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.
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||24-32 hours||Every day 12am, 4am, 8am, 12pm, 4pm|
|Daily||2 days||Every day 8pm or 12am|
|Weekly||1 week||Every Saturday 8pm or Sunday 12am|
In other words, we retain up to either 6 or 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 Hourly snapshots may be superseded by daily snapshots, and daily snapshots may be superseded by the weekly snapshot.
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.