RDF System Management Manual for J-series and H-series RVUs (RDF 1.10)

The RETAINCOUNT Configuration Parameter
The purger RETAINCOUNT parameter specifies how many image trail files (including the one
currently in use) must be retained on disk for each image trail. The default value for this parameter
is two.
This parameter is important because if you lose the primary system, the triple contingency protocol
will work only if one of the backup systems has retained all of the audit records that the other is
missing.
For example, assume that you have lost the original primary system (\A), you have successfully
completed a takeover on both backup systems (\B and \C), and the MAT positions displayed by
the respective 735 messages are:
\B: 735 LAST MAT POSITION: Sno 10, RBA 100500000
\C: 735 LAST MAT POSITION: Sno 10, RBA 100000000
500 kilobytes of audit records is missing on \C.
Suppose that the image trail files are relatively small, such that the audit record at MAT 10, RBA
100000010 was placed at the start of image trail file AA000025 on \B. If the purger on \B is
allowed to purge AA000025 before the takeovers occur, the triple contingency protocol will fail
because \C is missing some of the purged audit records (Sno 10, RBA 100000010 through Sno
10, RBA 100500000).
The RETAINCOUNT parameter is designed to prevent such a situation, although it is up to you to
set this value correctly.
You must determine how much time disparity to allow for in the event that one receiver falls behind
the other. Such a disparity would occur, for example, if the communications lines between the
primary system and one of the backup systems were to go down for some period of time. The
RETAINCOUNT parameter must be such that no image trail files that might be needed for triple
contingency are ever purged.
The best way to do determine the appropriate RETAINCOUNT value is to pick an acceptable time
differential such as 24 hours, 36 hours, or 48 hours, determine how many image trail rollovers
typically occur within that amount of time, and then set the RETAINCOUNT parameter to that
number of files.
For example, if you believe the two receiver processes will never be more than 36 hours apart in
their RDF processing and your image trail file sizes are such that rollovers occur only once every
24 hours, then you would be safe specifying a RETAINCOUNT of three for both backup systems.
In that situation, the purger process on both backup systems will always keep at least three image
trail files on disk (the one the receiver is currently writing to and the previous two). Assume, on the
backup system which is further ahead in its RDF processing, that files AA000010, AA000011,
and AA000012 are on disk, the receiver rolls over to file AA000013, and all updaters have just
begun reading file AA000013. Files AA000010 through AA000012 might be considered
expendable (and therefore be eligible for purging), but, because the RETAINCOUNT is set to three,
the purger process can only purge AA000010 (it must keep AA000011 and AA000012 on disk).
Thus, as long as the RTD times of the extractors on the two backup systems are less than 36 hours
apart, the triple contingency protocol will work successfully.
Similarly, if you believe the two receiver processes will never be more than 48 hours apart in their
RDF processing and your image trail file sizes are such that approximately 20 rollovers occur every
24 hours, then you should set the RETAINCOUNT to 40 on both backup systems.
You set the RETAINCOUNT parameter by issuing this command:
SET PURGER RETAINCOUNT num
where num is a number within the range 2 through 5000. The default is 2.
You can alter the RETAINCOUNT only when RDF is stopped.
You alter the RETAINCOUNT parameter by issuing this command:
ALTER PURGER RETAINCOUNT num
262 Triple Contingency