From: Uri Lublin <address@hidden>
This patchset let the user know the highest allocated byte of qcow2
images.
Actually it's the first unallocated byte after the highest byte written,
cluster-size aligned.
The highest allocated byte gives a maximal limit (easy to calculate)
to the number of bytes allocated for that image, and may hint how many
more allocations can be done before we reach end-of-file (end of host
block device).
Although there may be many free blocks below that number (allocated
and freed)
the file system can not deallocate those blocks, and they have to be
reused
by qemu. Also note that due to fragmentation those free blocks may not
be used on next allocations.
It can be useful for truncation of backing file images (ftruncate).
Also it may be useful for defragmentation later (although we'll need
the number of free blocks as well).