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.