Re: delete on snapshot does not free space

[Zdenek Kabelac <zdenek.kabelac@gmail.com>]

Dne 30. 04. 24 v 19:54 Michael DiDomenico napsal(a):
> this is probably expected behavior, but i'd like to confirm as i'm not
> a device-mapper expert
> 
> we're using COW snapshots overtop of a readonly block device.  this
> seems to be working fine, however, when a person deletes files the
> blocks are not freed.  case in point a RO block device with a COW
> filled up to 100% as shown in df.  when i deleted some files to free
> space the usage never decreased
> 
> to create the mount we do
> 
> echo "0 <realsize> linear /dev/loop1 0" | dmsetup create roimage-base-real
> echo "0 <cowsize> linear /dev/loop2 0" | dmsetup create roimage-snap-cow
> echo "0 <realsize> snapshot-origin /dev/mapper/roimage-base-real" |
> dmsetup create roimage-base
> echo "0 <realsize> snapshot /dev/mapper/roimage-base-real
> /dev/mapper/roimage-snap-cow n 1" | dmsetup create roimage-cow
> 
> mount /dev/mapper/roimage-cow /roimagemntpnt
> 
> this is working as the origin device is Read-Only, but we can change files.
> 
> so two questions come up
> 
> 1 when we delete files, how do we reclaim the space
> 2 should/can we roll the snapshots to reclaim space?

Please consider using  thin-pools - this should be giving you the features you 
are likely looking for.

Note - you can use thin-pool to  hold only snapshots  and use your read-only 
volume as the external origin for thin volume.

Regards

Zdenek