Torrey McMahon wrote: > Dana H. Myers wrote: >> Ed Gould wrote: >> >>> On Jan 26, 2007, at 12:13, Richard Elling wrote: >>> >>>> On Fri, Jan 26, 2007 at 11:05:17AM -0800, Ed Gould wrote: >>>> >>>>> A number that I've been quoting, albeit without a good reference, >>>>> comes from Jim Gray, who has been around the data-management industry >>>>> for longer than I have (and I've been in this business since 1970); >>>>> he's currently at Microsoft. Jim says that the controller/drive >>>>> subsystem writes data to the wrong sector of the drive without notice >>>>> about once per drive per year. In a 400-drive array, that's once a >>>>> day. ZFS will detect this error when the file is read (one of the >>>>> blocks' checksum will not match). But it can only correct the error >>>>> if it manages the redundancy. >>>>> >> >> >>> Actually, Jim was referring to everything but the trunk. He didn't >>> specify where from the HBA to the drive the error actually occurs. I >>> don't think it really matters. I saw him give a talk a few years ago at >>> the Usenix FAST conference; that's where I got this information. >>> >> >> So this leaves me wondering how often the controller/drive subsystem >> reads data from the wrong sector of the drive without notice; is it >> symmetrical with respect to writing, and thus about once a drive/year, >> or are there factors which change this? >> > > It's not symmetrical. Often times its a fw bug. Others a spurious event > causes one block to be read/written instead of an other one. (Alpha > particles anyone?)
I would tend to expect these spurious events to impact read and write equally; more specifically, the chance of any one read or write being mis-addressed is about the same. Since, AFAIK, there are many more reads from a disk typically than writes, this would seem to suggest that there would be more mis-addressed reads in a drive/year than mis-addressed writes. Is this the reason for the asymmetry? (I'm sure waving my hands here) Dana _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
