On 08/25/2010 08:23 AM, Avi Kivity wrote:
On 08/25/2010 03:46 PM, Anthony Liguori wrote:
If we had another disk format that only supported growth and metadata
for a backing file, can you think of another failure scenario?
btw, only supporting growth is a step backwards. Currently
file-backed disks keep growing even the guest-used storage doesn't
grow, since once we allocate something we never release it. But
eventually guests will start using TRIM or DISCARD or however it's
called, and then we can expose it and reclaim unused blocks.
You can do this in one of two ways. You can do online compaction or you
can maintain a free list. Online compaction has an advantage because it
does not require any operations in the fast path whereas a free list
would require ordered metadata updates (must remove something from the
first list before updating the l2 table) which implies a sync.
At a high level, I don't think online compaction requires any specific
support from an image format.
Regards,
Anthony Liguori
