Count Key Data; Fixed Block (Fb); Logical Volumes - IBM TotalStorage DS6000 Introduction And Planning Manual

Although sharing resources in the storage unit has advantages for storage
administration and resource sharing, there are additional implications for workload
planning. The benefit of sharing is that a larger resource pool (for example, disk
drives or cache) is available for critical applications. However, you must ensure that
uncontrolled or unpredictable applications do not interfere with critical work. This
requires the same kind of workload planning that you use when you mix various
types of work on a server.
If your workload is critical, consider isolating it from other workloads. To isolate the
workloads, place the data as follows:
v On separate RAID disk groups. S/390 or zSeries and open-systems data are
v On separate device adapters.
v In separate storage unit clusters, which isolates use of memory buses,

Count key data

In count-key-data (CKD) disk data architecture, the data field stores the user data.
Because data records can be variable in length, in CKD they all have an associated
count field that indicates the user data record size. The key field enables a
hardware search on a key. The commands used in the CKD architecture for
managing the data and the storage devices are called channel command words
(CCWs).

Fixed block (FB)

In fixed block (FB) architecture, the data (the logical volumes) are mapped over
fixed-size blocks or sectors.
With an FB architecture, the location of any block can be calculated to retrieve that
block. This architecture uses tracks and cylinders. On a physical disk there are
multiple blocks per track, and a cylinder is the group of tracks that exists under the
disk heads at one point in time without performing a seek operation.

Logical volumes

A logical volume is the storage medium that is associated with a logical disk. It
typically resides on one or more hard disk drives.
For the storage unit, the logical volumes are defined at logical configuration time.
For count-key-data (CKD) servers, the logical volume size is defined by the device
emulation mode and model. For fixed block (FB) hosts, the size is 100 MB to the
maximum capacity of a rank.
A logical device that has a non-removable media has one and only one associated
logical volume. A logical volume is composed of one or more extents. Each extent
is associated with a contiguous range of addressable data units on the logical
volume.
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Introduction and Planning Guide
automatically placed on separate arrays, which reduces the contention for disk
use.
microprocessors, and cache resources. Before you make this decision, verify that
the isolation of your data to a single cluster provides adequate data access
performance for your application.