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

[Avi Kivity <avi@redhat.com>]

  On 08/25/2010 05:14 PM, Anthony Liguori wrote:
>>
>>> At a high level, I don't think online compaction requires any 
>>> specific support from an image format.
>>>
>>
>> You need to know that the block is free and can be reallocated.
>
>
> Semantically, TRIM/DISCARD means that "I don't care about the contents 
> of the block anymore until I do another write."  Behind the scenes, we 
> can keep track of which blocks have been discarded in an in-memory 
> list whereas the first write to the block causes it to be evicted from 
> the discarded list.
>
> A background task would attempt to detect idle I/O and copy a block 
> from the end of the file to a location on the discarded list.  When 
> the copy has completed, you can then remove the L2 entry for the 
> discarded block (effectively punching a hole in the image), sync, and 
> then update the l2 entry for the block at the end of file location to 
> point to the new block location.  You can then ftruncate to reduce 
> overall file size.

That should work.

>
> 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.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.