Am 22.10.2010 15:45, schrieb Anthony Liguori: >>> On a physical system, if you don't have a battery backed disk and you >>> enable the WC on your disk, then even with cache=writethrough we're unsafe. >>> >> I don't think that's right. O_SYNC should guarantee that the volatile >> disk cache is flushed. >> > > If your filesystem does the right thing which an awful lot of them don't > today.
The list of really relevant filesystems is rather short, though. >>> Likewise, if you mount your filesystem with barrier=0, QEMU is unsafe. >>> >> Yeah, if you do something equivalent to cache=unsafe on a lower layer, >> then qemu can't do much about it. Maybe you can apply the same argument >> to NBD, even though it's unsafe by default. >> >> >>> QEMU can't guarantee safety. The underlying storage needs to be >>> configured correctly. As long as we're not introducing caching within >>> QEMU, I don't think we should assume we're unsafe. >>> >>> Do we have any place where we can add docs on a per-block format basis? >>> It would be good to at least mention for each block device how the >>> backing storage needed to be configured for safety. >>> >> docs/block-protocols.txt? >> > > Maybe docs/block/<name>.txt? Would be a good home for the qed spec too. I think spec and documentation for users should be kept separate. I thought that's the reason why docs/specs/ exists. And if you exclude specs, I'm not sure if there's a lot left to say for each format. Having ten files under docs/block/ which consist of two lines each would be ridiculous. If contrary to my expectations we actually do have content for it, docs/block/<name>.txt works for me as well. Kevin
