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?) > 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
