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Date: Mon, 28 Feb 2011 16:12:50 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block
	devices to be changed at runtime.
References: <20110228171956.05a84fb9@zephyr>
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To: Prerna Saxena <prerna@linux.vnet.ibm.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, Ananth Narayan <ananth@linux.vnet.ibm.com>, Christoph Hellwig <hch@lst.de>, qemu-devel <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

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.

Christoph, what's the status of these patches?

> 2. info block
> Now extended to display cache settings for available block devices.

Have you checked that libvirt is okay with this?

Kevin