You know you should

This chapter is philosophical and theoretical about backups. It discusses why you should back up, various concepts around backups, what kinds of things you should think about when setting up backups and what to do in the long term (verification, etc). It also discusses some assumptions Obnam makes and some constraints it imposes.

Why backup?

FIXME: Add some horror stories here about why backups are important. With references/links.

Backup concepts

This section covers core concepts in backups, and defines some terminology used in this book.

Live data is the data you work with or keep. It's the files on your hard drive: the documents you write, the photos you save, the unfinished novels you wish you'd finish.

Most live data is precious in that you'll be upset if you lose it. Some live data is not precious: your web browser cache probably isn't, for example. This distinction can let you limit the amount of data you need to back up, which can significantly reduce your backup costs.

A backup is a spare copy of your live data. If you lose some or all of your live data, you can get it back ("restore") from your backup. The backup copy is, by practical necessity, older than your live data, but if you made the backup recently enough, you won't lose much.

Sometimes it's useful to have more than one old backup copy of your live data. You can have a sequence of backups, made at different times, giving you a backup history. Each copy of your live data in your backup history is a generation. This lets you retrieve a file you deleted a long time ago, but didn't realise you needed until now. If you only keep one backup version, you can't get it back, but if you keep, say, a daily backup for a month, you have a month to realise you need it, before it's lost forever.

The place your backups are stored is the backup repository. You can use many kinds of backup media for backup storage: hard drives, tapes, optical disks (DVD-R, DVD-RW, etc), USB flash drives, online storage, etc. Each type of medium has different characteristics: size, speed, convenience, reliability, price, which you'll need to balance for a backup solution that's reasonable for you.

You may need multiple backup repositories or media, with one of them located off-site, away from where your computers normally live. Otherwise, if you house burns down, you'll lose all your backups too.

You need to verify that your backups work. It would be awkward to go to the effort and expense of making backups and then not be able to restore your data when you need to. You may even want to test your disaster recovery by pretending that all your computer stuff is gone, except for the backup media. Can you still recover? You'll want to do this periodically, to make sure your backup system keeps working.

There is a very large variety of backup tools. They can be very simple and manual: you can copy files to a USB drive using your file manager, once a blue moon. They can also be very complex: enterprise backup products that cost huge amounts of money and come with a multi-day training package for your sysadmin team, and which require that team to function properly.

You'll need to define a backup strategy to tie everything together: what live data to back up, to what medium, using what tools, what kind of backup history to keep, and how to verify that they work.

Backup strategies

You've set up a backup repository, and you have been backing up to it every day for a month now: your backup history is getting long enough to be useful. Can you be happy now?

Welcome to the world of threat modelling. Backups are about insurance, of mitigating small and large disasters, but disasters can strike backups as well. When are you so safe that no disaster will harm you?

There is always a bigger disaster waiting to happen. If you backup to a USB drive on your work desk, and someone breaks in and steals both your computer and the USB drive, the backups did you no good.

You fix that by having two USB drives, and you keep one with your computer and the other in a bank vault. That's pretty safe, unless there's an earth quake that destroys both your home and the bank.

You fix that by renting online storage space from another country. That's quite good, except there's a bug in the operating system that you use, which happens to be the same operating system the storage provider uses, and hackers happen to break into both your and their systems, wiping all files.

You fix that by hiring a 3D printer that prints slabs of concrete on which your data is encoded using QR codes. You're safe until there's a meteorite hits Earth and destroys the entire civilisation.

You fix that by sending out satellites with copies of your data, into stable orbits around all nine planets (Pluto is too a planet!) in the solar system. Your data is safe, even though you yourself are dead from the meteorite, until the Sun goes supernova and destroys everything in the system.

There is always a bigger disaster. You have to decide which ones are likely enough that you want to consider them, and also decide what the acceptable costs are for protecting against them.

A short list of scenarios for thinking about threats:

These questions do not cover everything, but they're a start. For each one, think about:

The threat modelling here is about safety against accidents and natural disasters. Threat modelling against attacks and enemies is similar, but also different, and will be the topic of the next episode in the adventures of Bac-Kup.

Backups and security

You're not the only one who cares about your data. A variety of governments, corporations, criminals, and overly curious snoopers are probably also interested. (It's sometimes hard to tell them apart.) They might be interested to find evidence against you, blackmail you, or just curious about what you're talking about with your other friends.

They might be interested in your data from a statistical point of view, and don't particularly care about you specifically. Or they might be interested only in you.

Instead of reading your files and e-mail, or looking at your photos and videos, they might be interested in preventing your access to them, or to destroy your data. They might even want to corrupt your data, perhaps by planting child porn in your photo archive.

You protect your computer as well as you can to prevent these and other bad things from happening. You need to protect your backups with equal care.

If you back up to a USB drive, you should probably make the drive be encrypted. Likewise, if you back up to online storage. There are many forms of encryption, and I'm unqualified to give advice on this, but any of the common, modern ones should suffice except for quite determined attackers.

Instead of, or in addition to, encryption, you could ensure the physical security of your backup storage. Keep the USB drive in a safe, perhaps, or a safe deposit box.

The multiple backups you need to protect yourself against earthquakes, floods, and roving gangs of tricycle-riding clowns, are also useful against attackers. They might corrupt your live data, and the backups at your home, but probably won't be able to touch the USB drive encased in concrete and buried in the ground at a secret place only you know about.

The other side of the coin is that you might want to, or need to, ensure others do have access to your backed up data. For example, if the clown gang kidnaps you, your spouse might need access to your backups to be able to contact your MI6 handler to ask them to rescue you. Arranging safe access to (some) backups is an interesting problem to which there are various solutions. You could give your spouse the encryption passphrase, or give the passphrase to a trusted friend or your lawyer. You could also use something like libgfshare to escrow encryption keys more safely.

Backup storage media considerations

This section discusses possibilities for backup storage media, and their various characteristics, and how to choose the suitable one for oneself.

There are a lot of different possible storage media. Perhaps the most important ones are:

We'll skip more exotic or unusual forms, such as microfilm.

Magnetic tapes are traditionally probably the most common form of backup storage. They can be cheap per gigabyte, but tend to require a fairly hefty initial investment in the tape drive. Much backup terminology comes from tape drives: full backup vs incremental backup, especially. Obnam doesn't support tape drives at all.

Hard drives are a common modern alternative to tapes, especially for those who do not wish pay for a tape drive. Hard drives have the benefit of every bit of backup being accessible at the same speed as any other bit, making finding a particular old file easier and faster. This also enables snapshot backups, which is the model Obnam uses.

Different types of hard drives have different characteristics for reliability, speed, and price, and they may fluctuate fairly quickly from week to week and year to year. We won't go into detailed comparisons of all the options. From Obnam's point of view, anything that can look like a hard drive (spinning rust, SSD, USB flash memory stick, or online storage) is usable for storing backups, as long as it is re-writable.

Optical disks, particularly the kind that are write-once and can't be updated, can be used for backup storage, but they tend to be best for full backups that are stored for long periods of time, perhaps archived permanently, rather than for a actively used backup repository. Alternatively, they can be used as a kind of tape backup, where each tape is only ever used once. Obnam does not support optical drives as backup storage.

Paper likewise works better for archival purposes, and only for fairly small amounts of data. However, a backup printed on good paper with archival ink can last decades, even centuries, and is a good option for small, but very precious data. As an example, personal financial records, secret encryption keys, and love letters from your spouse. These can be printed either normally (preferably in a font that is easy to OCR), or using two-dimensional barcode (e.g, QR). Obnam doesn't support these, either.

Obnam only works with hard drives, and anything that can simulate a read/writable hard drive, such as online storage. By amazing co-incidence, this seems to be sufficient for most people.

Glossary