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

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Message-ID: <4B9F5A96.1040705@redhat.com>
Date:	Tue, 16 Mar 2010 12:16:54 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Kevin Wolf <kwolf@...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>
Subject: Re: [PATCH][RF C/T/D] Unmapped page cache control - via boot parameter

On 03/16/2010 11:54 AM, Kevin Wolf wrote:
>
>> Is this with qcow2, raw file, or direct volume access?
>>
>> I can understand it for qcow2, but for direct volume access this
>> shouldn't happen.  The guest schedules as many writes as it can,
>> followed by a sync.  The host (and disk) can then reschedule them
>> whether they are in the writeback cache or in the block layer, and must
>> sync in the same way once completed.
>>
>> Perhaps what we need is bdrv_aio_submit() which can take a number of
>> requests.  For direct volume access, this allows easier reordering
>> (io_submit() should plug the queues before it starts processing and
>> unplug them when done, though I don't see the code for this?).  For
>> qcow2, we can coalesce metadata updates for multiple requests into one
>> RMW (for example, a sequential write split into multiple 64K-256K write
>> requests).
>>      
> We already do merge sequential writes back into one larger request. So
> this is in fact a case that wouldn't benefit from such changes.

I'm not happy with that.  It increases overall latency.  With qcow2 it's 
fine, but I'd let requests to raw volumes flow unaltered.

> It may
> help for other cases. But even if it did, coalescing metadata writes in
> qcow2 sounds like a good way to mess up, so I'd stay with doing it only
> for the data itself.
>    

I don't see why.

> Apart from that, wouldn't your points apply to writeback as well?
>    

They do, but for writeback the host kernel already does all the 
coalescing/merging/blah for us.

-- 
error compiling committee.c: too many arguments to function

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