RDF System Management Manual for H-Series RVUs (RDF 1.8)

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 information is missing on \C.
Suppose that the image trail files are relatively small, such that the audit record at MAT 10,
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 information (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 figure out 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 estimate the RETAINCOUNT value to specify 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 two image
trail files on disk (the one the receiver is currently writing to and the previous two). On the
backup system that is further ahead in its RDF processing, assume 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. Also assume that there are no long-running transactions that span
from file AA000010 to file AA000013. Files AA000010 through AA000012 might no longer needed,
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 24 hours apart, the triple contingency protocol will work
successfully.
Similarly, 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 approximately 20 rollovers
occur every 24 hours, then you should set the RETAINCOUNT to 31 on both backup systems.
Where Issued
Primary system only.
Security Restrictions
None.
RDF State Requirements
None.
Usage Guidelines
The SET PURGER command enters the parameter values specified for the purger in this command
into the RDF configuration table in memory. This table serves as an input buffer only, and so
RDFCOM Commands 213