Snapshots

On all the filesystems hosted on nas1.compute.dtu.dk and comp-nas3.compute.dtu.dk snapshots are made: A snapshot is a read-only copy of a filesystem at a specific time. Snapshots do not use one’s quota.

Snapshots are taken with different frequencies and are accessed differently depending on the server.

nas1.compute.dtu.dk

Frequency

Hourly: last 16 snapshots are kept (usually taken on min. 05)
Daily: last 6 are kept (usually taken just after midnight)
Weekly: last 2 are kept (usually taken in the night between Saturday and Sunday)

How to use them

From *nix

In the directory where the old file was, change to the folder .snapshot. In this directory there are a number of directories and the directory name indicates when the snapshot was taken.

drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-09-28_0010
drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-09-29_0010
drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-09-30_0010
drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-10-01_0010
drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-10-02_0010
drwx------ 23 root root 4096 Aug 18 08:55 daily.2016-10-03_0010
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0005
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0105
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0205
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0305
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0405
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0505
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0605
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0705
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0805
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_0905
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1005
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1105
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1205
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1305
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1405
drwx------ 23 root root 4096 Aug 18 08:55 hourly.2016-10-03_1505
drwx------ 23 root root 4096 Aug 18 08:55 weekly.2016-10-02_0015
drwx------ 23 root root 4096 Aug 18 08:55 weekly.2016-09-05_0015

Note

The folder .snapshot is not visible, i.e. it will not show in a ls -la listing.
If the folder .snapshot does not exist, it is because the parent folder was created after the latest snapshot was taken. Also, if only some of the “hourly” and “daily” folders exist, it is because the parent folder was not created at the time of the missing snapshot.

Tip

The command find will per default search the .snapshot directory when the command find . -name some-name is issued. Use the command find * -name some-name instead. This can speed up the process with a factor 10 or better.

From Windows

Either like in *nix change to a hidden folder ~snapshot and browse to the file of interest, or find the file/folder of interest, take properties of it and look at Previous Versions tab.

comp-nas3.compute.dtu.dk

Frequency

Nightly: 23:45
Each snapshot is suffixed with a number corresponding to day of the week (Monday=1, Tuesday=2, ….) and is kept for one week.

How to use them

From *nix

In the directory where the old file was, change to the folder .zfs/snapshot. In this directory there are 9 directories:

drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local1
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local2
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local3
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local4
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local5
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local6
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 local7
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 copied-280608-185052
drwxr-xr-x 38 hench unixusers 50 Jun 28 08:44 copied-280609-065138

The snapshot named local1 is the snapshot taken Monday night, local2 is Tuesday night, etc.

Note

The folder .zfs is not visible, i.e. it will not show in a ls -la listing.
If the folder .zfs/snapshot does not exist, it is because the parent folder was created after the latest snapshot was taken. Also, if only some of the snapshot folders exist, it is because the parent folder was not created at the time of the missing snapshot

Tip

The command find will per default search the .zfs directory when the command find . -name some-name is issued. Use the command find * -name some-name instead. This can speed up the process with a factor 10 or better.

From Windows

Accessing snapshots is currently not supported.

Last updated on 7. November, 2026

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