IBM System Storage DS3000, Руководство по программированию

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The
drive
parameter supports both high-capacity and low-capacity drive expansion enclosures. A
high-capacity drive expansion enclosure has drawers that hold the drives. The drawers slide out of the
drive expansion enclosure to provide access to the drives. A low-capacity drive expansion enclosure does
not have drawers. For a high-capacity drive expansion enclosure, you must specify the identifier (ID) of
the drive expansion enclosure, the ID of the drawer, and the ID of the slot in which a drive resides. For a
low-capacity drive expansion enclosure, you need to specify the ID of the drive expansion enclosure and
the ID of the slot in which a drive resides. For a low-capacity drive expansion enclosure, an alternative
method for identifying a location for a drive is to specify the ID of the drive expansion enclosure, set the
ID of the drawer to 0, and specify the ID of the slot in which a drive resides.
If you attempt to recover a logical drive using the drive parameter or the drives parameter and the
drives are in an unassigned state, the controller automatically creates a new subsystem. Use the
newLogicalDriveGroup
parameter to specify a name for the new subsystem.
You can use any combination of alphanumeric characters, underscore (_), hyphen (-), and pound (#) for
the names. Names can have a maximum of 30 characters.
The
owner
parameter defines which controller owns the logical drive. The preferred controller ownership
of a logical drive is the controller that currently owns the subsystem.
Preallocating Storage Capacity
With the
dssPreallocate
parameter, you can assign space in a logical drive for storing information that is
used to rebuild a logical drive. When you set the
dssPreallocate
parameter to
TRUE
, the storage space
allocation logic in the controller firmware preallocates space for future segment size changes. The
preallocated space is the maximum allowable segment size. The
dssPreallocate
parameter is necessary
for recovering logical drive configurations that are not retrievable from the controller database. To turn
off the preallocation capability, set
dssPreallocate
to
FALSE
.
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.
Cache Read Prefetch
Cache read prefetch lets the controller copy additional data blocks into cache while the controller reads
and copies data blocks that are requested by the host from 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
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IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide