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diff --git a/contrib/hanoi.cron.daily b/contrib/hanoi.cron.daily
new file mode 100644
index 0000000..a92b021
--- /dev/null
+++ b/contrib/hanoi.cron.daily
@@ -0,0 +1,68 @@
+#!/bin/bash
+
+# This algorithm implements a variation of the Towers of Hanoi rotation method
+# (see http://en.wikipedia.org/wiki/Backup_rotation_scheme#Towers_of_Hanoi).
+#
+# Unlike traditional ToH rotation, which uses a finite set of physical tapes,
+# we operate on a set of snapshots whose size doesn't necessarily have to be
+# bounded. Note that the number of snapshots only grows logarithmically with
+# time, which makes it very hard to fill your hard disk (even when running
+# unbounded).
+#
+# The result is that once we've run this for long enough, we'll find that for
+# recent dates (e.g. last few days) almost all snapshots are available, and the
+# older the date we're searching the more spread available snapshots will be.
+
+set -e
+
+. /etc/default/zfs
+
+# Does $1 belong to class $2 ?
+isclass ()
+{
+ if [ "$2" == "$AUTOSNAP_MAX_SNAPSHOTS" ] ; then
+ # Special-case. Treat all snapshots as if they belong to
+ # this class (in addition to their real class).
+ return 0
+ fi
+ local remainder=$((2 ** ($2 - 1)))
+ local divisor=$((${remainder} * 2))
+ [ $(($1 % ${divisor})) == ${remainder} ]
+}
+
+classify ()
+{
+ local creation="$1"
+ local creation_in_days="$(($(LANG=C date +%s -d "@${creation}") / 86400))"
+ local class="1"
+ # Find the class ${creation_in_days} belongs to.
+ while ! isclass "${creation_in_days}" "${class}" ; do
+ class=$((${class}+1))
+ done
+ echo "${class}"
+}
+
+for fs in $AUTOSNAP_FILESYSTEMS ; do
+ # Create today's snapshot.
+ echo zfs snapshot ${fs}@autosnap-$(date +%F)
+ zfs snapshot ${fs}@autosnap-$(date +%F)
+
+ # Remove any snapshots in the same class as today's addition.
+ LANG=C zfs list -r ${fs} -t snapshot -H -o name,creation \
+ | grep @autosnap- \
+ | while read name creation_human ; do
+ creation="$(LANG=C date +%s -d "${creation_human}")"
+ echo "$(classify "${creation}") ${creation} ${name}"
+ done \
+ | sort -nr \
+ | while read class creation name ; do
+ echo ${class} ${name}
+ if [ ${class} == "${previous_class}" ] ; then
+ echo zfs destroy "${name}"
+ zfs destroy "${name}"
+ fi
+ previous_class="${class}"
+ done
+done
+
+exit 0
--
1.7.10.4
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