linux-kernel - Disable aggregation of requests to a block device

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Message-ID: <ce513bcc0902190408j174807a6t73300c8b8fd3c708@mail.gmail.com>
Date:	Thu, 19 Feb 2009 14:08:20 +0200
From:	Erez Zilber <erezzi.list@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Disable aggregation of requests to a block device

Hi,

I'm running I/O to a block device (which is an iSCSI device, but that
shouldn't matter), usually with transfer length = 128kB. From time to
time, I see that 2 128kB commands get aggregated to a single 256kB
command, which is bad for me because the resources that I allocate are
based on 128kB commands. I could limit the command size by setting
/sys/block/sdX/queue/max_sectors_kb to 128kB, but sometimes I expect
larger commands (e.g. 512kB), and I don't want the block layer to
split them into smaller commands.

My question is: can I disable this aggregation? If yes, how? Else, is
there another solution?

Thanks,
Erez
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