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Date:	Wed, 8 Aug 2007 17:28:53 +0400
From:	Evgeniy Polyakov <>
To:	Jens Axboe <>
Cc:	Daniel Phillips <>,,,,
	Peter Zijlstra <>
Subject: Re: [1/1] Block device throttling [Re: Distributed storage.]

On Wed, Aug 08, 2007 at 02:17:09PM +0400, Evgeniy Polyakov ( wrote:
> This throttling mechanism allows to limit maximum amount of queued bios 
> per physical device. By default it is turned off and old block layer 
> behaviour with unlimited number of bios is used. When turned on (queue
> limit is set to something different than -1U via blk_set_queue_limit()),
> generic_make_request() will sleep until there is room in the queue.
> number of bios is increased in generic_make_request() and reduced either
> in bio_endio(), when bio is completely processed (bi_size is zero), and
> recharged from original queue when new device is assigned to bio via
> blk_set_bdev(). All oerations are not atomic, since we do not care about
> precise number of bios, but a fact, that we are close or close enough to
> the limit.
> Tested on distributed storage device - with limit of 2 bios it works
> slow :)

As addon I can cook up a patch to configure this via sysfs if needed.

	Evgeniy Polyakov
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