linux-kernel - Re: Linux 2.6.29

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Date:	Fri, 27 Mar 2009 11:03:01 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Theodore Tso <tytso@....edu>, Jens Axboe <jens.axboe@...cle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jeff Garzik <jeff@...zik.org>, 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

Christoph Hellwig wrote:
> On Fri, Mar 27, 2009 at 10:13:33AM -0400, Theodore Tso wrote:
>   
>> On Fri, Mar 27, 2009 at 08:57:23AM +0100, Jens Axboe wrote:
>>     
>>> Here's a simple patch that does that. Not even tested, it compiles. Note
>>> that file systems that currently do blkdev_issue_flush() in their
>>> ->sync() should then get it removed.
>>>
>>>       
>> That's going to be a mess.  Ext3 implements an fsync() by requesting a
>> journal commit, and then waiting for the commit to have taken place.
>> The commit happens in another thread, kjournald.  Knowing when it's OK
>> not to do a blkdev_issue_flush() because the commit was triggered by
>> an fsync() is going to be really messy.  Could we at least have a flag
>> in struct super which says, "We'll handle the flush correctly, please
>> don't try to do it for us?"
>>     
>
> Doing it in vfs_fsync also is completely wrong layering.  If people want
> it for simple filesystems add it to file_fsync instead of messing up
> the generic helper.  Removing well meaning but ill behaved policy from
> the generic path has been costing me far too much time lately.
>
> And please add a tuneable for the flush.  Preferable a generic one at
> the block device layer instead of the current mess where every
> filesystem has a slightly different option for barrier usage.
>   

I agree that we need to be careful not to put extra device flushes if 
the file system handles this properly. They can be quite expensive (say 
10-20ms on a busy s-ata disk).

I have also seen some SSD devices have performance that drops into the 
toilet when you start flushing their volatile caches.

ric

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