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

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices to be changed at runtime.
Date: Tue, 01 Mar 2011 14:22:52 +0100
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20101027 Fedora/3.0.10-1.fc12 Thunderbird/3.0.10

Am 01.03.2011 14:03, schrieb Anthony Liguori:
> On Feb 28, 2011 10:48 AM, "Kevin Wolf" <address@hidden
> <mailto: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
> <mailto: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?)
> Certainly good questions, but let me suggest not taking an HMP command
> and not a QMP commans because of interface concerns.
> My goal for 0.15 is to convert HMP to be implemented in terms of QMP. 
> To do that, a bunch of new QMP commands are needed.  They all won't be
> perfect but i'd rather support a bad QMP command forever than to
> continue to/ have people rely on HMP.

Okay, makes sense. So we should reject patches that add new HMP commands
without adding a QMP counterpart.

>> > 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 :).
>> Good point. That's a NACK for this patch as long as we haven't separated
>> WCE from the host cache setting.

Doesn't make a difference for this one, though, because it's NACKed anyway.


PS: Anthony, is there a specific reason why you started sending HTML emails?

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