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Date: Wed, 29 Jun 2011 10:26:52 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] Default cache mode
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To: Christoph Hellwig <hch@lst.de>
Cc: Kevin Wolf <kwolf@redhat.com>, Avi Kivity <avi@redhat.com>, Qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

On 06/29/2011 09:52 AM, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 09:20:03AM -0500, Anthony Liguori wrote:
>> Sorry, I meant having a new cache option (maybe diskcache).  Semantics
>> would be:
>>
>> diskcache=on,wce=on    0
>> diskcache=on,wce=off   O_SYNC
>> diskcache=off,wce=on   O_DIRECT
>> diskcache=off,wce=off  O_DIRECT | O_SYNC
>>
>> ignore_flush could be another option too.
>
> Please make it two options instead of a two-dimensional one.

Yeah, that was just for the table.  diskcache/hostcache belongs in 
-drive, wce belongs in -device.

> E.g. e hostcache=[on,off] option for the drive command line,
> where fake is the equivalent of the current code together with
>
>> I think developers usually want diskcache=on but I also agree that usually
>> in production, you want diskcache=off.
>
> Why would you want to not use O_DIRECT as a developer?

Relaunching the same guest:

cache=writethrough:

real	0m9.500s
user	0m4.720s
sys	0m3.780s

cache=none:

real	0m15.263s
user	0m4.870s
sys	0m6.390s

If you're doing kernel development where you're frequently launching the 
same guest with a different kernel, it using the page cache makes a big 
difference in guest boot time.

Regards,

Anthony Liguori