A filesystem technique that allows leisurely backups while continuing to use the system. This magic is accomplished by making a special copy of a directory or filesystem (the exact mechanics will vary with the product you are using) that doesn't really copy anything unless and until data changes at the source. Here's how it works: Let's say that you have created a snapshot of some directory in /snap. If you read files in /snap, they look just like the files they were created from. Actually, you are still reading from the original data blocks at this point.
However, if something is changed in your original source, the filesystem copies data to the /snap are before overwriting the new data. This keeps the things in /snap frozen in time at the moment you created them. Another advantage is that if there haven't been a lot of changes, /snap doesn't have to use much disk space: it is reading from the original data blocks until they change.
Some people have misunderstood this and have used mutliple snapshots as backups. You CAN do that, of course, but the general reason for snapshots is so that you can back up the snap to tape and then remove it.
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