Dell PowerVault MD3420 - Manual (Page 327)

Cache Read Prefetch
The cacheReadPrefetch parameter enables the RAID controller module to copy additional data blocks
into cache while the RAID controller module reads and copies data blocks that are requested by the host
from the physical disk 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.
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.
Set Virtual Disk Copy
Description
This command defines the properties for a virtual disk copy pair.
Syntax
set virtualDiskCopy target [targetName] [source
[sourceName]]
copyPriority=(highest | high | medium | low | lowest)
targetReadOnlyEnabled=(TRUE | FALSE)
copyType=(online | offline)
Parameters
Parameter Description
target Specifies the name of the target virtual disk for
which to define properties. You must put brackets
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