Re: Blocking I/O (was: Expensive op codes)

[Dave Murvin <davem@xxxxxxxx>]

>From what I read in the manuals (see "Sequential-Only Processing" in DB@ UDB 
>for
AS/400 Database Programming V4R4), a  News/400 article ("A Game Plan for
Performance" Sept. 1999) and various postings, the difference between SEQONLY 
and
NBRRCDS is as follows:

NBRRCDS controls the number of records that are moved as a unit between 
auxiliary
storage and main storage.  You should only be using this keyword for physical
files read sequentially and for input only logical files read sequentially where
the underlying physical file is ordered in the logical file sequence.  
Otherwise,
this can decrease performance due to buffers being flushed prematurely (read a
big block of records, process a record, flush the big block of records, read a
big block of records, etc.).

SEQONLY(*YES nnn) controls the number of records that are moved as a unit
between  main storage and your programs internal buffers.

My tests reading (READ op code)  the million+ record physical file did not show
much difference using the NBRRCDS keyword.  (341 records is a 64K block).

OVRDBF SEQONLY(*YES 341)
    Elapsed time 1:19, number of seconds 63
OVRDBF SEQONLY(*YES 341) NBRRCDS(341)
    Elapsed time 1:18, number of seconds 63

Same test with no overrides, using the default blocking of 4K buffer
    Elapsed time 2:44, number of seconds 112

The News/400 article also said that the reason that READE is slower is that the
compiler translates READE into QDBGETK (Get by Key) Machine Interface (MI)
program, which doesn't perform blocking.


Alan Campin wrote:

> <snip>
>
> As an aside here, do anybody understand the difference between "SEQ ONLY"
> and "NBRRCDS" on the OVRDBF command? I have got conflicting statement from
> IBM about this and the manuals don't seem clear. I thought I understood the
> SEQONLY and NBRRCDS but then got told it was the opposite of what I thought
> so not sure what to believe. Thanks.
>
> < big snip>

--
Dave Murvin
DRM Enterprises, Inc.
davem@drme.com


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