User`s guide

6.3 RAIDset and Mirrorset Rules and Important Information
The following list gives rules to remember about RAIDsets and mirrorsets:
• You must always have a write-back cache module when creating RAID level 5
RAIDsets or mirrorsets.
• Do not attempt to use any RAIDset or mirrorset commands with mismatched
cache modules. Both cache modules must be write-back cache, and both must
have the same number of megabytes.
• You must purchase licenses for write-back caching, RAID level 5 and
mirroring functionality. If you use these facilities without entering a valid
license key, you will receive an hourly error message at the terminal, and an
hourly error in the host error log. Once you enter your key, the error message
stops.
Note
You may activate write-back caching, RAID level 5, and mirroring via the
firmware licensing system (FLS) utility.
• NOWRITEBACK_CACHE (write-through caching) is automatically set
for units created from RAIDsets or mirrorsets. To increase the unit’s
performance, switch to WRITEBACK_CACHE.
• RAIDsets may be made up of 3 to 14 members. Mirrorsets may be from 2 to
6 members.
• For HSJ and HSD30 array controllers, RAIDsets and mirrorsets in a
dual-redundant configuration flush cache and failover to the companion
controller if the write-back cache module battery for one controller in the pair
has a low charge and CACHE_POLICY=B is set.
For HSZ40 array controllers, RAIDsets and mirrorsets in a dual-redundant
configuration do not failover when the write-back cache battery has a low
charge no matter which cache policy is set.
• RAIDsets and mirrorsets will go inoperative (and write-protected) if both
write-back cache modules’ batteries fail, or if a single cache’s batteries fail
in a nonredundant controller. Any stripesets will flush cache data and
become write-through (cache module acts as read cache) instead of write-back.
RAIDsets and mirrorsets become operational when the batteries become fully
charged again.
Note
When the power for the write-back cache module batteries is too low, a
console message is displayed. You can check the status of the batteries
at any time by entering the CLI SHOW THIS_CONTROLLER command
(or SHOW OTHER_CONTROLLER as appropriate). Depending on which
cache policy you choose, your RAIDsets and mirrorsets will either go
inoperative (CACHE_POLICY=A) or remain available in write-through
mode (CACHE_POLICY=B) when the batteries go low.
Working with RAID Arrays 6–7