Re: [Qemu-devel] qcow2 - safe on kill? safe on power fail?

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Journalling is so out of fashion.  It's better to sequence the 
> operations so that failure results in a leak instead of corruption.

Since the metadata is being updated synchronously, you could probably 
get away with a pretty simple journal.  Maybe even a single field that 
contains the offset you are allocating which then gets reset once the 
allocation was completed.

When QEMU starts up again, it can look at that field, and if it's not 0, 
check for anomalies in the allocation, prune that portion of the tree, 
and then start the guest.  That's a few more writes but it's already a 
slow path so it should be okay.

Regards,

Anthony Liguori