How the size of an extent (in blocks) is determined

Prasanna Srinivas D · New User Joined: 12 Oct 2007 Posts: 9 Location: chennai

hi

i would like to know how the size of an extent (in blocks) is determined. I did create some data sets with various block sizes...

Block Size(in bytes) -- Extent Size(in blocks)

16 -- 86
40 -- 78
800 -- 39

The DASD used here is 3990, OS is zOS and system is z800. The record size in all the cases where 8 bytes except for 800 where a record size of 80 bytes was used. The record format used was fixed blocked. As seen above, the extent size in bytes is not the same in every case. Then, on what basis is the extent size fixed?

enrico-sorichetti · Posted: Mon Oct 15, 2007 2:04 am

Remember that on dasd the minimum allocation unit is the TRACK, so
even if You ask for 1 800 bytes block, You will get one track worth of bytes

CICS Guy · Posted: Mon Oct 15, 2007 2:26 am

Also remember that between each block, there is an inter-block gap, which is kind of large...If your block size were full track, there would be only the one gap....smaller block sizes incur more inter-block gaps.....That is why the larger the blocksize, the smaller the actual number of tracks (extents) are required....