From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeff Moyer <jmoyer@redhat.com>
Subject: Re: [Lsf-pc] [LSF/MM TOPIC] a few storage topics
Date: Tue, 24 Jan 2012 14:14:14 -0500
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Cc: Andreas Dilger <adilger@dilger.ca>, Andrea Arcangeli <aarcange@redhat.com>,
        Jan Kara <jack@suse.cz>,
        "linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
        Mike Snitzer <snitzer@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        "dm-devel@redhat.com" <dm-devel@redhat.com>, fengguang.wu@gmail.com,
        Boaz Harrosh <bharrosh@panasas.com>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        "lsf-pc@lists.linux-foundation.org" <lsf-pc@lists.linux-foundation.org>
To: Chris Mason <chris.mason@oracle.com>
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In-Reply-To: <20120124190732.GH4387@shiny> (Chris Mason's message of "Tue, 24
	Jan 2012 14:07:32 -0500")
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Chris Mason <chris.mason@oracle.com> writes:

>> All three filesystems use the generic mpages code for reads, so they
>> all get the same (bad) I/O patterns.  Looks like we need to fix this up
>> ASAP.
>
> Can you easily run btrfs through the same rig?  We don't use mpages and
> I'm curious.

The readahead code was to blame, here.  I wonder if we can change the
logic there to not break larger I/Os down into smaller sized ones.
Fengguang, doing a dd if=file of=/dev/null bs=1M results in 128K I/Os,
when 128KB is the read_ahead_kb value.  Is there any heuristic you could
apply to not break larger I/Os up like this?  Does that make sense?

Cheers,
Jeff