Tim wrote:
On Mon, Apr 27, 2009 at 8:25 PM, Richard Elling <richard.ell...@gmail.com <mailto:richard.ell...@gmail.com>> wrote:I do not believe you can achieve five 9s with current consumer disk drives for an extended period, say >1 year.Just to pipe up, while very few vendors can pull it off, we've seen five 9's with Hitachi gear using SATA.
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 _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
