A photograph is something that stop the time. Whatever else you do, or how you change, after taking the photo isn't important, because you can always know how the things where at the time of the shot.

A snapshot in our case is the same thing, but related to file and directories. If we take a snapshot of a particular directory or file, we can always access this file as it was, no matter how much we changed the file or if the file was completely deleted. We can access this type of information at least until we delete the snapshot we took.

The filer that serve the IPG user create these snapshots automatically and in a trasparent fashion, the user's don't have to manage any configuration to activate this feature. The system generate a snapshot of the user's directory every hour and save it in a separate space (not really, but just to maintain the things simple). The users can access these data just browsing the homedir. No need to use some strange and unfriendly software. Maintaining a copy of every modify of a file is impossible, so the snapshots are created at regular intervals every hour and then every day and then every week. With this mechanism the user can access his/hers old file inside an hidden directory named .snapshot resident in the homedir. Inside this directory exists others directories that have name

• hourly.[0-3]
• nightly.[0-3]
• weekly.[0-1]

As the name suggest, these directory contain the files changed during the lasts 4 hours, 4 days and 2 weeks. If the wanted file was deleted/changed during this time, the user have to search these directories, in order to find it. Pay attention that inside the .snapshot directories a user can see the state of his/hers directory how it was, so if you look inside one of these directory you can see not only the files changed, but all the files you have currently. Amazing, isn'it? If the file is found, the users can copy it where it want and the restore is done.

Every user of the LTHC, LTHI and LCM labs can find inside his/hers homedir on the central filer an hidden directory named .snapshot. Inside this directory exists others directories:

ls .snapshot/
hourly.0
hourly.1
hourly.2
hourly.3
nightly.0
nightly.1
nightly.2
nightly.3
weekly.0
weekly.1

As the name suggest, every directory of the list contain the files changed during the lasts 4 hours, 4 days and 2 weeks (the 0 is something). If the wanted file was deleted/changed during this time, the user have to look inside these directories, in order to find it. If the file is found, the users can copy it where it want and the restore is done.

Don't try to use the files directly in the .snapshot dir.

<note warning> Pay attention that inside the .snapshot directories you can see the state of his/hers directory how it was at the time of snaphot, not only the files deleted </note>

In this case, we use a standard linux file server (which costs 10 times less then the NetApp) and an home-brewed snapshot solution developed using standard Unix hard-links and the rsync utility. For the moment we take hourly snapshots and we keep 10 snapshots distributed more or less logarithmically in time so that we have many recent snapshots and few old ones. The distribution over time is not regular because only meaningful snapshots (when something actually changed) are kept. Snapshots are sometimes only taken during the day, and from Monday to Friday. <note warning> Please consider this as an highly experimental feature for the moment. It will often be down for maintenance, bug-fixes, enhancement and various other reasons. </note>

Snapshots are presently only accessible from linux workstations using the automount directory /snap/USERNAME. Within this directory, the various snapshots are named after the date and time when they were created. For example:

[cangiani@licospc11 ~]$ ls -1 /snap/cangiani
2007-09-20_Thu_14h     ~ 1 hour ago
2007-09-20_Thu_13h     ~ 2 hours ago
2007-09-20_Thu_12h     ~ 3 hours ago
2007-09-20_Thu_08h     ~ 9 hours ago
2007-09-19_Wed_12h     ~ 1 day ago
2007-09-17_Mon_09h     ~ 3 days ago
2007-09-10_Mon_12h     ~ 10 days ago
2007-08-21_Tue_ 6h     ~ 1 month ago
2007-06-21_Thu_12h     ~ 3 months ago

Snapshot directories are mounted read-only so do not try to edit files therein. Note also that the snapshot service is not available to all users. In general it is only available to long term members of our labs (Professors, post-docs, and phD students).