Image resize for fat type of disk does not preallocate additional space

[akrasnov-drv]

problem

Currently (v.4.20.0.0) I see that sparse and fat disk types result in the same local disk format/size/type.
After digging into it I found the following flow for fat disks:

qemu-img create -o preallocation=full -f qcow2 newdisk template_size
qemu-img convert -O qcow2 -o preallocation=full -U --image-opts driver=qcow2,file.filename=template_file newdisk
qemu-img resize newdisk newsize

as a result new disk size is set to newsize, but its actual disk size is just of template_size
To be honest I see no reason to do create at all, it's overwritten by convert.
And to have fat newdisk there should be --preallocation=full in resize command too

qemu-img resize --preallocation=full newdisk newsize

This affects disk performance. In our CI environment CS disk becomes very fragmented, qemu-img map shows some 10K entries.
Also please consider using preallocated raw disks in place of preallocated qcow

Thanks,
Alex.

versions

CloudStack 4.20.0.0
Ubuntu 22.04.5 LTS
QEMU 1:6.2+dfsg-2ubuntu6.24
libvirt 8.0.0-1ubuntu7.10

The steps to reproduce the bug

1. Create VM with large fat disk from small template
2. Check qemu-img info to see virtual size is correct according to 1. but disk size is still of original template

What to do about it?

Add --preallocation=full to resize command for fat disk
Use raw instead of qcow

[weizhouapache]

makes sense

added to 4.19.3 milestone. cc @Pearl1594

[Suvrat1629]

@weizhouapache I was taking a look at this issue and I had some questions, like I tweaked the resize methods in QemuImg files and now I guess I have to tweak the LibvirtStorageAdaptor file. I am confused about how I will do this? Should i tweak the qemu.convert method to handle the provisioningType? Would changing the convert method break alot of things in the repo? Could you please guide me.
Thank you.

[akrasnov-drv]

Looks like preallocation for large disks makes significant load on cpu and may affect other VMs running
so worth doing
nice -n 19 qemu-img
or even choose some less busy core and do
nice -n 19 taskset -c $core qemu-img

Please consider using raw disks in place (or in addition to) qcow preallocated ones

[akrasnov-drv]

If you just add preallocation then nothing should be changed in VM config
but if you want to replace qcow with raw, then indeed VM xml needs update