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Date: Wed, 29 Jun 2011 16:52:48 +0200
From: Christoph Hellwig <hch@lst.de>
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Subject: Re: [Qemu-devel] Default cache mode
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Kevin Wolf <kwolf@redhat.com>, Avi Kivity <avi@redhat.com>, Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Qemu-devel@nongnu.org

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.

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?  Especially for
desktop setups where the VM images might be on a common use filesystem
not using O_DIRECT a deathly option for performance.