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Date: Wed, 29 Jun 2011 09:20:03 -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 08:55 AM, Christoph Hellwig wrote:
> On Wed, Jun 29, 2011 at 07:08:18AM -0500, Anthony Liguori wrote:
>> As long as we advertise wce and wce can be toggled from the guest, I don't
>> think the default is all that important.  I think cache=on is the right
>> default for most common use cases.
>
> What do you mean with cache=on?

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.

> We have
> cache=[none,writeback,writethrough,unsafe].  As discussed about a dozen
> times before these are an incomplete choice of the [odirect=on,off;
> osync=on,off, ignore cache flushes=on,off] matrix.

Yup, I agree that we need to deprecate 
cache=[none,writeback,writethrough,unsafe] and add more meaningful tunables.

> Not using O_DIRECT when it is available is almost always the wrong choice.
> It means an additional data copy, VM pressure and exercise of often suck
> fsync/O_SYNC codepatheses, with only a benefit if you have multiple VMs
> actually using the same dataset with the same COW backing image.

Can we set globals for -drive via -readconfig?

If so, we could defer the default value to a config file setting which I 
think would be the best possible scenario.

I think developers usually want diskcache=on but I also agree that 
usually in production, you want diskcache=off.

Regards,

Anthony Liguori