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

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Message-Id: <1234489622.3795.49.camel@sebastian.kern.oss.ntt.co.jp>
Date:	Fri, 13 Feb 2009 10:47:02 +0900
From:	Fernando Luis Vázquez Cao 
	<fernando@....ntt.co.jp>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, 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, 2009-02-12 at 15:30 -0600, Eric Sandeen wrote:
> Jan Kara wrote:
> > On Thu 12-02-09 11:13:37, Eric Sandeen wrote:
> 
> ...
> 
> >> 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
> 
> But nobody has asked for that, have they?  So why offer it up a this point?
> 
> They could use LD_PRELOAD to make fsync a no-op if they really don't
> care for it, I guess... though that's not easily per-fs either.
> 
> But do we really want to go out of our way to enable people to
> short-circuit data integrity paths and then file bugs when their files
> go missing? :)

Well, it is just a matter of using safe defaults. IMHO, a scenario where
the administrator wants to optimize writes to a certain partition and
_explicitly_ clears MS_FLUSHONFSYNC on that superblock is not completely
unreasonable.

> (I guess the blockdev tunable is similarly dangerous, but it more
> clearly meets the explicit need (writecache-safe devices))

If distributions use sane defaults and we document the mount option or
bdev tunable properly I guess it might make sense to allow system
administrators to shoot themselves in the foot.

(By the way, in this patch-set a patch for mount(8) is included.)

- Fernando

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