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Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices to be changed at runtime.
Date: Tue, 1 Mar 2011 09:55:05 +0000

On Mon, Feb 28, 2011 at 3:48 PM, Kevin Wolf <address@hidden> wrote:
> Am 28.02.2011 16:35, schrieb Stefan Hajnoczi:
>> On Mon, Feb 28, 2011 at 3:12 PM, Kevin Wolf <address@hidden> 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.
> Not sure why I wanted to change that during runtime, but agreed,
> allowing to change parameters using the monitor is generally a good thing.
> However, I'm not sure if a command for changing the cache mode is the
> right solution, or if it should be something like a command to change
> block device options. (For example, what about toggling read-only or
> snapshot mode?)

Yes, I think you're right.  We should aim for a general interface
rather than having to add many more specific interfaces in the future.

CQ: Do you see a relation to the "update" interface you added to
adjust drive options at runtime for FVD?


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