Am 20.09.2010 17:40, schrieb Anthony Liguori: > On 09/20/2010 10:08 AM, Kevin Wolf wrote: >>> If you're comfortable with a writeback cache for metadata, then you >>> should also be comfortable with a writeback cache for data in which >>> case, cache=writeback is the answer. >>> >> Well, there is a difference: We don't pollute the host page cache with >> guest data and we don't get a virtual "disk cache" as big as the host >> RAM, but only a very limited queue of metadata. >> >> Basically, in qemu we have three different types of caching: >> >> 1. O_DSYNC, everything is always synced without any explicit request. >> This is cache=writethrough. >> > > I actually think O_DSYNC is the wrong implementation of > cache=writethrough. cache=writethrough should behave just like > cache=none except that data goes through the page cache.
Then you have cache=writeback, basically. >> 2. Nothing is ever synced. This is cache=unsafe. >> >> 3. We present a writeback disk cache to the guest and the guest needs >> to explicitly flush to gets its data safe on disk. This is >> cache=writeback and cache=none. >> > > We shouldn't tie the virtual disk cache to which cache= option is used > in the host. cache=none means that all requests go directly to the > disk. cache=writeback means the host acts as a writeback cache. No, that's not the meaning of cache=none if you take the disk cache into consideration. It might be what you think should be the meaning of cache=none, but it's not what it means in any qemu release. > If your disk is in writethrough mode, exposing cache=none as a writeback > disk cache is not correct. The host's disk is writethrough? In this case it's being more conservative than needed, yes. >> We're still lacking modes for O_DSYNC | O_DIRECT and unsafe | O_DIRECT, >> but they are entirely possible, because it's two different dimensions. >> (And I think Christoph was planning to actually make it two independent >> options) > > I don't really think O_DSYNC | O_DIRECT makes much sense. Maybe, maybe not. It's just a missing entry in the matrix. Kevin
