250 IBM Midrange System Storage Hardware Guide
Logical drive
To locate this setting, highlight the logical drive in the Storage Manager menu and select
Logical Drive ï‚® Change ï‚® Cache Settings.... The window shown in Figure 4-113 will
appear.
Figure 4-113 Logical drive changes to cache settings
These settings have a large impact on the performance of the DS5000 storage subsystem
and on the availability of data. Be aware that performance and availability often conflict with
each other. If maximum performance is required, in most cases availability might have to be
compromised and vice versa.
The default settings are read and write cache for all logical drives, with cache mirroring to the
alternate controller for all write data. The write cache is only used if the battery for the
controller is fully charged. Read ahead is not normally used on the logical drives.
Read caching
Read caching allows read operations from the host to be stored in controller cache memory. If
a host requests data that is not in the cache, the controller reads the needed data blocks from
the disk and places them in the cache. Until the cache is flushed, any other requests for this
data are fulfilled with the cache data instead of initiating another read operation to the disk.
Write caching
The write caching parameter enables the storage subsystem to cache write data instead of
writing it directly to the disks. This can improve performance significantly, especially for
environments with random writes, such as databases. For sequential writes, the performance
gain varies with the size of the data written. If the logical drive is only used for read access, it
might improve overall performance to disable the write cache for this logical drive, and no
cache memory is reserved for the logical drive.
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