Take Control of Mac OS X Backups: Part One
Sidebar: Synchronization Utilities
Lots of utilities—including several that bill themselves as backup tools—perform a function called synchronization . As the name implies, synchronization means maintaining identical copies of a file, folder, or even an entire disk in two or more locations. Some synchronization utilities can run on a schedule, automatically “backing up” files from a location you specify to another volume. And some can create a bootable duplicate by synchronizing an entire disk to another disk.
There’s nothing wrong with synchronization—in fact, it can be incredibly useful in certain circumstances, such as keeping your PowerBook’s hard disk updated with documents you use frequently on your desktop Mac. As a quick and easy way of making an extra copy of certain files, it can serve as a type of primitive backup.
If you want to use a synchronization utility to make duplicates as part of your backup strategy, that is perfectly valid too. However, please do not mistake synchronization for a true backup—no matter what the utility’s advertising says.
What’s true of duplicates is equally true of individually synchronized files and folders: you get only the most recently modified version. You lack the ability to recover an older version of the file, which is a crucial part of a solid backup program. Also, if you don’t notice that a file is damaged before synchronizing it to another volume, you may end up with two useless copies. If you synchronize deletions, you lose your insurance against accidentally trashing files. And it’s all too easy to accidentally copy data in the wrong direction!
All that to say: a single copy of a single version of your data does not a backup make. By all means, synchronize if you wish, but not as a substitute for proper archives and complete, bootable duplicates.
Sidebar: Can a RAID Substitute for Duplicates?
RAID stands for Redundant Array of Independent (or Inexpensive) Disks; it’s a way of combining multiple physical hard drives into a single logical volume using either software or a special hardware controller. One way to configure a RAID, known as mirroring, is to have the same data written simultaneously to two or more drives. If any one drive fails, another can take over instantly and seamlessly with no loss of data and no down time; you can then replace the faulty drive at your leisure.
One developer of RAID software for Mac OS X uses the slogan “Better than Backup!” The logic is that the RAID gives you a bootable duplicate that’s always 100 percent up-to-date, without ever requiring you to run a backup program or worry about complicated restoration procedures in the event of a failure.
I have nothing against RAIDs, and if you need to keep a mission-critical computer running without any hiccups at all, a mirrored RAID might be just what you need. However, I strongly believe that a RAID is no substitute for multiple duplicates as described in this article. A mirrored RAID’s best feature is also its Achilles’ heel: because changes are reflected on all drives simultaneously, an accidentally deleted file will be immediately deleted on your “backup” drives too! Stand-alone duplicates—especially if you maintain two or three of them—reduce this risk greatly.
RAIDs address the problem of spontaneous drive failures, but they provide no insurance against human error, theft, natural disaster, or any of the other catastrophes that make backups so important. So, use a RAID if you wish, but only as a supplement to duplicates and archives.
Sidebar: Incremental or Differential?
Some backup programs distinguish between incremental and differential archiving schemes. Although not all software uses the terms in exactly the same way, the difference is typically that in an incremental backup, only the files changed or added since the last time the backup ran are added to the archive. With a differential backup, all the files changed or added since the initial full backup are added to the archive. Thus, differential backups take longer to run than incremental backups.
This distinction is important when backing up to tapes or other removable media, because it affects the speed with which a backup can be restored. When restoring from an incremental backup, the software must copy the entire initial backup and then step through each of the incremental backups to retrieve all the updated files. This can require a great deal of media swapping. A differential backup, on the other hand, can be restored more quickly because the software must copy only the original backup and the most recent one. When backing up to a hard drive, however, this distinction is less significant, because the random-access nature of a hard drive enables it to restore either sort of backup with roughly equal speed.