The FreeBSD 'zine
February 2000 : Backing up FreeBSD

Backing up FreeBSD
by David Lay <[email protected]>

Understanding Backups

It's probably fair to say that everyone who works with computers understands the importance of making regular backups. It's probably also fair to say that most people aren't in the habit of making any backups whatsoever. Establishing a backup schedule is one of the most important jobs you may ever have to do, and it can be a daunting task. This series of articles aims to demystify the process.

Sizing Up

The first step is establishing a backup schedule involves deciding how much data you have and how much of it you need to keep backed up. Generally you need to consider this from the viewpoint of a worst case scenario. Imagine all the disks in all your hosts spontaneously combusted and you lost every file. Now, some data may be easy to restore by simply re-installing software packages from scratch. Other data (such as personal data files) are probably either impractical or impossible to re-create in such a way. These irreplaceable files are the ones which you definately want to keep backed up. The files which you can restore via other means may be ommitted from the backup schedule - it's only a question of whether it's more convenient to install them from scratch or to restore them from a backup image. If it's easy to start from scratch, you can probably save some trouble (and storage space) by leaving them out.

Once you've determined what to backup it shouldn't be too difficult to work out how much storage space you'll need. This will be an important factor in choosing an appropriate backup medium. There are various backup media available, each with different storage capacities and data read/write speeds. Ideally, your backup medium should be large enough to hold all of your data on a single volume. Some backup software may be capable of splitting backup dumps across several volumes, but it's best to avoid complication where possible.

Backup Media

When considering backup media selection, there's three main choices: hard disk, CD, and magnetic tape.


The price per gigabyte of hard disk drives has steadily decreased over the past few years, and additional hard disk space is often the first backup medium considered. Although not the cheapest backup medium the price is affordable in most situations and the read/write speed greatly exceeds other media, but there's an important attribute of hard disk storage which makes it an inappropriate choice.

In my humble opinion, it's absolutely critical that backup images are stored offline. Remember that the purpose of keeping a backup is so that you may restore data in the event of a data loss. Obviously one way data loss might occur is due to a hardware component failure, but another way which is often overlooked is deliberate (or accidental) removal. It's rare to find a Unix sysadmin who can't tell at least one tale of woe involving a root shell and the 'r' and 'f' arguments to rm(1). There is also the (hopefully remote) possibility of a malicious intruder compromising a machine and deliberately removing files. Backup images held in online storage can be easily removed along with the original data. The best way to protect against this is to keep your backup images on removable media and store them in a safe place. Removable hard disks, such as Zip or Jaz drives address the offline storage issue, but their storage capacity is small compared with other media.

If you are contemplating hard disk backup, you may be better served by putting the additional disks into a fault tolerant RAID array. This probably makes better use of the disk, but don't forget that this is not a substitute for a backup scheme - RAID arrays aren't immune to an accidental rm -rf.


Writeable compact discs have become a commodity item. Drives are available from several different manufacturers, and the widespread acceptance of the format has made the drive price affordable to even home computer users. Blank CDs can be purchased for just a few dollars just about anywhere (even local supermarkets). There are drives capable of writing to CD media at up to 600 kilobytes per second, and most CD drives can read them many times faster. Unlike tape, the media is not sensitive to magnetic fields, and the discs themselves are virtually indestructible. The only drawback is the comparitively limited storage capacity of aroung 0.7 gigabytes, which is too small for many applications. Nevertheless, CD is an excellent medium for archival backups which need to be stored for many years (even though you must create an archive which spans several volumes).


Magnetic tape is still a popular backup medium. There has been a variety of different formats which have come and gone over the but DDS (Digital Data Storage) and DLT (Digital Linear Tape) are the two most common formats right now. Tape drive units are considered something of a high end device, so there tends to be fewer manufacturers to choose from and you will end up paying a higher price than for, say, a CD-R drive. The media cost is comparable to CD-R (a few dollar per gigabyte) but there's a huge difference in the storage capacity.

The DDS-3 format will hold 12 GB per tape, and the DLT format is capable of holding up to 40 GB per tape. Most tape drives support some form of hardware compression and manufacturer's specifications tend to quote the compressed capacity. Only the raw (uncompressed) capacity of a tape format should be used when making comparisons, because the effect of compression can be difficult to predict. The quoted compressed capacities may not always be achievable.


That's the brief overview of available backup media complete. The next article in this series will deal with using a tape drive under FreeBSD, for no other reason than that's the only backup medium I've got first hand experience with.

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