On 01/09/2013 11:10 PM, Paolo Bonzini wrote: > 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. > Ah I didn't get the current implementation right. I tried the 3.7 kernel and it works as expected (cache=writethrough result in a 'writethrough' cache in the guest). It looks fine to me to emulate writethrough as writeback + flush, since the profermance drop isn't big, though sheepdog itself support true writethrough cache (no flush). I am going to send v2 of directio patch for sheepdog driver. Thanks, Yuan
