On Mon, May 3, 2010 17:02, Richard Elling wrote: > 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
Yep, they say 30 years. That's probably in the same "years" where the MAM gold archival DVDs are good for 200, I imagine. (i.e. based on accelerated testing, with the lab knowing what answer the client wants). Although we may know more about tape aging, the accelerated tests may be more valid for tapes? But LTO-3 is a 400GB tape that costs, hmmm, maybe $40 each (maybe less with better shopping, that's a quick Amazon price rounded down). (I don't factor in compression in my own analysis because my data is overwhelmingly image filee and MP3 files, which don't compress further very well.) Plus a $1000 drive, or $2000 for a 3-tape changer (and that's barely big enough to back up my small server without manual intervention, might not be by the end of the year). Tape is a LOT more expensive than my current hard-drive based backup scheme, even if I use the backup drives only three years (and since they spin less than 10% of the time, they should last pretty well). Also, I lose my snapshots in a tape backup, whereas I keep them on my hard drive backups. (Or else I'm storing a ZFS send stream on tape and hoping it will actually restore.) >> 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. Yeah, sorry, I stumbled into using the same wrong figures lots of people were. -- David Dyer-Bennet, d...@dd-b.net; http://dd-b.net/ Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/ Photos: http://dd-b.net/photography/gallery/ Dragaera: http://dragaera.info _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
