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
