RPG and BLOCK(*YES)

We have a file with 17,960,960 records.  We were doing some time trials on
BLOCK(*YES).  And came up with the following.  I have some questions at the
end.


Test 1:  I,F,blocked, no key.  Takes 1 minute 31 seconds
     FITHL01    IF   E             DISK    BLOCK(*YES)

     C                   DOW       NOT %EOF(ITHL01)
     C                   READ(E)   ITHL01
     C                   EndDo

     C                   EXSR      LRSUB

Test 2:  I,F,unblocked, no key.  Takes 1 minute 42 seconds
     FITHL01    IF   E             DISK

     C                   DOW       NOT %EOF(ITHL01)
     C                   READ(E)   ITHL01
     C                   EndDo

     C                   EXSR      LRSUB

Test 3:  I,F,unblocked, key.  Takes 44 minutes 53 seconds
     FITHL01    IF   E           K DISK

     C     *loval        setll     ithl01

     C                   DOW       NOT %EOF(ITHL01)
     C                   READ(E)   ITHL01
     C                   EndDo

     C                   EXSR      LRSUB

Test 4:  I,F,blocked, key.  Takes 8 minutes, 37 seconds.
Test 5:  I,P, no key.  Takes 1 minute 0 seconds.
Test 6:  I,P, key.  Takes 6 minutes 42 seconds.

Times came from some processing in *INZSR and LRSUB storing times into a
data area.

Summary.  Processing by key adds a significant amount to the time.
Blocking, or using a primary file, takes a lot of this sting out.  Explains
that message at the bottom of a compile with a I,P file:
                          M e s s a g e   S u m m a r y
 * QRG7086 Severity:  00   Number:    1
           Message . . . . :   RPG handles blocking function for file.

Here are my questions:
When should you use BLOCK(*YES)?
Is there an overhead to blocking?  For example if I am only reading a
couple of records, does it take overhead to load a huge chunk of data for a
couple of records?
Should you use it when doing random chains all over the file?
Should you use it when doing a SETLL/READ but only reading a few records?
If not, then when is the cut over from when to use it and not to use it?

Rob Berendt

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