First, some background.

A Packstack answer file contains a parameter CONFIG_CINDER_VOLUMES_SIZE. My Rocky Packstack deployment uses the default size, which is 20GB.

Here is my cinder-volumes file:

$ ls -lsh /var/lib/cinder/cinder-volumes
256K -rw-r-----. 1 root root 21G May 14 19:33 /var/lib/cinder/cinder-volumes

The 256K figure is the actual space this file currently uses in the filesystem. It's a sparse file that fills up as you allocate Cinder volumes. I don't think that Linux has a mechanism to release space in a file.

The fix is to provide enough space in /var/lib/cinder so that the file can grow up to its 20GB. Or you set a smaller CONFIG_CINDER_VOLUMES_SIZE when deploying Packstack, knowing that this will limit the amount of volume data that you can create.

More background information: Packstack deploys LVM as the Cinder backend and enables thin provisioning by default. The cinder-volumes file is used as the LVM physical volume:

# pvs
  PV         VG             Fmt  Attr PSize   PFree
  /dev/loop1 cinder-volumes lvm2 a--  <20.60g 1012.00m
# lvs
  LV                  VG             Attr       LSize   Pool Origin Data%  Meta%  (...)
  cinder-volumes-pool cinder-volumes twi-a-tz--  19.57g             0.00   10.55

cinder-volumes-pool is the LVM pool from which thin logical volumes are allocated, each representing a Cinder volume.