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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [RFC][PATCH 0/2] Allow cache settings for block devices to be changed at runtime.
Date: Tue, 1 Mar 2011 07:03:18 -0600

On Feb 28, 2011 10:48 AM, "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?)

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.


Anthony Liguori

> > 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.
> Kevin

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