Viewing your ZFS snapshots as if they were directories
ZFS is a pretty sweet filesystem, an opinion shared by the majority of the internet it seems. BTRFS (No Wikipedia, it’s not “butter fuss”) is also a nice filesystem, and the fact it’s built-in to the kernel makes it far safer to use as a root filesystem.
One of my favourite features of BTRFS, even if it does also annoy me, is browsable snapshots. Snapshots are simply as read-only directories, and can be mounted wherever.
btrfs subvolume snapshot -r /mnt/tank/dbs/postgres/nextcloud /mnt/tank/dbs/postgres/nextcloud.$(date +%Y-%m-%d)
Whilst this makes accessing data really simple, as snapshots can be enumerated simply with ls, it also really clutters up directories and looks messy to me.
In contrast, ZFS snapshots are listed through zfs list -t snapshot command.
$ zfs list -t snapshot
...
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_00:00:02_daily 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_00:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_01:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_02:00:01_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_03:00:01_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_04:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_05:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_06:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_07:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_08:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_09:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_10:00:02_hourly 0B - 260M -
tank/dbs/postgres/nextcloud@autosnap_2020-08-30_11:00:02_hourly 0B - 260M -
Wouldn’t it be great if you could browse ZFS snapshots like they were directories, as you can with BTRFS?
…
Well, you can!
Inside your ZFS datasets exists an invisible .zfs directory. This directory isn’t “hidden”, it’s invisible, it won’t appear to ls -a, but you can cd into it (it won’t tab-complete).
/mnt/tank/dbs/postgres/nextcloud/.zfs$ ls
shares snapshot
Here we can see the shares and snapshots set up for the dataset. If we take a look inside the snapshot/ directory:
/mnt/tank/dbs/postgres/nextcloud/.zfs/snapshot$ ls -al
...
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_00:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_01:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_02:00:01_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_03:00:01_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_04:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_05:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_06:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_07:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_08:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_09:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_10:00:02_hourly
drwxrwxrwx 1 root root 0 Aug 30 12:01 autosnap_2020-08-30_11:00:02_hourly
Hey look, all our snapshots! And of course, each of our snapshots is a directory, so can be browsed easily:
/mnt/tank/dbs/postgres/nextcloud/.zfs/snapshot/autosnap_2020-08-30_00:00:02_hourly$ ls
base pg_commit_ts pg_hba.conf pg_logical pg_notify pg_serial pg_stat pg_subtrans pg_twophase pg_wal postgresql.auto.conf postmaster.opts
global pg_dynshmem pg_ident.conf pg_multixact pg_replslot pg_snapshots pg_stat_tmp pg_tblspc PG_VERSION pg_xact postgresql.conf postmaster.pid
These snapshots expose all the same file permissions and layouts as ZFS snapshots, because that’s what they are. You can copy files out of them, read files and all! For obvious reasons you can’t edit the files, but it definitely makes reviewing snapshots so much easier!
You can read more about this in the FreeBSD handbook (section 19.4.5.4).
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