Disk formats

In order to increase the net storage capacity and to improve the effective data transfer rate, new disk formats have been added. The following three criteria of a disk format are relevant for DMS:

• PAM key
  Disks with a PAM key (K disks) / disks without a PAM key (NK disks)

• Minimum allocation unit (min. AU) (minimum file size) 6 Kbytes / 8 Kbytes / 64 Kbytes

• Minimum transfer unit between disk and main memory 2 Kbytes / 4 Kbytes

The following disk formats must be differentiated from the viewpoint of DMS:

• K disk

• NK disk

• NK2 disk

• NK4 disk

With respect to allocation, a Net-Storage volume behaves like an NK2 disk with the minimum allocation unit of 8 KB.

K disk

Each 2-Kbyte data block on the disk is accompanied by a field outside the block reserved for block-specific management data, the so-called PAM key (16 bytes).
The information stored in the PAM key is typically evaluated by the DMS access methods SAM and ISAM; it is not required by the UPAM access method (unless the user part of the PAM key is accessed by users themselves). The minimum allocation unit is 6 Kbytes. K disks can be used to store files of any format. K disks are supported by all previously released BS2000 versions.

NK disk

NK (Non-Key) disks are formatted without a PAM key. Block-specific management information must either be maintained in the data block itself or dropped entirely. The term “NK disk” is a generic term for NK2 and NK4 disks.

NK2 disk

The minimum transfer unit between the disk and main memory is 2 Kbytes. In other words 2 Kbytes of data or a multiple thereof is transferred with each read/write operation. For NK2 disks, a further distinction is made on the basis of the minimum allocation unit.
There are disks with a minimum AU of 6 Kbytes, 8 Kbytes, and 64 Kbytes, respectively. The disks with a minimum AU of 8 or 64 Kbytes can be used for both NK2 and NK4 files.

NK4 disk

The minimum transfer unit between the disk and main memory is 4 Kbytes. In other words 4 Kbytes of data or a multiple thereof is transferred with each read/write operation. The minimum allocation unit is 8 Kbytes or 64 Kbytes. Only NK4 files can reside on NK4 disks.

The table below shows the various disk designations.

Designation of disk	PAM key K = Key NK= Non-Key	Minimum allocation unit (min. AU)	Minimum transfer unit (min. TU)
K disk	K	6 Kbytes	2 Kbytes
NK2 disk (6K) 1) = NK2 disk with min. AU of 6 Kbytes	NK	6 Kbytes	2 Kbytes
NK2 disk (8K) = NK2 disk with min. AU of 8 Kbytes	NK	8 Kbytes	2 Kbytes
NK2 disk (64K) = NK2 disk with min. AU of 64 Kbytes	NK	64 Kbytes	2 Kbytes
NK4 disk (8K) = NK4 disk with min. AU of 8 Kbytes	NK	8 Kbytes	4 Kbytes
NK4 disk (64K) = NK4 disk with min. AU of 64 Kbytes	NK	64 Kbytes	4 Kbytes

Table 38: Overview of various designations/formats for disks

1)	The minimum allocation unit is only indicated in the designation of the disk if it isrelevant for the described case. Otherwise, the terms NK2 disk and NK4 disk are used.

Notes

• The disk format is defined by systems support.

• The disk format in an SF pubset or volume set is homogeneous. For each disk format, there is also a corresponding pubset format.

• A user can obtain information on the formatting of his or her pubset with the SHOW-MASTER-CATALOG-ENTRY command or the STAMCE macro (on the program level).

• Only the K and NK2 formats with a minimum allocation unit of 6 Kbytes are supported for private disks.