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

Triple Contingency
Compaq NonStop™ RDF/IMP and IMPX System Management Manual—522204-001
10-4
The RETAINCOUNT Configuration Parameter
The RETAINCOUNT Configuration Parameter
The RETAINCOUNT purger process configuration parameter specifies how many
image trail files (including the one currently in use) must be retained on disk for each
image trail. The default for this parameter is two.
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 do determine 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 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