[Qemu-devel] Re: [RFC][STABLE 0.13] Revert "qcow2: Use bdrv_(p)write_sync for metadata writes"

[Anthony Liguori]

On 08/25/2010 09:36 AM, Avi Kivity wrote:
> > If you tried to maintain a free list, then you would need to sync on 
> > TRIM/DISCARD which is potentially a fast path.  While a background 
> > task may be less efficient in the short term, it's just as efficient 
> > in the long term and it has the advantage of keeping any fast path fast.
>
> You only need to sync when the free list size grows beyond the amount 
> of space you're prepared to lose on power fail.  And you may be able 
> to defer the background task indefinitely by satisfying new 
> allocations from the free list.

Free does not mean free.  If you immediately punch a hole in the l2 
without doing a sync, then you're never sure whether the hole is there 
on disk or not.  So if you then allocate that block and put it somewhere 
else in another l2 table, you need to sync the previous l2 change before 
you update the new l2.

Otherwise you can have two l2 entries pointing to the same block after a 
power failure.  That's not a leak, that's a data corruption.

Regards,

Anthony Liguori