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

[Vladimir Sementsov-Ogievskiy]

Hi Sam!

10.06.2020 08:28, Sam Eiderman wrote:
> Hi,
>
> 168468fe19c8 ("qemu-img: Add --target-is-zero to convert") has added a
> nice functionality for cloud scenarios:
>
> * Create a virtual disk

What is the format of your target?

> * 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?

Yes, the feature is only for really zero-initialized target, it will skip 
"allocated" zeroes as well.

What you want - copying only allocated blocks of backing-supporting format - looks like 
"top" mode of
mirror and backup block jobs. Didn't you considered using qemu itself (in 
stopped mode, i.e. cpus are
not running) or new qemu-storage-daemon instead of qemu-img? With this approach 
you'll have the whole
power of QMP commands to manage block-layer, including block-jobs.

--
Best regards,
Vladimir