Information Technology Reference
In-Depth Information
find particularly important as you read this and compare that list
against the latest version of the tables in the online appendix.
However, please keep in mind that you don't necessarily need
all
these features in your backup software, and that in some cases,
a combination of two or more programs might serve you best.
Duplication
I know, we're talking about versioned backups at the moment, but
because you'll also need bootable duplicates, you may want software
that does both (instead of using two separate programs). Most backup
apps that can create a bootable duplicate can also update a duplicate
incrementally (rather than recopy every single file each time).
Change Detection
In general, backup software doesn't start copying files immediately
when it runs. It must first figure out what to copy—that is, apart from
the first full backup, it needs to know which files are new or different
since it performed the last incremental update. A backup program
typically starts each run by scanning all the folders and files you've
asked it to back up to determine not only what has changed, but how
much space your backup will require and whether your backup will fit
on the destination volume.
Some programs take a
long
time to scan, whereas others use any of
several tricks to reduce or eliminate scanning time. Quite a few apps
use the FSEvents (file system events) notification system in Mac OS X
to determine which files have changed recently, while others run in the
background and use their own methods to watch for file changes.
Whatever the mechanism, the result is that new or modified files can
be copied immediately, and full scans (to identify changes that may
have slipped through the cracks somehow) can occur infrequently.
Among the backup programs that detect file changes instantly so they
can perform incremental updates without lengthy scans every time are
CrashPlan, NTI Shadow, Synchronize Pro X, Synk (Standard and Pro
editions), and Time Machine. Other programs, like SuperDuper, scan
and copy in a single pass for greater efficiency.
