linux-kernel - 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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Date:	Tue, 7 Jan 2014 07:58:30 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Jan Kara <jack@...e.cz>
Cc:	Sergey Meirovich <rathamahata@...il.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Gluk <git.user@...il.com>
Subject: Re: Terrible performance of sequential O_DIRECT 4k writes in SAN
 environment. ~3 times slower then Solars 10 with the same HBA/Storage.

On Mon, Jan 06, 2014 at 09:10:32PM +0100, Jan Kara wrote:
>   This is likely a problem of Linux direct IO implementation. The thing is
> that in Linux when you are doing appending direct IO (i.e., direct IO which
> changes file size), the IO is performed synchronously so that we have our
> life simpler with inode size update etc. (and frankly our current locking
> rules make inode size update on IO completion almost impossible). Since
> appending direct IO isn't very common, we seem to get away with this
> simplification just fine...

Shouldn't be too much of a problem at least for XFS and maybe even ext4
with the workqueue based I/O end handler.  For XFS we protect size
updates by the ilock which we already taken in that handler, not sure
what ext4 would do there.

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