Information Technology Reference
In-Depth Information
Versioned Backup Pruning
Whereas a rolling backup scheme saves a fixed number of complete
backups, deleting older ones as newer ones are added, many backup
programs offer a more sophisticated way of saving space: they
prune
(or erase) older files or snapshots from versioned backups when
certain conditions are met. For example, a program might let you
choose the maximum number of copies of any given file to save; once
you reach that limit, it prunes the oldest one after it copies a new one.
Or it may go by age—extra versions older than, say, 30 days are deleted
automatically to make space for newly backed-up files.
Time Machine does its own sort of pruning: it keeps hourly backups for
4 hours, daily backups for a week, and weekly backups until it runs out
of space. That means almost every time it runs, it purges at least some
older files. And when your backup disk is almost full, it deletes further
files to ensure that, if possible, you always have at least a day's worth
of hourly backups and a week's worth of daily backups—even if you're
seriously low on disk space.
Without a pruning feature, you could get stuck when your backup
media runs out of space—you'd have to migrate your backups to a
larger drive or manually erase backed-up files to make room for new
ones. So pruning can be a valuable feature, but use it with caution; you
don't want to erase files you might need to recover later. That's why
I'm not enthusiastic about backup programs and online services that
enforce pruning after a given period of time, such as 30 days—I prefer
to have a choice as to whether and when pruning should occur. So,
especially when you're considering an online storage provider for
your backups, pay attention to the flip side of pruning—
data retention
.
That is, be sure you know how long the service retains old versions and
deleted files, and whether you can adjust the retention period.
File Format, Compression, and Encryption
To oversimplify somewhat, most software employs one of two basic
methods to copy files when performing a backup. One way is to copy
each file in a stand-alone Finder-readable format, so the backed-up
