Gary Mills wrote:
On Fri, Jan 26, 2007 at 11:05:17AM -0800, Ed Gould wrote:
On Jan 26, 2007, at 9:42, Gary Mills wrote:
How does this work in an environment with storage that's centrally-
managed and shared between many servers?
It will work, but if the storage system corrupts the data, ZFS will be
unable to correct it. It will detect the error.
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.
Our Netapp does double-parity RAID. In fact, the filesystem design is
remarkably similar to that of ZFS. Wouldn't that also detect the
error? I suppose it depends if the `wrong sector without notice'
error is repeated each time.
If the wrong block is written by the controller then you're out of luck.
The filesystem would read the incorrect block and ... who knows. Thats
why the ZFS checksums are important.
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