User`s guide

64 007-4522-003
4: Using the RAID Controller
Controller Cache and Availability
The RAID controller has a write cache of 512 MB. This physical memory is used to
increase the performance of data retrieval and storage operations.
The controller can report to the operating system that a write is complete as soon as the
controller receives the data. Enabling write cache (write-back cache) improves
performance, but exposes the data to loss if a system crash or power failure occurs before
the data in the cache is written to disk. To prevent data loss, use an uninterruptable
power supply (UPS).
In systems using dual-active RAID controllers, data is copied to the cache of the partner
controller before the write complete is reported to the host initiator. During the time the
data is being written to the partner controller, the system is exposed to possible data loss
if a system crash or power failure occurs. Again, a UPS is recommended to preserve data
integrity.
Caution: No UPS has been tested, qualified, or approved by SGI.
RAID Levels and Availability
Table 4-4 summarizes RAID levels offered by the RAID controller and the advantages
(and disadvantages) of the RAID levels as they apply to availability.
Table 4-4 RAID Levels and Availability
RAID Level Fault Tolerance Type Availability
0 None Data is striped across a set of multiple disk drives. If a disk drive in the set ceases to
function, all data contained on the set of disk drives is lost. (This configuration is not
recommended if fault tolerance is needed.)
1 Mirrored Data is written to one disk drive, and then the same data is written to another disk drive.
If either disk drive fails, the other one in the pair is automatically used to store and retrieve
the data.
3 and 5 Striped Data and parity are striped across a set of at least three disk drives. If any fail, the data (or
parity) information from the failed disk drive is computed from the information on the
remaining disk drives.
!