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? I'm putting together a new
IMAP server that will eventually use 3TB of space from our Netapp via
an iSCSI SAN. The Netapp provides all of the disk management and
redundancy that I'll ever need. The server will only see a virtual
disk (a LUN). I want to use ZFS on that LUN because it's superior
to UFS in this application, even without the redundancy. There's
no way to get the Netapp to behave like a JBOD. Are you saying that
this configuration isn't going to work?
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.
I would suggest exporting two LUNs from your central storage and let
ZFS mirror them. You can get a wider range of space/performance
tradeoffs if you give ZFS a JBOD, but that doesn't sound like an
option.
--Ed
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