On 24-Jul-09, at 6:41 PM, Frank Middleton wrote:
On 07/24/09 04:35 PM, Bob Friesenhahn wrote:
Regardless, it [VirtualBox] has committed a crime.
But ZFS is a journalled file system! Any hardware can lose a flush;
No, the problematic default in VirtualBox is flushes being *ignored*,
which has a different failure mode. A host crash under this regime
can potentially corrupt *any* journaled and transactional system
(starting with filesystems and RDBMS) in a manner that does not occur
on properly functioning bare metal that honours flushes, because
their ordering assumptions no longer hold.
Whether this is 'possible' with a guest-only crash is arguable - I
don't want to speak for Miles, but I suspect he was reasoning that a
guest crash would not interact with ignore-flush, as all requested
issued I/O up until the crash "should" finally complete - making a
guest crash similar to a "real" crash. But the virtualised stack is
complex enough that I don't know if we can be certain about that.
I would say that ignoring flushes is still a suspect.
it's just more likely in a VM, especially when anything Microsoft
is involved,
I originally saw the problem on a Ubuntu system, 6 months ago. The
subsystems which broke were ext3fs and InnoDB - both supposedly
"journaling".
and the whole point of journalling is to prevent things
like this happening.
It can ONLY do that when flushes/barriers/ordering are respected.
--Toby
...
HTH -- Frank
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