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Re: Terrible performance of sequential O_DIRECT 4k writes in SAN environ

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: Terrible performance of sequential O_DIRECT 4k writes in SAN environment. ~3 times slower then Solars 10 with the same HBA/Storage.
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 21 Jan 2014 09:18:43 +1100
Cc: Sergey Meirovich <rathamahata@xxxxxxxxx>, xfs@xxxxxxxxxxx, Jan Kara <jack@xxxxxxx>, linux-scsi <linux-scsi@xxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Gluk <git.user@xxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20140120135855.GA26280@xxxxxxxxxxxxx>
References: <CA+QCeVQRrqx=CrxyuAe7k0e0y4Nqo7x_8jtkuD99VM8L9Dxp+g@xxxxxxxxxxxxxx> <20140106201032.GA13491@xxxxxxxxxxxxx> <20140107155830.GA28395@xxxxxxxxxxxxx> <CA+QCeVRiwHU+C5utaLQXf_MpjoYMYEF4LKRyDPaqcd=H6n-RRw@xxxxxxxxxxxxxx> <20140108140307.GA588@xxxxxxxxxxxxx> <CA+QCeVRuvNtZ8+9D-NtMOD=B9UEA5HMvKaGdXOQCjO-KTnZdbw@xxxxxxxxxxxxxx> <20140115220721.GY3469@dastard> <20140120135855.GA26280@xxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Mon, Jan 20, 2014 at 05:58:55AM -0800, Christoph Hellwig wrote:
> 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.

Oh, right, I got an off-by-one when reading the code - the EOF
zeroing only occurs when the offset is beyond EOF, not at or beyond

> > 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.

I'm probably not going to get to it before then, either, so check
back in a week?


Dave Chinner

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