Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Kevin Wolf <kwolf@redhat.com>]

Am 13.09.2010 13:34, schrieb Avi Kivity:
>   On 09/13/2010 01:28 PM, Kevin Wolf wrote:
>>
>>> Anytime you grow the freelist with qcow2, you have to write a brand new
>>> freelist table and update the metadata synchronously to point to a new
>>> version of it.  That means for a 1TB image, you're potentially writing
>>> out 128MB of data just to allocate a new cluster.
>> No. qcow2 has two-level tables.
>>
>> File size: 1 TB
>> Number of clusters: 1 TB / 64 kB = 16 M
>> Number of refcount blocks: (16 M * 2 B) / 64kB = 512
>> Total size of all refcount blocks: 512 * 64kB = 32 MB
>> Size of recount table: 512 * 8 B = 4 kB
>>
>> When we grow an image file, the refcount blocks can stay where they are,
>> only the refcount table needs to be rewritten. So we have to copy a
>> total of 4 kB for growing the image file when it's 1 TB in size (all
>> assuming 64k clusters).
>>
>> The other result of this calculation is that we need to grow the
>> refcount table each time we cross a 16 TB boundary. So additionally to
>> being a small amount of data, it doesn't happen in practice anyway.
> 
> Interesting, I misremembered it as 8 bytes per cluster, not 2.  So it's 
> actually fairly dense (though still not as dense as a bitmap).

Yes, refcounts are 16 bit. Just checked it with the code once again to
be 100% sure. But if it was only that, it would be just a small factor.
The important part is that it's a two-level structure, so Anthony's
numbers are completely off.

Kevin