
	Ric Wheeler <rwheeler@...hat.com>
Subject: Re: vfs: Add MS_FLUSHONFSYNC mount flag

On Thu 12-02-09 11:13:37, 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@....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?
  Hum, that's an interesting idea. Yes, probably it makes even more sence
than a global 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.
  Yes, but OTOH we should give sysadmin a possibility to enable / disable
it on just some partitions. I don't see a reasonable use for that but people
tend to do strange things ;) and here isn't probably a strong reason to not
allow them.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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