User`s guide

50 007-4522-003
3: Features of the RAID Controller
information on a replacement disk drive is questionable or invalid, the disk drive will be
labeled unconfigured offline or dead.
If a drive fails in a RAID level that uses a hot spare, drive roaming allows the controller
to keep track of the new hot spare, which is the replacement for the failed drive.
Caution: Mixing controllers or disk drives from systems running different versions of
firmware presents special situations that may affect data integrity. If a new disk drive
containing configuration data is added to an existing system while power is off, the
controller may incorrectly adopt the configuration data from the new drive. This may
destroy the existing valid configuration and result in potential loss of data. Always add
drives with the power supplied to the system to avoid potential loss of data.
Data Caching
RAID controllers can be operated with write cache enabled or disabled. This section
describes the modes in the following subsections:
• “Write Cache Enabled (Write-back Cache Mode)” on page 50
• “Write Cache Disabled (Write-through or Conservative Cache Mode)” on page 51
Write caching is set independently for each system drive in the system management
software.
Write Cache Enabled (Write-back Cache Mode)
If write cache is enabled (write-back cache mode), a write completion status is issued to
the host initiator when the data is stored in the controller’s cache, but before the data is
transferred to the disk drives. In dual-active controller configurations with write cache
enabled, the write data is always copied to the cache of the second controller before
completion status is issued to the host initiator.
Enabling write cache enhances performance significantly for data write operations; there
is no effect on read performance. However, in this mode a write complete message is sent
to the host system as soon as data is stored in the controller cache; some delay may occur
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