Re: [Qemu-devel] [PATCH] qemu-img: return allocated size for block device with qcow2 format

[Eric Blake <eblake@redhat.com>]

On 05/02/2018 10:13 AM, Max Reitz wrote:

>> We also recently added 'qemu-img measure', which DOES report how many
>> clusters are in use.  Is any of that reusable here?
> 
> It only tells you that information for a hypothetical new image, though,
> doesn't it?

It has two uses: with just a size, estimate the overhead needed to 
create a file with the given format and exposing the given size to the 
guest (both sparse and fully allocated sizes are estimated); and with a 
pre-existing image, compute the exact overhead needed for 'qemu-img 
convert' to create a new image with the given format and the same 
guest-visible contents.  The latter use, where it uses an existing guest 
image as the starting point to measure, indeed only tells you what a 
hypothetical new image will occupy (the used cluster count, not the 
wr_highest_offset count) - but isn't that what you want?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org