IBM System Storage DS3000, Programming Manual

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Cache Read Prefetch
The
cacheReadPrefetch
command lets the controller copy additional data blocks into cache while the
controller reads and copies data blocks that are requested by the host from the disk drives 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 subsystem that you use determine the number of additional data blocks that the
controller 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 controller 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 controller writes on a single disk drive
in a logical drive before writing data on the next disk drive. 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 controller at run time. If the value that you entered is not valid, the controller returns a
list of valid values. Using a single disk drive for a single request leaves other disk drives available to
simultaneously service other requests.
If the logical drive 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 disk drives in the subsystem
that are used for data transfers.) In this case, multiple disk drives are used for the same request, but each
disk drive 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 disk drives that are required to satisfy a data transfer request.
Security Type
The
securityType
parameter is valid for drives that are capable of full disk encryption (FDE). With FDE,
the controller firmware can create a key and activate the Full Disk Encryption feature. The Full Disk
Encryption feature encrypts data as the data is written to the drive and decrypts the data as the data is
read from the drive. Without the key created by the controller, the data written to the drive is
inaccessible.
Before you can set the
securityType
parameter to capable or enabled , you must create a storage subsystem
security key. Use the
create storageSubsystem securityKey
command to create a storage subsystem
security key. These commands are related to the security key:
v create storageSubsystem securityKey
v enable subsystem [subsystemName] security
v export storageSubsystem securityKey
v import storageSubsystem securityKey
v set storageSubsystem securityKey
v start secureErase (drive | drives)
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IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide