On Feb 28, 2011 10:48 AM, "Kevin Wolf" <kw...@redhat.com> wrote: > > Am 28.02.2011 16:35, schrieb Stefan Hajnoczi: > > On Mon, Feb 28, 2011 at 3:12 PM, Kevin Wolf <kw...@redhat.com> 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. Regards, 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 >
