> On Sun, Aug 22, 2010 at 4:54 PM, james <j...@tigger.ws> 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. >
The best plan is a 22 tape plan (or 20 if you don't cover the weekend). 1 tape for each week day Monday to Wednesday and Friday to Sunday 1 tape each for Thursdays 1 to 3 and 5 1 tape each for January to December which is used on Thursday week 4 This plan allows you to recover mostly everything for a maximum of a year which would cover most situations. Substitute Thursday for whatever day you want but use the same regime. Rick -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
