linux-kernel - Re: [PATCH][RF C/T/D] Unmapped page cache control

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Message-ID: <20100317165229.GA29548@lst.de>
Date:	Wed, 17 Mar 2010 17:52:29 +0100
From:	Christoph Hellwig <hch@....de>
To:	Avi Kivity <avi@...hat.com>
Cc:	Chris Webb <chris@...chsys.com>, balbir@...ux.vnet.ibm.com,
	KVM development list <kvm@...r.kernel.org>,
	Rik van Riel <riel@...riel.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Kevin Wolf <kwolf@...hat.com>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

On Wed, Mar 17, 2010 at 06:22:29PM +0200, Avi Kivity wrote:
> They should be reorderable.  Otherwise host filesystems on several 
> volumes would suffer the same problems.

They are reordable, just not as extremly as the the page cache.
Remember that the request queue really is just a relatively small queue
of outstanding I/O, and that is absolutely intentional.  Large scale
_caching_ is done by the VM in the pagecache, with all the usual aging,
pressure, etc algorithms applied to it.  The block devices have a
relatively small fixed size request queue associated with it to
facilitate request merging and limited reordering and having fully
set up I/O requests for the device.

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