Backups: Tapes or Hard Disks?

If you do more with your PC than view things obtained from Steam or iTunes, it is important that you keep them safe somehow.

There are two cases to consider:

  • You want to protect setups you have downloaded from harm and the need to reconfigure everything.
  • You have work of your own you need to protect, as well as downloaded items.

Simple Backups

In the first case, the simplest option is to get an external USB drive and use it as a backup disk. Small 2.5” drives are commonly available and frequently come equipped with a Windows backup program of some sort. Users of Windows 7 also have a backup program built in, which can be used as well.

Some people prefer to make a simple copy of the files to a different filesystem. This has the benefit that it is very simple to browse the backups - no additional software needed. One program that makes this sort of copy very easy is DeltaCopy, which enables you to synchronise two folders (which can be on different drives) with a minimum of copying, by only copying the (parts of) files that are actually different. DeltaCopy does include a scheduler, so you can maintain your copy automatically.

There are two very important things to note:

  • A backup is only any good if you keep it up to date.
  • A backup that deletes the file you want restored, or has saved the corrupted file, is not very useful.
  • A backup that dies with the system it’s backing up is no good to anyone.

The ultimate solution to the first point is the mirrored disk. This is where the operating system keeps a perfect copy of one drive on another drive in the same computer. Also called RAID-1, it boosts the protection of your data from one of the disk drives “going bad” - suffering mechanical or magnetic failure for example. Mirroring requires that both drives are of the same size, and for most home setups, it requires that both drives are in the same cabinet. Consumer disks have an average failure rate of around 1 byte per 11 TB transferred in normal use.

As you might guess, there are a number of problems with mirroring as a backup strategy. If your purpose in having a backup is so that, if you delete or overwrite a file, you can revert it to its earlier state, mirrored disks is no good: the mirror is a perfect copy of “now”, not earlier. Mirroring in the “home” sense also fails if your PC is stolen or is bust by lightning - it’s as likely as not that both disks die at the same time.

We learn from this that there are actually two “use cases” of backups: to keep a copy of the current state, so we can continue should the normal hardware fails, and to keep a copy of an earlier state, so that we can continue in case of user error or software failure.

The traditional backup solution was to use tape drives; in the past this was a cheap way to store very much more data than most hard disks stored, so you could store several copies. It was simple therefore to copy the stuff to tape - say each night - and if something happened, you have a complete history, one copy per day. The “grandfather, father, son” method grew up, in which some day’s tapes were recycled to the next week, but a few were kept back - say every Friday’s. And then the Friday tapes were recycled except, say one a month. This meant you have a frequent copy of the recent past, and a less frequent copy of the less recent past.

This system has in recent times broken down, not because it’s no good but because hard disk capacities have grown much faster than tape drive capacities, and because tape drives are still expensive while hard disks are (relatively) cheap. And people manage to fill up all that space with stuff they’d prefer didn’t disappear on them.

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