Dell PowerVault MD3620f Command Line Interface Manual page 157

disks into cache. This action increases the chance that a future request for
data can be fulfilled from cache. Cache read prefetch is important for
multimedia applications that use sequential data transfers. The configuration
settings for the storage array that you use determine the number of additional
data blocks that the RAID controller module reads into cache. Valid values for
the cacheReadPrefetch parameter are TRUE or FALSE.
You do not need to enter a value for the cacheReadPrefetch parameter or the
segmentSize parameter. If you do not enter a value, the RAID controller
module firmware uses the usageHint parameter with fileSystem as the default
value. Entering a value for the usageHint parameter and a value for the
cacheReadPrefetch parameter or a value for the segmentSize parameter does
not cause an error. The value that you enter for the cacheReadPrefetch
parameter or the segmentSize parameter takes priority over the value for the
usageHint parameter.
Segment Size
The size of a segment determines how many data blocks that the RAID
controller module writes on a single physical disk in a virtual disk before
writing data on the next physical disk. Each data block stores 512 bytes of
data. A data block is the smallest unit of storage. The size of a segment
determines how many data blocks that it contains. For example, an 8 KB
segment holds 16 data blocks. A 64 KB segment holds 128 data blocks.
When you enter a value for the segment size, the value is checked against the
supported values that are provided by the RAID controller module at run
time. If the value that you entered is not valid, the RAID controller module
returns a list of valid values. Using a single physical disk for a single request
leaves other physical disks available to simultaneously service other requests.
If the virtual disk is in an environment where a single user is transferring large
units of data (such as multimedia), performance is maximized when a single
data transfer request is serviced with a single data stripe. A data stripe is the
segment size that is multiplied by the number of physical disks in the disk
group that are used for data transfers. In this case, multiple physical disks are
used for the same request, but each physical disk is accessed only once.
For optimal performance in a multiuser database or file system storage
environment, set your segment size to minimize the number of physical disks
that are required to satisfy a data transfer request.
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