==> In article <[EMAIL PROTECTED]>, Steve Harris <[EMAIL PROTECTED]> writes:
> at 1% , 1-(0.99**30), or about .25 > at 2%, 1-(0.98**30) , or about .45 > at 3%, 1-(0.97**30), or about .60 (Please feel free to correct my maths if I'm > wrong - probability was never my strong point) I think your math is good; I'd add though that there's a -GREAT- deal of locality of reference: Or in other words, if 3% of your files change a day, then for user filespace probably 95% of the files that change tomorrow will be the files that chaged yesterday, and so on. I think that 25 - 30% for userdir space is probably about right. > Now, of course its often the same files which change day after day, so real > experience should be better than this, but at the time, I decided that the > overhead of mainitianing two TSMs (and two clients per node) wasn't worth the > benefit, and went with archives. But I would disagree with your logic; In place of the incremental possibilities, which would have led you at worst above to backing up 60% every month, you're choosing to back up 100% every month, without fail. I think that this represents a substantially more costly strategy. In fact, given the monthly-for-five-years figure and your numbers above, it's about twice as expensive in facilities to archive monthly than to run incrementals with similar retention characteristics; If my guess is closer to correct, it's three- to four- times the cost. For a small amount of data, this may be cheaper than the human organizational time to build two sets of nodes; but by the time you get even medium size, I think it would be pretty expensive. - Allen S. Rout
