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

[Kevin Wolf <kwolf@redhat.com>]

Am 12.09.2010 19:09, schrieb Anthony Liguori:
> For a 1PB disk image with qcow2, the reference count table is 128GB.  
> For a 1TB image, the reference count table is 128MB.   For a 128GB 
> image, the reference table is 16MB which is why we get away with it today.

This is physical size. If you have a 1 PB disk, you're probably okay
with using 128 GB of it for metadata (and I think it's less than that,
see below)

> 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.

Kevin