Comment 1 for bug 2063105

@Bartosz it seems you were lucky with 24GB ends being with a multiple of 16KiB (ZFS uses K and G to mean KiB and GiB btw):

$ lxc config device override testvm root size=24GB
Device root overridden for testvm
$ zfs get -p volsize default/virtual-machines/testvm.block
NAME PROPERTY VALUE SOURCE
default/virtual-machines/testvm.block volsize 24000004096 local

15 and 16GB are not multiple of 16KiB. You can generally avoid such issues by defining the storage sizes by using MiB or GiB units. Like here:

$ lxc config device override testvm root size=15GiB
Device root overridden for testvm
$ lxc config device set testvm root size=16GiB

Could you check if using GiB works for you too? I'll mark the bug as incomplete until you can test and report. Thank you.