Re: [Qemu-devel] [RFC] Proposed qcow2 extension: subcluster allocation

[Alberto Garcia]

On Fri 07 Apr 2017 07:10:46 PM CEST, Max Reitz wrote:
>> === Changes to the on-disk format ===
>> 
>> The qcow2 on-disk format needs to change so each L2 entry has a bitmap
>> indicating the allocation status of each subcluster. There are three
>> possible states (unallocated, allocated, all zeroes), so we need two
>> bits per subcluster.
>
> You don't need two bits, you need log(3) / log(2) = ld(3) ≈ 1.58. You
> can encode the status of eight subclusters (3^8 = 6561) in 13 bits
> (ld(6561) ≈ 12.68).

Right, although that would make the encoding more cumbersome to use and
to debug. Is it worth it?

> One case I'd be especially interested in are of course 4 kB
> subclusters for 64 kB clusters (because 4 kB is a usual page size and
> can be configured to be the block size of a guest device; and because
> 64 kB simply is the standard cluster size of qcow2 images
> nowadays[1]...).

I think that we should have at least that, but ideally larger
cluster-to-subcluster ratios.

> (We could even get one more bit if we had a subcluster-flag, because I
> guess we can always assume subclustered clusters to have OFLAG_COPIED
> and be uncompressed. But still, three bits missing.)

Why can we always assume OFLAG_COPIED?

> If course, if you'd be willing to give up the all-zeroes state for
> subclusters, it would be enough...

I still think that it looks like a better idea to allow having more
subclusters, but giving up the all-zeroes state is a valid
alternative. Apart from having to overwrite with zeroes when a
subcluster is discarded, is there anything else that we would miss?

> By the way, if you'd only allow multiple of 1s overhead
> (i.e. multiples of 32 subclusters), I think (3) would be pretty much
> the same as (2) if you just always write the subcluster information
> adjacent to the L2 table. Should be just the same caching-wise and
> performance-wise.

Then (3) is effectively the same as (2), just that the subcluster
bitmaps are at the end of the L2 cluster, and not next to each entry.

Berto