On 08/25/2010 06:06 PM, Anthony Liguori wrote:
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.
L2 certainly needs to be updated before the block is reused. But
that's not different from a file format without a free list.
The batching I was referring to was only for free list management,
same as the allocation issue which started this thread.