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.
The quote from Jim seems to be related to the leaves of the tree
(disks).
Anecdotally, now that we have ZFS at the trunk, we're seeing that the
branches are also corrupting data. We've speculated that it would
occur,
but now we can measure it, and it is non-zero. See Anantha's post for
one such anecdote.
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.
--Ed
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