Re: NBRRCDS and BLOCK(*YES)

[Ken Sims <mr2087@xxxxxxxxx>]

Hi Uma -

> 8) Now my worry is, where ever I refer to NBRRCDS details it mentions that
> " if the file is processed by a keyed order, then the physical order of
> data should be same as the keyed order else NBRRCDS will worse the
> performance"
What NBRRCDS does is tell the operating system how many records to bring
from the disk at one time.  This is not the same as blocking for two reasons:

1. Blocking involves a private memory area specific to the open data path
being used (and therefore cannot be used with update files, otherwise every
record in the block would have to be locked); NBRRCDS just pages in the
records in.  Every job that wants any of these records benefits by
this.  If updating, only the record actually being read is locked.  Records
could be paged back out before the program actually reads them.

2. Blocking is done is in sequence that the records are being accessed:
keyed or arrival.  NBRRCDS is always done in arrival sequence.  That's why
the warning.

For example, let's say that NBRRCDS is 20.  You are reading by key.  The
first key happens to point to record 1.  Since NBRRCDS is 20, the read
actually brings records 1-20 from the disk into memory, and puts record
number 1 in the program's buffer.  If the second key points to any of
records 2-20, then the record is already in memory and just has to be put
in the program's buffer.  However if the second key happens to point to
record 100, then the record brings in records 100-119 and puts record 100
in the program's buffer.

IF the system has enough memory and few enough other jobs running, records
2-20 may still be hanging around in memory when the program finally reaches
the key that points to one of them.  If so, then NBRRCDS helped.  If not,
then NBRRCDS hurt because records were brought into memory unnecessarily.

If reading by key, the more the arrival sequence matches the key sequence,
the more help from NBRRCDS.  NBRRCDS will always help with arrival sequence
unless the system is so active that the records are paged back out between
one read and the next.

Ken
http://www.ke9nr.net/
Opinions expressed are my own and do not necessarily represent the views of
my employer or anyone in their right mind.