On Tue, Apr 28, 2009 at 4:52 PM, Richard Elling <richard.ell...@gmail.com>wrote:
> > Well done! Of course Hitachi doesn't use consumer-grade disks in > their arrays... > > I'll also confess that I did set a bit of a math trap here :-) The trap is > that if you ever have to recover data from tape/backup, then you'll > have no chance of making 5-9s when using large volumes. Suppose > you have a really nice backup system that can restore 10TBytes in > 10 hours. To achieve 5-9s you'd need to make sure that you never > have to restore from backups for the next 114 years. Since the > expected lifetime of a disk is << 114 years, you'll have a poor > chance of making it. So the problem really boils down to how sure > you can be that you won't have an unrecoverable read during the > expected lifetime of your system. Studies have shown [1] that you > are much more likely to see this than you'd expect. The way to > solve that problem is to use double parity to further reduce this > probability. Or, more simply, BAARF. > > [1] http://www.cs.wisc.edu/adsl/Publications/corruption-fast08.pdf > > -- richard Your *trap* assumes COMPLETE data loss. I don't' know what world you live in, but the one I live in doesn't require a restore of 10TB of data when *ONE* block is bad. You've also assumed that the useful life of the data is 114 years, also false in the majority of primary disk systems. Then there's the little issue with you ignoring parity when you quote "a disk drives life". I'll stick with the 3 year life cycle of the system followed by a hot migration to new storage, thank you very much. --Tim
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