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.
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.
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. Or is it random?
We're having a debate related to this, data would be appreciated :-)
Do you get small, random read performance equivalent to N-2 spindles
for an N-way double-parity volume?
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.
That would double the amount of disk that we'd require. I am actually
planning on using two iSCSI LUNs and letting ZFS stripe across them.
When we need to expand the ZFS pool, I'd like to just expand the two
LUNs on the Netapp. If ZFS won't accomodate that, I can just add a
couple more LUNs. This is all convenient and easily managable.
Sounds reasonable to me :-)
-- richard
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