Add script to report the space that would be reclaimed by deleting older snapshots

ROOT/DEBIAN/control

@@ -3,6 +3,6 @@ Priority: optional
 Section: otherosfs
 Maintainer: Chemistry COs <support@ch.cam.ac.uk>
 Architecture: all
-Version: 0.9-ch95
+Version: 0.9-ch96
 Depends: zfs-dkms, postgresql-13 | postgresql-9.5 | postgresql-9.4 , liblockfile-simple-perl, libdbi-perl, libjson-perl, libzfs-perl-chem, libnet-openssh-perl, libdbd-pg-perl, mbuffer, rsync, nfs-kernel-server, pv, libwww-curl-perl
 Description: a backup system using ZFS (repository 'backup-scheduler')

ROOT/usr/local/bin/analyse-snapshot-usage

+#!/bin/bash
+
+# This script runs "zfs destroy" commands. We explicitly specify "-nv" options,
+# and I can't currently think of a way it would go wrong, but there is no reason
+# for this script to be run as root and I want to avoid accidents.
+if [ $EUID -eq 0 ]; then
+   echo "You appear to be running this script as root. Please run it as unprivileged user instead."
+   exit 1
+fi
+
+if [ $# -ne 1 ] ; then
+  echo usage: $0 zfsname
+  echo
+  echo Prints out a report of the space that would be reclaimed by successfully
+  echo deleting snapshots of a ZFS, from the oldest through to the newest.
+fi
+
+ZFS=$1
+
+if ! zfs list $ZFS >/dev/null 2>&1 ; then
+ echo could not list $ZFS
+ exit 1
+fi
+
+OLDEST=$(zfs list -H -t snapshot -oname $ZFS | head -n 1 | cut -d '@' -f 2)
+
+ALLSNAPS=$(zfs list -H -t snapshot -oname $ZFS | cut -d '@' -f 2)
+
+# NB the following two commands give the same output:
+# zfs destroy -nv $ZFS@1654118566
+# zfs destroy -nv $ZFS@1654118566%1654118566 # i.e. specifying the same snapshot as the start and end of the range
+# so we don't need to special-case how we handle ${OLDEST}
+for SNAP in $ALLSNAPS; do 
+  RECLAIM=$(zfs destroy -nv ${ZFS}@${OLDEST}%${SNAP} | grep "would reclaim" | awk '{print $3}')
+  echo "$OLDEST -> $SNAP : $RECLAIM"
+done