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Attention: There are different replacement FRU part numbers for the DS4700
Express controller with certain cache sizes. The DS4700 Express Storage
Subsystem profile has the information on the installed cache memory size in the
DS4700 Express controllers. Refer to the storage subsystem profile and see “Parts
listing” on page 188 for the correct FRU part number for the DS4700 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 DS4700 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
be off and one or both amber Battery Needs Attention LEDs will be lit.)
Note: Always use the DS4000 Storage Manager client to check your cache
memory settings before assuming a hardware failure.
Figure 103 shows the locations of the Cache Active LEDs on the front of the RAID
controllers.
Table 47. Description of Figure 103
Number Description
1 Cache Active LED
ds470032
1
1
Figure 103. Cache Active LEDs
Chapter 4. Operating the storage subsystem 141