Why PVE move/clone disk using qemu-img instead of dd on raw disk?

[CoiaPrant]

I'm using PVE with lvm storage, when I move or clone disk, it's using qemu-img. but qemu-img convert is too slow.

I move disk from ssd to hdd (both lvm). 20G disk spend at least 2 hours.
When I using dd. its spend about 10min.

Why qemu-img convert so slow? Should pve using dd to replace qemu-img on raw?

 

[LnxBil]

qemu-img is probably used because it is the tool that can do all conversions, is therefore the only necessary code path and it has a progress bar. Maybe qemu-img uses per default a sync write and dd not and that could accomodate for the different speeds? I have not experienced any slowliness with this on my lvm-based systems

 

[_gabriel]

no slow here too.
qemu should faster because skip / writing zeroes where dd write zeroes ...

edit: correction

 

[CoiaPrant]

HDD is very slow, SSD is acceptable.
dd with status=progress can show progress.

 

[LnxBil]

qemu should faster because skip zeroes where dd read zeroes ...

qemu-img will also read zeros, yet it may not write them, where dd writes everything you feed into (yes, there are dd options for that).