On 10/23/2010 8:22 PM, Anil wrote:
We have Sun STK RAID cards in our x4170 servers. These are battery
backed with 256mb cache.
What is the recommended ZFS configuration for these cards?
Right now, I have created a one-to-one logical volume to disk mapping
on the RAID card (one disk == one volume on RAID card). Then, I mirror
them using ZFS. No hardware mirror. What I am a little confused with
is if it is better to not do any logical volumes on the RAID card and
only use ZFS mirrors for creating the pools.
If you have a RAID card that has battery-backed cache, most cards make a
distinction between a JBOD mode and a RAID volume mode. I'd have to
look at the specs for your card, but most cards put into a JBOD mode
will act similarly to a stupid HBA - that is, the one-board cache will
be used solely for read-ahead cache, and not for any write-caching.
When using the "one-disk-per-volume", the card acts as a "normal" RAID
controller, and can potentially use the one-board cache as a write cache
(see below).
Normally, doing what you are is the preferred method of operation, as it
gives ZFS the chance to do its magic (and use all its advantages), AND
also gives the bonus of having the fast write cache of the RAID
controller. It's how I configure these things for virtually all my systems.
The one real down side of doing it your way is that the drives now are
non-portable. That is, you can't move them to another HBA of a different
brand, and you most likely will only ever be able to move them to
another physical controller of identical brand AND model. And, of
course, you have to make sure that you re-power the RAID controller
within its battery lifespan after a power loss, or you lose data.
Please see configuration output here:
http://pastebin.ca/1971460
Also, what exactly does "write-back" mean?
Thanks
"write-back" and "write-through" are the two major modes most RAID cards
support in respect to how they treat their on-board cache and write
requests. "Write-back" mode indicates that the RAID card will
acknowledge a committed write as soon as the CACHE has been updated, but
BEFORE the data makes it all the way out to the disks. "Write-through"
mode will not acknowledge a committed write operation until the data
actually makes it to disk (it "writes-through" the cache).
--
Erik Trimble
Java System Support
Mailstop: usca22-123
Phone: x17195
Santa Clara, CA
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