Embarassing. Can't figure out why blank ZVOL is using space?

[diskdiddler]

I am messing around with ZSTD compression for fun and I'm moving ZVOLs around my SSDS in my system.

I have 3 VMs using 3 ZVOLs with maximum compression. They're fantastic - only 21GB for them. (before snapshots)
I am creating 3 'blank' ZVOLS to migrate them to another SSD, the 3 blank ZVOLS I'm creating the same way I created the current ones they're using.

When I do so, instantly, all drive space is allocated. I'm not ticking 'sparse' or 'force size'.
The resulting zvols are using ALL allocated space, immediately, before even ZFS send / recving data to them.



To be clear the ones at the bottom ARE VM ZVOLS, running and live, consuming a total of 21.9GB. The disks are /allocated/ 10/10/80GB - but they're consuming 21.9GB

Why, when I go to create the top ones, does it instantly allocate 101.5GB and when I send / recv into those ZVOLs they get even larger still?
I'm clearly missing something here.
See screenshot:

I 100,000% created the bottom disks, only yesterday, using the same method and they consumed only 21GB.

 

[Patrick M. Hausen]

I'm not ticking 'sparse' or 'force size'.

If you don't tick "sparse" then the size of the ZVOL will be reserved instantly. That is a feature. Just enable "sparse" if you want a growing ZVOL.

 

[diskdiddler]

I swear on my life, I did not originally tick that box, so I'm baffled how they can be created as sparse now.

 

[Patrick M. Hausen]

Try zfs set refreservation=none <volume name> - that will turn a preallocated volume into a sparse one.