[Qemu-block] QCOW2 support for LZO compression

[Peter Lieven]

Hi,


I am currently working on optimizing speed for compressed QCOW2 images. We use 
them for templates and would also like to use them

for backups, but the latter is almost infeasible because using gzip for 
compression is horribly slow. I tried to experiment with different options

to deflate, but in the end I think its better to use a different compression 
algorithm for cases where speed matters. As we already have probing

for it in configure and as it is widely used I would like to use LZO for that 
purpose. I think it would be best to have a flag to indicate that compressed

blocks use LZO compression, but I would need a little explaination which of the 
feature fields I have to use to prevent an older (incompatible) Qemu opening

LZO compressed QCOW2 images.


I also have already some numbers. I converted a fresh Debian 9 Install which 
has an uncomressed QCOW2 size of 1158 MB with qemu-img to a compressed QCOW2.

With GZIP compression the result is 356MB whereas the LZO version is 452MB. 
However, the current GZIP variant uses 35 seconds for this operation

where LZO only needs 4 seconds. I think is is a good trade in especially when 
its optional so the user can choose.


What are your thoughts?


Peter