On Sun, Aug 22, 2010 at 4:54 PM, james <[email protected]> wrote: > I've spent many hours with pencil and paper, I certain, but am asking in case > someone older-n-wiser can offer sage words: > > If I want to backup a system for n days, and be able to recover any particular > days files the only way that I can see is to have a daily backup for n days. > > Tower of Hanoi (for eg) says you can backup 2^^n-1 days with n tapes but i can > break that. Simple EG starting with day 4 sequence ie backup: > > C A B A C A B A (http://en.wikipedia.org/wiki/Backup_rotation_scheme) > > On day 2 you create a file, which you remove on day 3. On day 6 you try to > restore day 2 .... > > On day 6 you have: > A from day 5 > B from day 2 (or worse from day 6) > C from day 4 > > So the file created on day 2 backed up on day 3 is lost. > Can anybody point to the boat (I've missed) or confirm my vision.
Yup, that's the biggest failings with most "commercially acceptable" backup regimes. It's an offset of cost (in tapes) against reliability. If you want to be 99% guaranteed[1] to be able to recover any file which was saved, your only option is to take a daily full backup. Any grandfather/father/son schema will eventually lead to the possibility for files going missing. The cost gets ridiculous if you want to keep your data for a long period - you need a new tape for each and every single day you backup - and you have to store them somewhere. Commercially, I usually make users aware that there is guaranteed recovery for XX days (a week), and then the possibility of loss if circumstances like you've outlined above occur (I.E. file saved at start of week, deleted in middle of week and not on weekly tape cycle.). I used to run 4 daily tapes (Mon-Thurs) and 6 weekly tapes (weeks 1-6) before going to monthly tapes - which means I could guarantee *any* file for a week, then *most* files for 6 weeks, then it was pot luck if the file was on the monthly tape. Last place I worked found this acceptable, some places (including current $POE) don't and wear the extra cost in tapes. And that can be a *lot* of cost if you're talking large amounts of data - LTO4 tapes run to about $50 a pop (maybe less if you buy in bulk), LTO5 is worse. DaZZa -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
