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Date: Mon, 20 Jan 2014 05:58:55 -0800
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: Terrible performance of sequential O_DIRECT 4k writes in SAN
	environment. ~3 times slower then Solars 10 with the same HBA/Storage.
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To: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>, linux-scsi <linux-scsi@vger.kernel.org>, Gluk <git.user@gmail.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, xfs@oss.sgi.com, Christoph Hellwig <hch@infradead.org>, Sergey Meirovich <rathamahata@gmail.com>

On Thu, Jan 16, 2014 at 09:07:21AM +1100, Dave Chinner wrote:
> Yes, I think it can be done relatively simply. We'd have to change
> the code in xfs_file_aio_write_checks() to check whether EOF zeroing
> was required rather than always taking an exclusive lock (for block
> aligned IO at EOF sub-block zeroing isn't required),

That's not even required for supporting aio appends, just a further
optimization for it.

> and then we'd
> have to modify the direct IO code to set the is_async flag
> appropriately. We'd probably need a new flag to say tell the DIO
> code that AIO beyond EOF is OK, but that isn't hard to do....

Yep, need a flag to allow appending writes and then defer them.

> Christoph, are you going to get any time to look at doing this in
> the next few days?

I'll probably need at least another week before I can get to it.  If you
wanna pick it up before than feel free.

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