rhkra...@gmail.com wrote: > I'm not really clear on the concept of a snapshot (for backup) -- I've done a > little googling but haven't found an explanation that "satisfies" me. > > Starting from a beginning, I suppose I could copy the entire contents of > whatever I wanted to make a snapshot of (by any of a variety of tools -- dd, > cp, ...) and call that a snapshot, although the more common name for it would > be a "full backup".
Let's look at the larger circumstances. In ordinary usage, there are tens to thousands of processes runnning on your system. Some of them are emitting logs or writing files. Taking a backup takes some time. During that time, some files get written, some get opened, and some are related to each other (by the processes) in ways which are inconsistent until all of them are written. A snapshot differs from a backup in two important regards: - first, it requires the filesystem to bring writes to a halt. There is now a consistent view. - second, it doesn't actually copy things. It just records their state and, when done, allows future writes to continue -- writes which are not part of this snapshot. As a result, you can take a snapshot and then: - discard it (trivial) - look through it and copy off any file or group of files, thus getting what they contained at the time of the snapshot, not the what they contain now (excellent for recovering from an accidental delete) - copy all of it off elsewhere, producing a consistent full backup. Does that help? -dsr-
