Array Accelerator - Compaq RAID Array 4100 User Manual

Array Accelerator

The Array Accelerator consists of the 16-MB Read Cache on the controller
board and 48-MB Read/Write ECC battery-backed removable cache.
The RA4000 Controller Array Accelerator improves performance on data
access to the drive array by serving as a posted-write cache and read-ahead
cache. The write cache can accept data from the server, then write the data to
the drives at a later time. The read-ahead cache uses a multi-threaded
algorithm to predict the next likely read operation for the array. That data is
then read into the Array Accelerator from the drives. When the RA4000
Controller receives a read request for the cached data, it is sent immediately
into the system. You may disable the Array Accelerator feature using Array
Configuration Utility.
The Array Accelerator has Error Checking and Correcting (ECC) memory,
providing a high level of data integrity. ECC memory detects and corrects hard
and soft memory errors without affecting performance.
Onboard rechargeable batteries in the Array Accelerator ensure that the data
temporarily held (cached) there is safe, even with equipment failure or power
outage. This is particularly important for data that has been cached by a
posted-write cache but has not yet been written to the hard drives. The
batteries preserve data in the Array Accelerator for up to four days.
IMPORTANT: The rechargeable batteries on a new RA4000 Controller may be discharged
when you install the board. If this is the case, the Array Accelerator is disabled during
power up. This problem requires no action on your part. The Array Accelerator
automatically enables when the batteries are charged to 90% of their capacity.
It may take up to 36 hours for the internal circuitry to fully charge the batteries. The
RA4000 Controller functions properly during this time, although without the performance
advantage of the Array Accelerator.
The Array Accelerator, with backup batteries, is located on a removable
daughterboard. In the unlikely event of an RA4000 Controller failure, you may
detach the Array Accelerator from the failed RA4000 Controller and attach it
to a replacement RA4000 Controller. If the configuration information stored in
the Array Accelerator matches that stored on the drives, any valid posted-write
data being held in the cache is written automatically to the drives attached to
the replacement RA4000 Controller. Thus, data held in the Array Accelerator
memory is recovered despite failure of the main RA4000 Controller.
WARNING: There is risk of fire and burns if the battery pack is not handled
properly. For precautions regarding batteries, see Appendix A, "Regulatory
Compliance Notices."
Introduction 1-9