Information Technology Reference
Selectors and Exclusions
Versioned backups may not include every file on your hard disk. If
storage space is at a premium or if you want to save time on network
backups, you might choose to include only part of your data in
versioned backups (while putting all of it in your bootable duplicates).
You can almost always do this manually, by selecting one or more
specific files or folders to include or exclude. But some backup
programs go further, letting you create patterns indicating which
files should be included (selectors) or excluded (exclusions) from a
particular folder or volume based on file names, sizes, Finder labels,
extensions, modification dates, and other factors.
Snapshots and File Lists
When it comes time to restore files from a versioned backup, you must
be able to locate the versions you're looking for easily. Some backup
programs facilitate such restorations by offering snapshots —lists
of all the files being backed up as they existed at the time of each
backup. Even though a certain file may not have been copied during
a particular backup run (because it hadn't changed since the previous
backup), it will appear in the snapshot. You can typically restore all
the files in a given snapshot, or delete a given snapshot, in a single
One way of creating snapshots without relying on a separate catalog
or file list is to use a Unix feature called a hard link (see the sidebar
The Magic of Hard Links ), which gives the appearance of a file or
folder existing in more than one place even though only one copy is
taking up any real space. When backup software creates hard links to
all the files or folders it didn't copy in their entirety on a given run, you
get a versioned backup that essentially functions as its own snapshot.
Time Machine and Personal Backup (among others) use this approach.
Although snapshots are extremely useful, you may want to access
your backed up files in other ways too. Some backup software uses
a hierarchical file list that shows you every file and folder you've
backed up—and then, for each file, every version it's stored over time.