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?)
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