RDF/IMP and IMPX System Management Manual (RDF 1.3+)

Entering RDFCOM Commands
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
8-60
Command Overview
RETAINCOUNT num
specifies how many image trail files must be retained on disk for each image trail
(including the image trail file currently in use). num must be within the range 2 to
5000. If you do not explicitly set this configuration parameter, the default value is 2.
This configuration parameter is only relevant for the triple contingency feature;
otherwise, this parameter should be left at its default value.
The importance of this parameter is as follows. If you lose the primary system, the
triple contingency protocol will work only if all of the missing audit information at the
backup system which is further behind in its RDF processing is still on disk on the other
backup system.
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 as follows:
\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