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 (
	"bufio"
	"fmt"
	"os"
	"regexp"
)

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() {
	//fmt.Println(snapshotLineRegex.MatchString("dpool/www@zfs-auto-snap_frequent-2020-08-04-1830\t0B\t-\t201M\t-"))
	input := bufio.NewScanner(os.Stdin)
	for input.Scan() {
		if snapshotLineRegex.MatchString(input.Text()) {
			fmt.Println(snapshotLineRegex.FindStringSubmatch(input.Text()))
			fmt.Println(snapshotLineRegex.SubexpNames())
		} 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!

ZFS Backup Tool Part 1

I haven’t seen a lot of tools that are designed to backup ZFS snapshots to removable media. So, I am writing my own. I am going to document the process here.

The basic loop for a backup tool is

  1. Read a list of snapshots on the source
  2. Read a list of snapshots on the destination
  3. Find the list of snapshots on the source that are not on the destination. These are the non backed up snapshots.
  4. Find the list of snapshots on the destination that are not on the source. These are the aged out snapshots.
  5. Copy all non backed up snapshots to the destination, preferably one at a time to make recovery from IO failure easier.
  6. Remove the aged out snapshots.

I am designing this tool to only backup snapshots taken by zfs-auto-snapshot. These are named <pool|filesystem>@zfs-auto-snap_<time interval>-<year>-<month>-<day>-<hour><minute>. The command zfs -Hrt snapshot <source poolname> will generate a list of all snapshots in a pool in a machine parseable format.

Issuing the command zfs send -ci <old snapshot> <pool|filesystem>@<new snapshot> will send an incremental snapshot from old to new to the commands standard output. I can estimate the amount of data to be transferred by replacing -ci with -cpvni in the zfs send command.

Issuing the command zfs receive -u <backup location> will store a snapshot from its standard input to the backup location.

Snapshots are removed by zfs destroy -d <pool|filesystem>@<snapshot name>. The snapshot name is the portion of the snapshot pattern mentioned above after the @ symbol.