On May 3, 2010, at 2:38 PM, David Dyer-Bennet wrote: > On Sun, May 2, 2010 14:12, Richard Elling wrote: >> On May 1, 2010, at 1:56 PM, Bob Friesenhahn wrote: >>> On Fri, 30 Apr 2010, Freddie Cash wrote: >>>> Without a periodic scrub that touches every single bit of data in the >>>> pool, how can you be sure >>>> that 10-year files that haven't been opened in 5 years are still >>>> intact? >>> >>> You don't. But it seems that having two or three extra copies of the >>> data on different disks should instill considerable confidence. With >>> sufficient redundancy, chances are that the computer will explode before >>> it loses data due to media corruption. The calculated time before data >>> loss becomes longer than even the pyramids in Egypt could withstand. >> >> These calculations are based on fixed MTBF. But disk MTBF decreases with >> age. Most disks are only rated at 3-5 years of expected lifetime. Hence, >> archivists >> use solutions with longer lifetimes (high quality tape = 30 years) and >> plans for >> migrating the data to newer media before the expected media lifetime is >> reached. >> In short, if you don't expect to read your 5-year lifetime rated disk for >> another 5 years, >> then your solution is uhmm... shall we say... in need of improvement. > > Are they giving tape that long an estimated life these days? They > certainly weren't last time I looked.
Yes. http://www.oracle.com/us/products/servers-storage/storage/tape-storage/036556.pdf http://www.sunstarco.com/PDF%20Files/Quantum%20LTO3.pdf > And I basically don't trust tape; too many bad experiences (ever since I > moved off of DECTape, I've been having bad experiences with tape). The > drives are terribly expensive and I can't afford redundancy, and in thirty > years I very probably could not buy a new drive for my old tapes. > > I started out a big fan of tape, but the economics have been very much > against it in the range I'm working (small; 1.2 terabytes usable on my > server currently). > > I don't expect I'll keep my hard disks for 30 years; I expect I'll upgrade > them periodically, probably even within their MTBF. (Although note that, > though tests haven't been run, the MTBF of a 5-year disk after 4 years is > nearly certainly greater than 1 year.) Yes, but MTBF != expected lifetime. MTBF is defined as Mean Time Between Failures (a rate), not Time Until Death (a lifetime). If your MTBF was 1 year, then the probability of failing within 1 year would be approximately 63%, assuming an exponential distribution. -- richard ZFS storage and performance consulting at http://www.RichardElling.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
