Cache Memory And Cache Battery; Cache Memory - IBM DS4800 User, Installation And Maintenance Manual

Cache memory and cache battery

Each RAID controller in the DS4800 Storage Subsystem contains either 2 GB, 4
GB, or 8 GB of cache memory to store read and write operations. (Both RAID
controllers in the DS4800 must have the same amount of cache memory.) The
interconnect-battery unit contains two Lithium Ion battery packs that can maintain
the data in the RAID controller caches for up to three days in the event that the
DS4800 Storage Subsystem loses power.
Important::
Do not replace the battery until it is marked failed by the controller. If the batteries
are shown as expired, use the reset battery age function in the SM client
subsystem management window to reset the age.
Attention:
controller with certain cache sizes. The DS4800 storage subsystem profile has the
information on the installed cache memory size in the DS4800 controllers. Refer to
the storage subsystem profile and see "Parts list" on page 166 for the correct FRU
part number for the DS4800 controller. In addition, the DS4800 model 80A/H has
different replacement FRU part numbers than the DS4800 model 82A/H controllers,
even though they both have the same cache size.

Cache memory

Cache memory is memory on the RAID controller that is used for intermediate
storage of read and write data on the DS4800 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 80 on page 130 shows the locations of the Cache Active LEDs on the front
of the RAID controllers.
b. No – Contact your IBM service representative. Print a storage subsystem
profile, and save the storage subsystem support data. The profile and
support data are helpful when troubleshooting storage subsystem
problems. (For information about printing a storage subsystem profile and
saving the support data, refer to the Subsystem Management Window
online help.)
There are different replacement FRU part numbers for the DS4800
be off and one or both amber Battery Needs Attention LEDs will be lit.)
memory settings before assuming a hardware failure.
Chapter 4. Operating the storage subsystem
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