linux-ext4 - Re: Query about DIO/AIO WRITE throttling and ext4 serialization

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Message-ID: <4DE8555E.7040404@redhat.com>
Date:	Thu, 02 Jun 2011 22:30:38 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	Christoph Hellwig <hch@...radead.org>
CC:	Vivek Goyal <vgoyal@...hat.com>, "Ted Ts'o" <tytso@....edu>,
	Dave Chinner <david@...morbit.com>, linux-ext4@...r.kernel.org
Subject: Re: Query about DIO/AIO WRITE throttling and ext4 serialization

On 6/2/11 8:02 PM, Christoph Hellwig wrote:
> On Thu, Jun 02, 2011 at 08:54:03PM -0400, Vivek Goyal wrote:
>> Just wondering why ext4 and XFS behavior are different and which is a
>> more appropriate behavior. ext4 does not seem to be waiting for all
>> pending AIO/DIO to finish while XFS does.
> 
> They're both wrong.  Ext4 completely misses support in fsync or sync
> to catch pending unwrittent extent conversions, and thus fails to obey
> the data integrity guarante.  

I'm not sure about that.

ext4_sync_file() does ext4_flush_completed_IO() which does:

 * When IO is completed, the work to convert unwritten extents to
 * written is queued on workqueue but may not get immediately
 * scheduled. When fsync is called, we need to ensure the
 * conversion is complete before fsync returns.
 * The inode keeps track of a list of pending/completed IO that
 * might needs to do the conversion. This function walks through
 * the list and convert the related unwritten extents for completed IO
 * to written.

Granted, I get easily lost in ext4's codepaths here, which is actually
why I suggested Vivek pose these questions to the list ;)

-Eric
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