On Jan 17, 2011, at 9:37 PM, Aaron McCaleb wrote: > The "churn" of the data > does not influence the impact of loss of the data. What it does > impact is the likelihood of an undesirable event. In particular, it > impacts the likelihood that the data which is backed up might not be > sufficient to restore the desired state of the data...which could be > either because all of the backups are too recent...or all of the > backups are too stale. So as I see it, the rate of change of the data > is completely independent of the subjective valuation of that data.
The churn also gives you an idea of how much time it would take to back up the data with any given backup system, how much time it might take to restore it, and perhaps informs you as to what type of backup system you might need -- is slow tape robot okay, or do you need fast disk-to-disk backup, or do you need full-blown Continuous Data Protection that theoretically gives you an always-on and always-live backup of all your data, and if there is a failure in the primary system then you can just switch to using the secondary system with minimal hiccups. So, there's lots of things that churn can tell you about your backups. For example, in the case of 1m+ user mailboxes on the GNN mail system in 1996, we found that over 50% of them changed in a twelve hour period of time, and any backup that was over 24 hours old was pretty much useless. This had a huge impact on us when the primary system died because of a Field Engineer who did some things to the RAID controllers used by the mail system, and we didn't have a secondary fail-over system to switch to, and even though we had recently instituted nightly backups, they took nearly 20 hours to run and they ended up being pretty much useless anyway. If we'd had CDP, we would have been golden. If we had more frequent backups, we might have been okay. As it was, we -- and our 1m+ users -- were well and truly screwed. -- Brad Knowles <[email protected]> LinkedIn Profile: <http://tinyurl.com/y8kpxu> _______________________________________________ Tech mailing list [email protected] https://lists.lopsa.org/cgi-bin/mailman/listinfo/tech This list provided by the League of Professional System Administrators http://lopsa.org/
