Beginners start here: Backup Gotchas

Sean McNamara
5 November, 2008
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Backing up is one of those things you should never stop thinking about. Although I recently covered backup basics here on the AMW site, I thought it worthwhile delving a bit deeper into aspects of backups which don’t seem to be thought about very much.

The most important thing to remember in computing is that the primary law which governs their operation isn’t Moore’s Law but Murphy’s law.

On that basis, it’s worth expanding on a couple of points from my prior post, as well as making a couple of new observations to help you get into a backing up mood.

To illustrate these points, I’m creating a new computing term – the “SMIP”, for “Sean McNamara’s Index of Paranoia” (which in true computer industry fashion is an acronym annoyingly similar to but not to be confused with another acronym, in this case “MIPS“).

As with most computing terms and acronyms, it’s really just a fancy name for something exceedingly mundane – in this case, the total number of uncorrupted copies of a file (which is only mundane until you need one of those copies).

SMIPs can be applied to individual files (with each file having a SMIP which refers to how many valid copies of that file exist anywhere) or to arbitrarily large collections of files (anywhere from two files to the entire contents of a hard disk, in which case it can also be thought of as the SMIP of the computer).

High SMIPs are good (and the higher the better), low SMIPs are bad, with a SMIP of 0 being the worst case scenario and a SMIP of 1 really not being good enough at all, thank you very much. The minimum SMIP you should be working towards is 2.

Too many backups are never enough
or “A SMIP of n+1 is better than a SMIP of n, but a SMIP of n+2 is even better”
Having multiple copies of a file is the ultimate goal of any backup strategy and is A Good Thing. Having even more copies than that is even better. And if you can get just another few copies made, all the better. Honestly, too many backups are never enough. Keep your SMIPs up to keep you happy when you need to recover some data.

A backup of bad data is worse than no backup of good data
or “A SMIP of 0 is (much) worse than a SMIP of 1″
Just because you’re backing up doesn’t mean you’re backing up good data! You should be checking your important original data occasionally to make sure it’s OK, otherwise that file’s SMIP could be depressingly low.

A bad backup of good data is no backup at all
or “A SMIP of 2 where the backup is corrupt is really a SMIP of 1″
You should really be testing the viability of your backups by recovering data from them occasionally. There’s nothing worse than your SMIPs being lower than you thought!

Don’t do a Francis Ford Coppola
or “Scattering your SMIPs is good”
In a break-in of his Buenos Aires home, Francis Ford Coppola lost not just his computer, but also his backup drive which was in the office with his computer. As far as catastrophic loss through theft or natural disaster goes, co-located copies don’t really increase your SMIP at all. Keep some copies offsite to keep your SMIPs safe from catastrophe, or at least keep some of your backups in separate rooms.

For me a central tenet of computing is that data will be lost one way or another – just as the entropy of the universe will never decrease, SMIPs left by themselves will always decrease over time and will, at some point in the future, degrade to 0 (this is “Sean McNamara’s Second law of computing”). Your ability to recover from the inevitable loss of data can only be enhanced by being thorough and paranoid when it comes to your backup strategy.

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