On Thu, Dec 08, 2005 at 10:57:35AM -0600, Rob Landley wrote:
> What we really want are write barriers (which I don't believe Linux exposes
> to
> userspace but I'm not sure), and for UBD to propogate through actual fsync
> requests to the underlying OS.
The real problem is if the *host* crashes. UML can't (or currently
doesn't try to) order writes down to the disk. Without AIO, It does
order writes into the host page cache. With AIO, it issues O_DIRECT
requests in order, but can't guarantee that that's the order in which
they will reach the disk.
In both cases, as long as the host stays up, everything is good. If
the UML crashes, then writes will have been sent to the host in the
correct order, and the consistency guarantees needed by the filesystem
should be respected.
If the host crashes, then some number of issued requests may not have
reached the disk, and there is the possibility that the ubd image may
not be recoverable. However, I have crashed a number of hosts, and
never seen this, and I don't remember ever hearing of it happening.
Jeff
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