Snapshots

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Snapshots are a implementation of copy-on-write that allows for a file system to quickly take a point-in-time copy of the file system and provide access to the data through a .snapshot directory. Snapshots provide a fast, user-accessible way to recover data that has been accidentally deleted or corrupted within a recent time window -- rather than having to retrieve the data from comparatively slow tape backups. They also help to span the time gap between full backups.

We provide Snapshots on our ZFS and FluidFS filers to certain file systems. If you are ever unsure if a particular volume has Snapshots enabled, please contact the Help Desk.

Snapshot Retention Policy

Our core file systems in the department are on a 4 hour snapshot cycle.

Snapshot Name Retention Length When is it taken?
Hourly 32 hours Every day 12am, 4am, 8am, 12pm, 4pm
Daily 2 days Every day 8pm
Weekly 1 week Every Saturday 8pm

In other words, we retain up to 8 hourly snapshots, 2 daily snapshots and 1 weekly snapshot. Hourly snapshots may be superseded by daily snapshots, and daily snapshots may be superseded by the weekly snapshot.

Snapshot Restoring

If you have deleted a file by mistake and you need to get it back, you can use the snapshots directory to recopy the file.

This directory can typically be found in your home directory. It generally will not be visible, even when viewing hidden directories.

It will be either:

  • .snapshots for the FluidFS filer
  • .zfs/snapshot for the ZFS filer

The inside of one of these will look something like this on a FluidFS filesystem:

sattwood@zaphod:~$ pwd
/nfshomes/sattwood
sattwood@zaphod:~$ cd .snapshots
sattwood@zaphod:~/.snapshots$ ls
daily_2018_06_14__20_00   hourly_2018_06_14__12_00  hourly_2018_06_15__04_00  weekly_2018_06_09__20_00
daily_2018_06_15__20_00   hourly_2018_06_14__16_00  hourly_2018_06_15__08_00  
hourly_2018_06_14__08_00  hourly_2018_06_14__00_00  hourly_2018_06_15__12_00  

Or this, on a ZFS filesystem:

sattwood@swirl:~$ pwd
/nmhomes/sattwood
sattwood@swirl:~$ cd .zfs/snapshot
sattwood@swirl:~/.zfs/snapshot$ ls
zfs-auto-snap_daily-2018-06-13-01h00   zfs-auto-snap_hourly-2018-05-28-00h00
zfs-auto-snap_daily-2018-06-14-01h00   zfs-auto-snap_hourly-2018-05-29-00h00
zfs-auto-snap_daily-2018-06-15-01h00   zfs-auto-snap_hourly-2018-06-03-16h00
zfs-auto-snap_hourly-2018-05-24-00h00  zfs-auto-snap_hourly-2018-06-10-08h00
zfs-auto-snap_hourly-2018-05-25-00h00  zfs-auto-snap_hourly-2018-06-15-12h00
zfs-auto-snap_hourly-2018-05-26-00h00  zfs-auto-snap_weekly-2018-06-09-03h00

For an example of file restoration, please see this page.