Snapshot Backup vs Mirror Backup

The biggest feature of snap2 is the snapshot type backup.  A backup snapshot captures the state of all files at a given point in time. It is essentially a full backup, but due to the use of Linux hard links, only new and modified files actually take up space on disk as new snapshot backups are made.

You could for example have a "snapshot" of your home directory as it was yesterday, and for every day of the week, and for every week for several weeks.  You could then easily recover previous versions of files from any of those "snapshots."

Suppose you have 5 gigs in your home directory.  Making a "ordinary" daily full backup of 5 gigs would would be very slow and would probably require an impossibly large amount of bandwidth and hard drive space.  However, this could be done with snap2 because in any given "snapshot" backup, only new and modified files are really copied.  All the other unchanged files (most files are usually unchanged from the previous backup) are just hard-linked to a single copy on the backup disk.  This makes snapshot backups extremely fast and efficient in disk use and bandwidth.

In sum, you would use snapshot backups when you want access to previous versions of files, even files you have deleted from your hard drive.

mirror-type backups, on the other hand, are used when all you need is a single backup copy.  This is the sort of backup I use for my Opera web browser and mail client files.  I don't consider it necessary to have access to previous 'snapshots' of these files.  Mirror backups, of course, use less disk space that snapshot backups. Like snapshot backups, only new and modified files are stored each time a backup is made.

It is important to remember that when you delete files from directories included in the mirror backup, they will be deleted from backup storage the next time the mirror backup is run.  If this behavior is not to your liking, you should use the snapshot type backup.

For both snapshot and mirror backups, the "engine" under the hood of snap2 is the wonderful rsync program.  rsync transmits only changed portions of files, and it compresses the data it transmits!

Planning your Backup

Although snap2 can deal with huge snapshots, you would not want to back up every file on your hard drive.  In general, you should back up only those files which you could not easily replace after a hard drive disaster.  Usually this means that should not back up any part of the operating system.  Only back up your data files and  difficult-to-replace files.  If you back up to a second hard drive (perhaps a USB connection external hard drive), you will have room to back up considerable amounts of music and photos.  However, for space-eating important-but-not-unique files such as music, photos, videos, and CD images (which can easily amount to hundreds of gigabytes), you may consider burning to CD or DVD for backup, or rely on RAID drives with no other backup.

For more advice on what to backup, see Introduction to Backing Up and Restoring Data from the Linux Documentation Project.


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