ZFS Backup Tool Part 2

Recognizing a snapshot made by zfs-auto-snapshot.

First, what does a list of these snapshots look like?

robert@mars:~/src/go/zfs_backup$ zfs list -Hrt snapshot dpool
dpool@zfs-auto-snap_monthly-2020-05-12-1245     96K     -       148K    -
dpool@zfs-auto-snap_monthly-2020-06-11-1248     8K      -       23.3G   -
dpool@zfs-auto-snap_monthly-2020-07-11-1245     0B      -       23.3G   -
dpool@zfs-auto-snap_weekly-2020-07-26-1242      0B      -       30.5G   -
dpool@zfs-auto-snap_daily-2020-07-27-1238       4.74G   -       31.3G   -
dpool@zfs-auto-snap_daily-2020-08-02-1235       0B      -       143G    -
dpool@zfs-auto-snap_weekly-2020-08-02-1240      0B      -       143G    -
dpool@zfs-auto-snap_hourly-2020-08-03-1117      0B      -       143G    -
dpool@zfs-auto-snap_daily-2020-08-03-1236       0B      -       143G    -
dpool@zfs-auto-snap_frequent-2020-08-04-2030    0B      -       143G    -
dpool@zfs-auto-snap_frequent-2020-08-04-2045    0B      -       143G    -
dpool@zfs-auto-snap_frequent-2020-08-04-2100    0B      -       143G    -
dpool@zfs-auto-snap_frequent-2020-08-04-2115    0B      -       143G    -
dpool@zfs-auto-snap_hourly-2020-08-04-2117      0B      -       143G    -
dpool/home@zfs-auto-snap_hourly-2020-08-04-1717 0B      -       96K     -
dpool/home@zfs-auto-snap_hourly-2020-08-04-1817 0B      -       96K     -
dpool/home@zfs-auto-snap_hourly-2020-08-04-1917 0B      -       96K     -
dpool/home@zfs-auto-snap_hourly-2020-08-04-2017 0B      -       96K     -
dpool/home@zfs-auto-snap_frequent-2020-08-04-2030       0B      -       96K     -
dpool/home@zfs-auto-snap_frequent-2020-08-04-2045       0B      -       96K     -
dpool/home@zfs-auto-snap_frequent-2020-08-04-2100       0B      -       96K     -
dpool/home@zfs-auto-snap_frequent-2020-08-04-2115       0B      -       96K     -
dpool/home@zfs-auto-snap_hourly-2020-08-04-2117 0B      -       96K     -
dpool/home/robert@zfs-auto-snap_frequent-2020-08-04-2115        0B      -       69.1G   -
dpool/home/robert@zfs-auto-snap_hourly-2020-08-04-2117  0B      -       69.1G   -
dpool/plex@snap1        116G    -       442G    -
dpool/plex@zfs-auto-snap_monthly-2020-05-12-1245        8K      -       344G    -

I trimmed the previous list down a bit. So what is a regular expression that will match this? The first question is which regular expression library am I using? I am writing this tool in Go. Thus I will use the regexp Go package. Go’s regexp package is based on Google’s RE2 library. The syntax for it is here.

I will start with the snapshot names. The part after the @. Those start with zfs-auto-snap so "zfs-auto-snap" will match it.

The next section is which timer made the snapshot. This section can also be called the increment. The valid timers are yearly, monthly, weekly, daily, hourly, and frequent for a default install of zfs-auto-snapshot. The regex "yearly|monthly|weekly|daily|hourly|frequent" will match these timers. However, I would like to get which timer created the snapshot without further parsing. That is the perfect job for a capturing sub match. After adding the capturing sub match, the regex looks like "(?P<increment>yearly|monthly|weekly|daily|hourly|frequent)".

The final section is the timestamp of the snapshot. Like with the timer section, it is useful not to have to parse this data a second time. With the sub matches "(?P<year>[[:digit:]]{4})-(?P<month>[[:digit:]]{2})-(?P<day>[[:digit:]]{2})-(?P<hour>[[:digit:]]{2})(?P<minute>[[:digit:]]{2})" will work.

With the snapshot names completed, I need to capture the zfs tree structure before the @ symbol. I haven’t found a reliable regular expression that will capture that tree but "(?:[[:word:]-.]+)+(?:/?[[:word:]-.]+)*" will recognize a subset of all valid zfs trees. Avoid using anything it won’t recognize, or you may end up with inaccessible files.

Including some test code the tool’s source code looks like this so far.

package main

import (

const zfsRegexStart string = "zfs-auto-snap"
const zfsRegexIncrement string = "(?P<increment>yearly|monthly|weekly|daily|hourly|frequent)"
const zfsRegexDateStamp string = "(?P<year>[[:digit:]]{4})-(?P<month>[[:digit:]]{2})-(?P<day>[[:digit:]]{2})-(?P<hour>[[:digit:]]{2})(?P<minute>[[:digit:]]{2})"

var zfsRegex = regexp.MustCompile(zfsRegexStart + "_" + zfsRegexIncrement + "-" + zfsRegexDateStamp)

func testSnapshot(possible string, increment string) (bool, bool) {
	var matches = zfsRegex.FindStringSubmatch(possible)
	if matches == nil {
		return false, false
	var isASnapshot = true
	if matches[1] == increment {
		return isASnapshot, true
	return isASnapshot, false

func isAYearlySnapshot(possible string) bool {
	_, isYearly := testSnapshot(possible, "yearly")
	return isYearly

func isAMonthlySnapshot(possible string) bool {
	_, isMonthly := testSnapshot(possible, "monthly")
	return isMonthly

func isAWeeklySnapshot(possible string) bool {
	_, isWeekly := testSnapshot(possible, "weekly")
	return isWeekly

func isADailySnapshot(possible string) bool {
	_, isDaily := testSnapshot(possible, "daily")
	return isDaily

func isAnHourlySnapshot(possible string) bool {
	_, isHourly := testSnapshot(possible, "hourly")
	return isHourly

func isAFrequentSnapshot(possible string) bool {
	_, isFrequent := testSnapshot(possible, "frequent")
	return isFrequent

const poolNameRegex string = "(?:[[:word:]-.]+)+(?:/?[[:word:]-.]+)*"

var snapshotLineRegex = regexp.MustCompile("^" + poolNameRegex + "@" + zfsRegex.String() + ".*$")

func main() {
	input := bufio.NewScanner(os.Stdin)
	for input.Scan() {
		if snapshotLineRegex.MatchString(input.Text()) {
		} else {
			fmt.Printf("%s\t%s\n", input.Text(), "Is not a snapshot.")
	if err := input.Err(); err != nil {
		fmt.Fprintln(os.Stderr, "reading Standard Input:", err)

I will continue this tomorrow. See you then!