Clarification regarding new qemu-img convert --target-is-zero flag

[Sam Eiderman]

Hi,

168468fe19c8 ("qemu-img: Add --target-is-zero to convert") has added a
nice functionality for cloud scenarios:

* Create a virtual disk
* Convert a sparse image (qcow2, vmdk) to the virtual disk using
--target-is-zero
* Use the virtual disk

This saves many unnecessary writes - a qcow2 with 1MB of allocated
data but with 100GB virtual size will be converted efficiently.

However, does this pose a problem if the virtual disk is not zero initialized?

Theoretically - if all unallocated blocks contain garbage - this
shouldn't matter, however what about allocated blocks of zero? Will
convert skip copying allocated zero blocks in the source image to the
target since it assumes that the target is zeroed out first thing?

Sam