Il 09/01/2013 14:04, Liu Yuan ha scritto: > > > 2 The upper layer software which relies on the 'cache=xxx' to choose > > > cache mode will fail its assumption against new QEMU. > > > > Which assumptions do you mean? As far as I can say the behaviour hasn't > > changed, except possibly for the performance. > > When users set 'cache=writethrough' to export only a writethrough cache > to Guest, but with new QEMU, it will actually get a writeback cache as > default.
They get a writeback cache implementation-wise, but they get a writethrough cache safety-wise. How the cache is implemented doesn't matter, as long as it "looks like" a writethrough cache. In fact, consider a local disk that doesn't support FUA. In old QEMU, images used to be opened with O_DSYNC and that splits each write into WRITE+FLUSH, just like new QEMU. All that changes is _where_ the flushes are created. Old QEMU changes it in the kernel, new QEMU changes it in userspace. > We don't need to communicate to the guest. I think 'cache=xxx' means > what kind of cache the users *expect* to export to Guest OS. So if > cache=writethrough set, Guest OS couldn't turn it to writeback cache > magically. This is like I bought a disk with 'writethrough' cache > built-in, I didn't expect that it turned to be a disk with writeback > cache under the hood which could possible lose data when power outage > happened. It's not by magic. It's by explicitly requesting the disk to do this. Perhaps it's a bug that the cache mode is not reset when the machine is reset. I haven't checked that, but it would be a valid complaint. Paolo > > So Guest-WCE only works when users provide us a writeback capable disk.
