ZFS clean empty snapshots

#!/bin/bash

fsWithSnapshots=$(zfs list -Hr -t snapshot tank/share |grep '@' |cut -d '@' -f 1 |uniq)

for fs in $fsWithSnapshots ; do
  # Changed to now sort newest to oldest.  This will mean that newer snapshots without deltas will get removed.
  emptySnapshot=$(zfs list -Hr -d1 -t snapshot -o name,used -S creation $fs |sed '$d' |awk ' $2 == "0B" { print $1 }' )
  for snapshot in $emptySnapshot ; do

    # Added safety check.  Verify the size of the snapshot prior to destroying it
    used=$(zfs list -Hr -d1 -t snapshot -o used $snapshot)
    if [[ $used != "0B" ]]; then
      continue
    fi

    echo "Destroying empty snapshot $snapshot"
    zfs destroy $snapshot
  done
done

Thanks - very useful!
Changed the 'tank/share' to my vdev/share and worked!

FYI this script is broken, and will delete non-empty snapshots. See the explanation here: https://www.mail-archive.com/zfs-discuss@opensolaris.org/msg17752.html

This works fine and carefully deletes zero sized snaps one by one

https://github.com/fracai/zfs-rollup/blob/master/clearempty.py

zfs list -t snapshot | awk '$2 == 0 {print $1}' | xargs -r -n 1 /sbin/zfs destroy

change line 7 to

emptySnapshot=$(zfs list -Hr -d1 -t snapshot -o name,used -s creation $fs | sed '$d' | awk ' $2 == "0B" { print $1 }' )

and it works again 👍

FYI this script is broken, and will delete non-empty snapshots. See the explanation here: https://www.mail-archive.com/zfs-discuss@opensolaris.org/msg17752.html

^ This is correct. The script will delete snapshots that aren't actually zero bytes.

Here's a revised version -- it should avoid deleting snapshots that actually have data, and should preserve the first snapshot in which the change appeared:

#!/bin/bash

fsWithSnapshots=$(zfs list -Hr -t snapshot tank/share |grep '@' |cut -d '@' -f 1 |uniq)

for fs in $fsWithSnapshots ; do
  # Changed to now sort newest to oldest.  This will mean that newer snapshots without deltas will get removed.
  emptySnapshot=$(zfs list -Hr -d1 -t snapshot -o name,used -S creation $fs |sed '$d' |awk ' $2 == "0B" { print $1 }' )
  for snapshot in $emptySnapshot ; do

    # Added safety check.  Verify the size of the snapshot prior to destroying it
    used=$(zfs list -Hr -d1 -t snapshot -o used $snapshot)
    if [[ $used != "0B" ]]; then
      continue
    fi

    echo "Destroying empty snapshot $snapshot"
    zfs destroy $snapshot
  done
done

Thanks! I have updated the gist (especially now that it attracts so many people!) as well as my own scripts.

is still safe this script?

Hi
The reason this script (and surprisingly, all other scripts which fulfil the same task) has to iterate over the snapshots in reverse order and recheck the size is, that you are using the "used" property instead of the "written" property. The latter is exactly then zero when the snapshot is empty. And in case you wonder, it's from 2011: https://www.illumos.org/issues/1645

zfs create zroot/snaptest
zfs set mountpoint=/snaptest zroot/snaptest
cd /snaptest/

dd if=/dev/random bs=1M count=1 of=a
zfs snap zroot/snaptest@1

dd if=/dev/random bs=1M count=1 of=b
zfs snap zroot/snaptest@2

zfs snap zroot/snaptest@3

dd if=/dev/random bs=1M count=1 of=c
zfs snap zroot/snaptest@4

zfs snap zroot/snaptest@5

dd if=/dev/random bs=1M count=1 of=d
rm a
zfs snap zroot/snaptest@6

rm b
rm c
zfs snap zroot/snaptest@7

zfs snap zroot/snaptest@8

rm d
zfs snap zroot/snaptest@9

zfs list -o name,used,written zroot/snaptest -r -tall

NAME USED WRITTEN
zroot/snaptest 4.44M 0
zroot/snaptest@1 64K 1.10M
zroot/snaptest@2 0B 1.07M
zroot/snaptest@3 0B 0
zroot/snaptest@4 0B 1.07M
zroot/snaptest@5 0B 0
zroot/snaptest@6 64K 1.07M
zroot/snaptest@7 0B 64K
zroot/snaptest@8 0B 0
zroot/snaptest@9 0B 64K

https://gist.github.com/SkyWriter/58e36bfaa9eea1d36460?permalink_comment_id=3552336#gistcomment-3552336

Here's a revised version -- it should avoid deleting snapshots that actually have data, and should preserve the first snapshot in which the change appeared:

jimsalterjrs/sanoid#616 (comment)
https://www.reddit.com/r/zfs/comments/ja1k7n/sanoid_dont_create_empty_0b_snapshots/