Take Control of Mac OS X Backups: The Online Appendixes

The features on this page deal primarily (though not exclusively) with versioned backups, by which I mean storing multiple versions in a backup. When you keep adding to your backups, as opposed to replacing files, it helps to have some ways of managing your media usage (for example, deleting older files from your backups). It’s also important to be able to see what files you’ve backed up and to get back to a particular version easily. For fine-grained control over which files go in your backup, some programs let you specify patterns such that matching files or folders are automatically included in, or excluded from, your backups. And an increasing number of programs, when updating your backups, can copy only the portions of files that have changed, rather than the whole files. All those features are covered here. Click a column heading for more information about the feature being described.

Note: The top portion of the table lists programs that offer bootable duplicates, versioned backups, or both. The bottom portion of the table lists other backup, synchronization, and file copying tools. Enterprise-grade software is not shown here.

Last updated: August 14, 2010

Feature Explanations for This Table

Rolling Versioned Backups: (Previously called “rotating archives.”) Let’s say you put 10 files in your backup. Over time, because you change file #3 quite often, you accumulate a lot of copies of that file in your backup. If it’s a big file, that can result in your backup occupying a significant amount of space. Some backup programs can automatically delete only older files in your backup to make space for new ones. For example, you might be able to specify that your backup contains a maximum of 10 versions of any particular file, or that any additional copies more than 90 days old are deleted. Any scheme whereby a backup program automatically and selectively deletes older files from an backup gets a “Yes” in this column.

Rolling Backups: Some programs update backups additively but not incrementally. In other words, if you’re backing up your Documents folder, the program copies the entire folder every time it runs, regardless of how few files in it have changed. When backup programs do this, they sometimes give you the option of deleting the oldest complete backup after a certain number of runs (as opposed to deleting individual files) to make space on your backup media. I refer to this type of procedure as a rolling backup.

Snapshots: Snapshots apply only to programs that create versioned backups. By “snapshot,” I mean a picture of what your entire disk (or, at least, the entire set of files you’re backing up) looked like at the moment each backup session ran. With snapshots, you can easily see how many files appeared at a given time, and in one fell swoop, restore your disk to the way it appeared then. In most cases, programs that store snapshots give you a list or menu of all the times at which backups occurred, and when you select one of those, you can generally see all the versions of files in your backup as they appeared at that time—even those files that hadn’t changed recently and therefore weren’t copied on that particular run.

File List: For the purpose of this table, when I refer to a “file list,” I simply want to know whether there’s some way I can see, from within the backup program, what files it backed up (rather than having to dig around in the Finder). Not all file lists are created equal. Some are spiffy—color-coded, searchable, and intelligently hierarchical. Others are confusing and tedious to use. But I don’t distinguish niceness here, only whether or not there’s something in the application that could reasonably be construed as a file list.

Selectors: It goes without saying that virtually every backup program lets you select what folders or files you back up. But in this table, when I say “selectors,” I mean pattern-based rules for selecting files. For example: “Back up all the files with an extension of ‘.doc’ that are less than 500 MB in size.” Of the programs that offer selectors, some are much more sophisticated than others, but the point is that you have a way of specifying, usually with a few clicks, what kinds of files will be included in your backups. A close relative of the selector is the preset, as typified by Backup’s Quick Picks or Memeo LifeAgent’s Smart Picks. Presets are basically prebuilt selectors that choose all files of a particular sort—for example, all the files that make up your Address Book database, or all your Mail files, or every MP3 file anywhere on your computer.

Exclusions: Once again, I’m not merely talking about the fact that you can manually deselect or exclude a particular file or folder. By “exclusion” here I mean a pattern-based rule for selecting files that will not be backed up. For example: “Omit all files over 2 GB” or “Omit files with a Red label in the Finder.”

Sub-File Updates: Until recently, almost all Macintosh backup software performed versioned backups on a file-by-file basis. In other words, if just 10 bytes of a 10 GB file change, that marks the file as modified, and the whole file must be copied on the next backup run. Some software, however, can perform sub-file incremental updates. In some cases, the software copies only the individual bytes that have changed since the last backup, and in other cases it copies slightly larger units called blocks. (A block is a unit of storage typically equal to 4096 bytes (4K) on modern Macs.) With backup applications that perform these sub-file (byte-level or block-level) updates, if only 10 bytes of a file change, only those 10 bytes, or the block(s) containing those 10 bytes, are added to the backup—a tiny amount of data. The advantage of such an approach is that backups go much faster after the initial run and take up far less storage space; this is particularly important when backing up over the Internet. The disadvantage is that restoring a file requires the backup software to reconstruct it by putting together the pieces from all its incremental backups. If even a single one of those incremental bits were to become damaged or lost, you might be unable to restore the file. Note that this last objection doesn’t apply to programs based on rsync (marked with footnote [b]); these copy only the changed bits, but then, on the receiving end, reconstruct the entire updated file in the backup. So such programs save bandwidth and time but not storage space; on the other hand, they virtually eliminate the risk of restoration problems from missing or damaged pieces.