Values For The Cacheflushmodifier Parameter - IBM System Storage DS3000 Programming Manual

The cache block size value defines the size of the data block that is used by the controller in transferring
data into or out of the cache. You can set the cache block size to either 4KB or 16KB. The value that you
use applies to the entire storage subsystem and all of the logical drives in the storage subsystem. For
redundant controller configurations, this value includes all logical drives owned by both controllers. Use
smaller cache block sizes for systems that require transaction processing requests or I/O streams that are
typically small and random. Use larger cache block sizes for large I/O, sequential, high-bandwidth
applications. The choice of block size affects read/write performance. Large data transfers take longer in
4-KB block sizes than 16-KB block sizes. This example shows how to set the cacheBlockSize parameter:
c:\...\...\client>smcli 123.45.67.89 -c "set storageSubsystem cacheBlockSize=16;"
To prevent data loss or corruption, the controller periodically writes cache data to the disk drives (flushes
the cache) when the amount of unwritten data in the cache reaches a predefined level, called a start
percentage. The controller also writes cache data to the disk drives when data has been in the cache for a
predetermined amount of time. The controller writes data to the disk drives until the amount of data in
the cache drops to a stop percentage level. Use the set storageSubsystem command to set the start value
and the stop value as percentages of the filled and unfilled, respectively, capacity of the cache. For
example, you can specify that the controller start flushing the cache when it reaches 80 percent full (the
default, which can be overridden) and stop flushing the cache when it reaches 84 percent empty. This
example shows how to set these parameters:
c:\...\...\client>smcli 123.45.67.89 -c "set storageSubsystem
cacheFlushStart=80 cacheFlushStop=84;"
Low start percentages and low stop percentages provide for maximum data protection. For both low start
percentages and low stop percentages, the chance that data requested by a read command is not in the
cache is increased. When the data is not in the cache, the cache hit percentage for writes and I/O requests
decreases. Low start values and low stop values also increase the number of writes that are necessary to
maintain the cache level. Increasing the number of writes increases the system overhead and further
decreases performance.
Use the set logicalDrive command to change settings for the cache flush modifier, cache without
batteries enabled, mirror cache enabled, the read ahead multiplier, read cache enabled, and write cache
enabled. Use this command to set properties for all of the logical drives or for a specific logical drive in a
subsystem. The command takes this form:
set (allLogicalDrives | logicalDrive [logicalDriveName] |
logicalDrives [logicalDriveName1 ... logicalDriveNameN]
logicalDrive <wwID>) |
cacheFlushModifier=cacheFlushModifierValue |
cacheWithoutBatteryEnabled=(TRUE | FALSE) |
mirrorCacheEnabled=(TRUE | FALSE) |
readCacheEnabled=(TRUE | FALSE) |
writeCacheEnabled=(TRUE | FALSE) |
cacheReadPrefetch=(TRUE | FALSE)
The cacheFlushModifier parameter defines the amount of time that data stays in the cache before it is
written to the disk drives. The following table lists the values for the cacheFlushModifier parameter.
Table 4-9. Values for the cacheFlushModifier Parameter
Value
Immediate
250
500
750
1
1500
Meaning
Data is flushed as soon as it is placed into the cache.
Data is flushed after 250 ms.
Data is flushed after 500 ms.
Data is flushed after 750 ms.
Data is flushed after 1 s.
Data is flushed after 1500 ms.
Chapter 4. Configuring a Storage Subsystem
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