linux-kernel - Re: Linux 2.6.29

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Date:	Wed, 25 Mar 2009 15:32:13 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...e.de>, David Rees <drees76@...il.com>,
	Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Jens Axboe wrote:
> On Tue, Mar 24 2009, Jeff Garzik wrote:
>> Linus Torvalds wrote:
>>> But I really don't understand filesystem people who think that "fsck" 
>>> is the important part, regardless of whether the data is valid or not. 
>>> That's just stupid and _obviously_ bogus.
>> I think I can understand that point of view, at least:
>>
>> More customers complain about hours-long fsck times than they do about  
>> silent data corruption of non-fsync'd files.
>>
>>
>>> The point is, if you write your metadata earlier (say, every 5 sec) and 
>>> the real data later (say, every 30 sec), you're actually MORE LIKELY to 
>>> see corrupt files than if you try to write them together.
>>>
>>> And if you write your data _first_, you're never going to see 
>>> corruption at all.
>> Amen.
>>
>> And, personal filesystem pet peeve:  please encourage proper FLUSH CACHE  
>> use to give users the data guarantees they deserve.  Linux's sync(2) and  
>> fsync(2) (and fdatasync, etc.) should poke the block layer to guarantee  
>> a media write.
> 
> fsync already does that, at least if you have barriers enabled on your
> drive.

Erm, no, you don't enable barriers on your drive, they are not a 
hardware feature.  You enable barriers via your filesystem.

Stating "fsync already does that" borders on false, because that assumes
(a) the user has a fs that supports barriers
(b) the user is actually aware of a 'barriers' mount option and what it 
means
(c) the user has turned on an option normally defaulted to off.

Or in other words, it pretty much never happens.

Furthermore, a blatantly obvious place to flush data to media -- 
fsync(2), fdatasync(2) and sync_file_range(2) -- should cause the block 
layer to issue a FLUSH CACHE for __any__ filesystem.  But that doesn't 
happen either.

So, no, for 95% of Linux users, fsync does _not_ already do that.  If 
you are lucky enough to use XFS or ext4, you're covered.  That's it.

	Jeff



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