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Snapshots

Introduction

The CS Department uses a Network Appliance FAS940 file server ("The NetApp") to provide network file service to both Unix and Windows clients. This server has a feature known as snapshots which allows for online file recovery of recently changed or deleted files. This document will show you how this mechanism works and how to use it.

How does it work?

The NetApp uses a custom filesystem which provides for the creation of snapshots at specified intervals. For more technical information on their implementation, you can read the technical report.

A snapshot is a read-only copy of all the files and directories in the NetApp filesystem. The NetApp creates a snapshot at regular intervals and rotates them on a schedule. Snapshots can be accessed as quickly and easily as the live filesystem.

When a snapshot is created, it does not occupy any additional disk space. It consists of the same disk blocks in use by the live filesystem. The snapshot and the live filesystem share these blocks, none of which may be modified until the snapshot is deleted. Subsequent changes to the live filesystem are accomplished using unallocated disk blocks that are not part of the snapshot, so the snapshot remains frozen.

From a user's point of view, at regular intervals, the NetApp "takes a snapshot" of the current filesystem. This snapshot looks like a read-only copy of the filesystem at that point. There is a .snapshot directory in every directory, including each user home directory.

The snapshot mechanism is independent of our normal tape backup mechanism.

Using Snapshots

How you use the snapshots depends on whether you're using Windows or Unix.

Other Useful Snapshot Information

Here are a few other useful pieces of information about snapshots:

• Files found in .snapshot do not count against your quota.

• Unless you specify otherwise, most programs which crawl the filesystem will report the files in the .snapshot directory. We have modified /usr/local/bin/find and /usr/local/bin/du to ignore these directories (use /usr/bin/find and /usr/bin/du to get the original behavior), but all other programs like this will require some attention.

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Page Owner: Phirum Peang

Last Modified: Thu Sep 2 09:12:53 2004