On 09/08/2010 01:56 PM, Blue Swirl wrote:
That's a bit big, for example CD images are only 640M and there were
smaller disks. But I guess you mean the smallest maximum size limited
by the cluster_size etc, so the actual images may be even smaller.
Yes. The smallest image is one cluster. The smallest cluster is 4k so
the smallest image is 4k.
I don't think cluster sizes much greater than 64k actually make sense. We
don't need an image format that supports> 1PB disks.
File system developers could want to try images in exabyte ranges.
Isn't the purpose of an image format that you can create a virtual
disk that can appear to be bigger than the disk space needed?
$ qemu-img create -f qed -o table_size=16,cluster_size=1M exabyte.qed
$((1024*1024))T
Formatting 'exabyte.qed', fmt=qed size=1152921504606846976
cluster_size=1048576 table_size=16 copy_on_read=off
I still contend it's insane to do, but it does work and only requires a
1M cluster size.
Generally speaking, max image size is:
(cluster_size * table_size / 8) * (cluster_size * table_size / 8) *
cluster_size
Or:
(2^x * 2^y / 2^3) * (2^x * 2^y / 2^3) * 2^x
valid values for y are [0...4]. Valid values for x are [12...31]
Solve for each range and you have 2^30...2^107 but you can't have an
image > ~2^64.
There's an awful lot of flexibility with just something as simple as a
two level table.
Regards,
Anthony Liguori
