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

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Date:	Sun, 22 Feb 2009 18:42:11 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	Theodore Tso <tytso@....edu>, Pavel Machek <pavel@....cz>,
	Eric Sandeen <sandeen@...hat.com>, Jan Kara <jack@...e.cz>,
	Fernando Luis V?zquez Cao <fernando@....ntt.co.jp>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	kernel list <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>, fernando@....ac.jp,
	Ric Wheeler <rwheeler@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: vfs: Add MS_FLUSHONFSYNC mount flag

Theodore Tso wrote:
> On Sun, Feb 22, 2009 at 03:15:33PM +0100, Pavel Machek wrote:
>> Sounds like posix violation to
>> me... '/sys/fsync_does_not_really_sync'?
> 
> It's as much a Posix violation as noatime.  :-)
> 
>> Perhaps it is better done at glibc level? Environment variables
>> already mostly have semantics you want.....
> 
> Environment variables own't allow us to switch fsync's on and off
> while the process is running, so that the system can run "safe" while
> it is on AC mains, but "low power" when on battery.

Overall, for each filesystem, we really should _default_ to a safe mode 
where fsync(2), fdatasync(2), sync_file_range(2) and transaction commits 
are guaranteed to be committed to stable storage -- i.e. SYNC CACHE / 
FLUSH CACHE, a forced unit access (FUA) bit, or guarantee that the 
underlying storage has disabled write-back caching.

Correctness should come before performance.  Linux has not had 
credibility here, in the ATA+ext[23] space at least, and it is embarrassing.

Modern SATA and SCSI disks can do tagged command queueing with the FUA 
bit, which does a lot to mitigate the performance loss seen with a less 
capable "FLUSH CACHE + barrier" setup.

	Jeff

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