Tao Of Backup Wailing Wall Story

Don't use streaming compression.

I lost most of the data on my workstation, including applications, email, documents, and several substantial software systems I had created. Here's how.

To backup my workstation, I would write the entire contents of the disk to a single tape. I was very careful to maintain two backup tapes, one onsite and one offsite. Before overwriting a backup tape, I would first test the alternate tape by restoring from it. I performed a full backup just once a month, because incremental backups tended to store little more than email, and I decided I could live with that loss. Not only did this provide physical separation, but also the confidence that I could recover from a failure during a backup.

One day, the external data disk failed. This was not just a partial failure, but a full spindle meltdown, and there was no hope of recovering any data from it. To recover, I turned to the onsite backup, only to discover that it had accidentally been re-used to back up a different system. This left only the offsite backup, but, to my horror, it turned out that the tape had been written with "streaming compression" and that, because of this, a minor unrecoverable error early in the tape had rendered the rest of the blocks on the tape unrecoverable.

I had coverage, frequency, separation, and testing, but in the end I just didn't have enough backup tapes to overcome the run of bad luck I encountered. Here's what I learnt:

* Two backups aren't enough.

* Physical security of your onsite or offsite backups is not necessarily sufficient; in any organisation, you must also ensure that procedures are in place to reduce the risk of accidentally reusing a tape.

* Streaming compression may save you time and money in the short term, but may cost far more in the long term. I don't use streaming compression any more. :-/

     Andrew Rutherford
    Adelaide, Australia, Fri 08-May-1998 2:43am

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