half track blocking

[claudeb]

I don't understand the mechanics behind "half track blocking" strategy. I know that the goal is to efficiently use the space on disk.
on a 3390 disk, there are 15 tracks per cylinder. Each track holds 56664 bytes.
1- what is this magical number 27,920 block size ?
2- what is a DASD ?
3- 3332 cylinders = 2.832 Gb is this true ?
Thanks a lot

 

[slade]

Hi Claude,

The half track blocking concept is based on 2 facts:

The max blksize limit (until LBI for tape) is 32K.
DASD track capacity has now exceeded 32K but not 64K.

The reasoning is: blocking up the data to half track will utilize the space available maximally while staying under the 32K blksize limit.

While track capacity stayed under 32K full track processing was the way to go. As track capacities grow (if there are no "scientific" limitations), we'll probably see 1/3 or 1/4 track blocking.

DASD = D irect
A ccess
S torage
D evice

I don't have the manual/card available, but I'd venture that 2.832 Gb sounds like the capacity of a unit of 6 or 8 drives. Can't say for sure w/o the manual.

HTH, Jack.

 

[claudeb]

Thanks Slade. Once again you were there to help.
This site is helpful too.

http://programmingstuff.tripod.com/blksize.htm