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Cache memory and cache battery
Each RAID controller in the DS4200 Express Storage Subsystem contains 1 GB of
cache memory to store read and write operations. (Both RAID controllers in the
DS4200 Express must have the same amount of cache memory.) The battery unit
in each controller can maintain the data in the RAID controller caches for up to
three days in the event that the DS4200 Express Storage Subsystem loses power.
Attention:
DS4700 controller FRUs. If a new controller is not compatible with an existing
controller, it will be held in reset. Refer to the storage subsystem profile and see
"Parts listing" on page 140 for the correct FRU part number for the DS4200 Express
controller.
Cache memory
Cache memory is memory on the RAID controller that is used for intermediate
storage of read and write data on the DS4200 Express RAID controllers. Using
cache memory can increase system performance. The data for a read operation
from the host might be in the cache memory from a previous operation (thus
eliminating the need to access the drive itself), and a write operation is completed
when it is written to the cache, rather than to the drives.
The RAID controller has a Cache Active LED that displays the current status of the
cache. The LED is on if there is data in the cache, and it is off if there is no data in
the cache.
If caching is enabled and the Cache Active LED never comes on during I/O activity,
it indicates one of the following conditions:
v The cache memory from either controller A or controller B has failed.
v The cache sizes in controller A and controller B are not the same.
v The battery has failed. (In this case, one or both of the green Battery LEDs will
Note: Always use the DS4000 Storage Manager client to check your cache
Figure 67 shows the locations of the Cache Active LEDs on the front of the RAID
controllers.
Figure 67. Cache Active LEDs
The DS4200 Express controller FRU is not compatible with the
be off and one or both amber Battery Needs Attention LEDs will be lit.)
memory settings before assuming a hardware failure.
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Chapter 4. Operating the storage subsystem
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