User guide

Introduction
Capacity Expansion refers to the ability to add storage capacity to an existing
array by adding hard drives.
The Array Configuration Utility lets you perform Capacity Expansion, even in
fault-tolerant configurations, without requiring a data backup and restore
cycle. If you are using Microsoft Windows NT or Novell NetWare operating
systems, the storage expansion can also be performed online, without leaving
the server operating system. For more information on storage expansion, see
Appendix E, “Hard Drive Arrays.”
The batteries in the Array Accelerator can be dangerous if they are
not handled properly. For precautions regarding batteries, see Appendix A,
“Regulatory Compliance Notices.”
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. The Array Accelerator feature may be disabled by the Array
Configuration Utility if desired.
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.