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. 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 :). Stefan
