Surface mode is recommended with the 7906M disk drive. Due to the physical structure of the disk drive, there is no advantage in access time when cylinder mode is used. There is, however, a significant disadvantage to using cylinder mode. Cylinder mode requires that LUs span all four surfaces of the drive, two of which are removable. When the removable platter (surfaces 0 and 1) is removed and replaced with another platter, the LUs (which cover all four surfaces) will contain mixed data from the old fixed platter and the new removable platter. The results may be interesting but not usable. Surface mode avoids such problems, since LUs in surface mode do not cross surface boundaries. If you replace the top platter, the information on the bottom platter is undisturbed. Further, the VCP ROMs will only boot a system from a 7906M if it is in surface mode.
LU integrity is also preserved in shortened cylinder mode, because the cylinders include only the two sides of either the fixed or the removable platter. Better performance can be expected in surface mode than in shortened cylinder mode, but if your need is to read data that has been stored in shortened cylinder mode, you can configure the drive for that purpose. To configure the disk in shortened cylinder mode, use device type 34B (DT:34B in the DVT statement), and specify driver DDM30 (%DDM30).
While up to 31 LUs may be defined for one 7906M disk drive, a good plan is to define no more than six. The total number of tracks on a 7906M is 1644. Of these, you should use approximately 1600 to make up your LUs and distribute the remaining tracks over the LUs as spares. The FORMT disk utility program lets you use spare tracks to replace bad tracks. FORMT is described in the RTEA Utilities Reference manual.
Using the Worksheet
The 7906M Disk Configuration Worksheet, along with an example of its use, is shown at the end of this section. The worksheet contains a schematic diagram of the four surfaces of the 7906M drive. Use the worksheet to map the drive in such a way that the tracks and spares are contiguous for each LU and the tracks and spares of one LU do not overlap those of another LU. Using the rough rule of thumb of allowing 2 or 3 spare tracks per 100 data tracks, allocate the tracks to the LUs as desired and fill in the driver parameters to match. You can then use this information to fill in the driver parameter portion of the IFT/DVT Worksheet.
Configuring a Disk Subsystem C 27