We can use du or ncdu to find directories or files taking up the space:

# Search all directories under the root filesystems
du -sh /*

# When we find a large directory, run du on it, e.g.:
du -sh /var/*

ncdu features a simple text interface and can also delete the files itself.

Should du or ncdu take too long to run (tens of seconds or minutes), we're most likely dealing with too many files. In this case, search the directories on which du gets stuck by hand, one by one.

If you're deleting data from NAS, but the free space is not increasing, it's most likely held by a snapshot. Data taken by snapshots is reducing NAS disk size. It's necessary to delete all snapshots to free the space. You can do it in vpsAdmin, menu Backups → NAS.

If you can't find what's taking the space and you have mounted NAS, it's possible that the data is hidden beneath the NAS mountpoint. Data could be written to the directory when NAS wasn't mounted and ended up on the VPS's disk. Later, when NAS gets mounted, the data on the VPS gets overlapped.

You can check this either by temporarily unmounting NAS and then looking at the mountpoint:

umount /mnt/nas
du -sh /mnt/nas
df -h /mnt/nas

When df shows the filesystem as tank/ct/<id vps>, the data is on the VPS's disk.

If you do not wish to umount NAS, e.g. because it's being used, there is another way. We can make a bind-mount of the root filesystem, in which NAS will not be mounted:

mkdir tmp-rootfs
mount --bind / tmp-rootfs

du -sh tmp-rootfs/mnt/nas
df -h tmp-rootfs/mnt/nas

df should show the filesystem as tank/ct/<id vps>. Clean up the data in tmp-rootfs/mnt/nas and remove the bind-mount:

umount tmp-rootfs
rmdir tmp-rootfs