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Date: Mon, 28 Feb 2011 15:35:56 +0000
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Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block
	devices to be changed at runtime.
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Kevin Wolf <kwolf@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, Ananth Narayan <ananth@linux.vnet.ibm.com>, Prerna Saxena <prerna@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>

On Mon, Feb 28, 2011 at 3:12 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 28.02.2011 12:49, schrieb Prerna Saxena:
>> The following patchset introduces monitor commands:
>>
>> 1. set_cache DEVICE CACHE-SETTING
>> Change cache settings for block device, DEVICE, through the monitor.
>> (Available options : 'none', 'writeback', 'writethrough')
>> Eg,
>> (qemu)set_cache ide0-hd0 none
>> -> Changes cache setting for ide0-hd0 to 'none'
>
> Not sure if adding this interface is a good idea. I see that you only
> add it for HMP, and we may consider that, but it's definitely not
> suitable for QMP.
>
> One reason is that none/writethrough/writeback/unsafe isn't really what
> we want to use long term. We want to separate advertising a write cache
> (which is guest visible) from things like whether to use O_DIRECT or not.
>
> In the past, Christoph mentioned that he had patches to make these
> separate and even let the guest change the "write cache enabled" flag,
> which would probably solve most of the use cases of this patch.

Toggling host page cache at runtime is useful too because it saves
having to restart VMs.  I agree that the guest should control the
emulated drive cache at runtime and we probably don't want to allow
toggling that from the host - it could be dangerous :).

Stefan