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Figure 3–29 Cache Setting Screen
You can use the LUN=x option to specify which LUN to change. If no LUN is specified, changes will
be applied to all the LUNs. Valid LUN values are 0 to 1023. The default value will apply changes to all
LUNs.
3.3.1.1 Cache Segment Size
A large cache segment size may give better performance for large I/O requests and a small cache segment
size may give better performance for small I/O requests. For optimal performance, the cache segment
size should be larger than the average host I/O request size. You may use the STATS LENGTH command
to determine the average host I/O request size. The cache segment size should not be changed during
heavy I/O conditions because the system will temporarily halt all I/O requests while the changes are
taking effect.
Use the CACHE SIZE=x command to set the cache segment size for the specified LUN in kilobytes
(kbs). Valid segment sizes are 128, 256, 512, 1024, and 2048 kilobytes (kbs). The default value is 1024.
This command should not be issued under heavy I/O conditions because the system will momentarily
halt all I/O requests while the changes are taking effect.
3.3.1.2 Writeback Cache Settings
Writeback caching allows the system to increase the performance of handling write I/O requests by
storing the data in cache and saving the data to the disks at a later time.
CACHE WRITEBACK=ON|OFF enables or disables writeback caching for the specified LUN. Default
setting is ON.
CACHE WRITELIMIT=x specifies the maximum percentage of the cache that can be used for writeback
caching. The system will force all writeback requests to be flushed to the disks immediately if the
percentage of writeback data in the cache exceeds this value. Valid range is <0...100>. Default value is
75.
3.3.1.3 Prefetch Settings
When the system receives a request, it can read more data than has been requested. PREFETCH tells the
system how much data to look ahead. This will improve performance if your system needs to perform
sequential reads. For random I/O applications, however, use the smallest prefetch value.
CACHE PREFETCH=x sets the prefetch that will occur on read commands for the specified LUN. Valid
range is 0 to 65535. Default setting is 1.
Current Cache Settings
Maximum
PrefetchLUN
Write
Caching
On
----------------------------------------------------
x1Enabled0
15000 [1]: cache
640.0 Mbytes of Cache Installed
MF
Bit
Prefetch
Ceiling
65535
Onx1Enabled1 65535
Onx1Enabled2 65535
Onx1Enabled3 65535
writeback limit: 75%