Re: [Qemu-devel] [Bug 322602] Re: Snapshot usage makes qcow2 image unusable due to large tables

[Kevin Wolf <kwolf@redhat.com>]

Am 11.06.2010 09:59, schrieb Jes Sorensen:
> Could you please let us know whether this is still a problem and if it
> isn't, lets close this bug.

This looks like a purely hypothetical thing - have you seen this happen
in reality, and if so, with how many snapshots?

This is reported against a very old version, so we have to assume 4k
clusters. This means that a refcount block holds the refcounts for 4k /
2 = 2k clusters. Let's assume a refcount table of only one cluster, so
we can describe (4k / 8) * 2k = 1M clusters with this, which makes up an
image size of 4G.

To hold a 20G virtual disk plus some metadata we'll therefore need
something like 6 clusters = 24k. To make the refcount table consume just
1 MB, you'll therefore need at least 42 snapshots, each fully allocated
on its own, consuming 1 TB for the image. I doubt that there are too
many machines which can handle a 1 TB image file on disk, but not a 1 MB
refcount table in RAM.

Nowadays, of course, we're using 64k clusters by default. With a
refcount table of 64k we describe 16 TB there, with a 1 MB refcount
table it's 256 TB.

Kevin