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Date: Mon, 07 Nov 2011 15:05:57 +0530
From: Supriya Kannery <supriyak@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] [v8 Patch 0/6]Qemu: Host pagecache setting from
 cmdline and monitor
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To: Zhi Yong Wu <zwu.kernel@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Christoph Hellwig <hch@lst.de>

On 11/07/2011 02:19 PM, Zhi Yong Wu wrote:
> On Mon, Nov 7, 2011 at 4:38 PM, Supriya Kannery
> <supriyak@linux.vnet.ibm.com>  wrote:
>> On 11/04/2011 07:59 AM, Zhi Yong Wu wrote:
>>>
>>> On Sun, Oct 30, 2011 at 6:33 PM, Supriya Kannery
>>> <supriyak@linux.vnet.ibm.com>    wrote:
>>>>
>>>> Currently cache setting of a block device cannot be changed
>>>> without restarting a running VM. Following patchset is for
>>>> enabling dynamic change of cache setting for block devices
>>>> through qemu monitor. Code changes are based on patches
>>>> from Christoph Hellwig and Prerna Saxena.
>>>>
>>>> This patchset introduces
>>>> a. monitor command 'block_set_hostcache' using which host
>>>>    pagecache setting for a block device can be changed
>>>>    dynamically.
>>>
>>> I got a bit confusion. Is it used to change host pagecache setting on
>>> hyperviser or on guest?
>>> This block device said by you is for guest, right?
>>>
>>
>>   This command is used for changing pagecache setting of
>> the image files in host which are acting as block devices
>> for guest.
> So what is the difference between it and cache option?
>

Cache=xx sets a combination of status flags for image files
among which host pagecache is just one. Intention here is
to gradually replace cache=xx with more explicit settings
like hostcache, flush, WCE.