linux-kernel - Re: [PATCH] issue storage device flush via sync

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Date:	Wed, 25 Mar 2009 20:58:43 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Eric Sandeen <sandeen@...deen.net>
CC:	Jeff Garzik <jeff@...zik.org>,
	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: [PATCH] issue storage device flush via sync_blockdev() (was Re:
 Linux 2.6.29)

Eric Sandeen wrote:
> Jeff Garzik wrote:
>   
>> On Wed, Mar 25, 2009 at 01:40:37PM -0700, Linus Torvalds wrote:
>>     
>>> On Wed, 25 Mar 2009, Jeff Garzik wrote:
>>>       
>>>> It is clearly possible to implement an fsync(2) that causes FLUSH CACHE to be
>>>> issued, without adding full barrier support to a filesystem.  It is likely
>>>> doable to avoid touching per-filesystem code at all, if we issue the flush
>>>> from a generic fsync(2) code path in the kernel.
>>>>         
>>> We could easily do that. It would even work for most cases. The 
>>> problematic ones are where filesystems do their own disk management, but I 
>>> guess those people can do their own fsync() management too.
>>>
>>> Somebody send me the patch, we can try it out.
>>>       
>> This is a simple step that would cover a lot of cases...  sync(2)
>> calls sync_blockdev(), and many filesystems do as well via the generic
>> filesystem helper file_fsync (fs/sync.c).
>>
>> XFS code calls sync_blockdev() a "big hammer", so I hope my patch
>> follows with known practice.
>>
>> Looking over every use of sync_blockdev(), its most frequent use is
>> through fsync(2), for the selected filesystems that use the generic
>> file_fsync helper.
>>
>> Most callers of sync_blockdev() in the kernel do so infrequently,
>> when removing and invalidating volumes (MD) or storing the superblock
>> prior to release (put_super) in some filesystems.
>>
>> Compile-tested only, of course :)  But it should be work :)
>>
>> My main concern is some hidden area that calls sync_blockdev() with
>> a high-enough frequency that the performance hit is bad.
>>
>> Signed-off-by: Jeff Garzik <jgarzik@...hat.com>
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 891e1c7..7b9f74a 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -173,9 +173,14 @@ int sync_blockdev(struct block_device *bdev)
>>  {
>>  	int ret = 0;
>>  
>> -	if (bdev)
>> -		ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
>> -	return ret;
>> +	if (!bdev)
>> +		return 0;
>> +	
>> +	ret = filemap_write_and_wait(bdev->bd_inode->i_mapping);
>> +	if (ret)
>> +		return ret;
>> +	
>> +	return blkdev_issue_flush(bdev, NULL);
>>  }
>>  EXPORT_SYMBOL(sync_blockdev);
>>     
>
> What about when you're running over a big raid device with
> battery-backed cache, and you trust the cache as much as much as the
> disks.  Wouldn't this unconditional cache flush be painful there on any
> of the callers even if they're rare?  (fs unmounts, freezes, unmounts,
> etc?  Or a fat filesystem on that device doing an fsync?)
>
> xfs, reiserfs, ext4 all avoid the blkdev flush on fsync if barriers are
> not enabled, I think for that reason...
>
> (I'm assuming these raid devices still honor a cache flush request even
> if they're battery-backed?  I dunno.)
>
> -Eric
>   
I think that Jeff's patch misses the whole need to protect transactions, 
including meta data, in a precise way. Useful for thing like unmount, 
not to give us strong protection for transactions or for fsync().

This patch will be adding overhead here - you will still need flushing 
at the transaction commit layer of the specific file systems to get any 
reliable transactions.

Having looked at the timing of barrier flushes on slow s-ata drives with 
an analyser a few years back, the first one is expensive (as you would 
expect with a large drive cache of 16 or 32 MB) and the second was 
nearly free.

Moving the expensive flush to this layer guts the transaction building 
blocks and costs the same....

ric



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