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.