RFC: Automated ZFS snapshots using Hanoi algorithm
See attached patch, the comment header explains everything. I'm using
this myself and find it very useful. I'd like to add this to zfsutils
(but NOT enable it by default, of course).
Can you comment / test / improve / give feedback in general?
# 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.
--
Robert Millan
Index: debian/zfsutils.cron.daily
===================================================================
--- debian/zfsutils.cron.daily (revision 0)
+++ debian/zfsutils.cron.daily (revision 0)
@@ -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
Index: debian/zfsutils.install
===================================================================
--- debian/zfsutils.install (revision 3922)
+++ debian/zfsutils.install (working copy)
@@ -4,3 +4,4 @@
cddl/usr.bin/ztest/ztest /usr/bin
cddl/usr.sbin/zdb/zdb /usr/sbin
debian/local/bash_completion.d/zfsutils /etc/bash_completion.d
+debian/local/default/zfs /etc/default
Index: debian/local/default/zfs
===================================================================
--- debian/local/default/zfs (revision 0)
+++ debian/local/default/zfs (revision 0)
@@ -0,0 +1,9 @@
+# Space-separated list of filesystems you want automated snapshots for.
+#AUTOSNAP_FILESYSTEMS=""
+
+# Maximum number of automated snapshots that may be preserved at any
+# given time. This variable is optional; if it isn't set, snapshots
+# will increase without limit. However, 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).
+#AUTOSNAP_MAX_SNAPSHOTS="2"
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