bwp@utgpu.UUCP (08/24/87)
I'd like to add a couple of comments to the recent volume shadowing discussion. Paul Clayton in a recent message said that dismounting a disk from a shadow set will leave open ISAM (and possibly other types of) files in an inconsistent state. In general, that is the case. With careful application design this problem can be avoided. A combination of: - not using the RMS "deferred write" option - waiting for active writes to complete - allowing no further writes to be initiated - allowing all non-atomic transactions be completed and allowing no new non-atomic transactions to begin will leave a prolog 3 ISAM file consistent on disk (with the file still open for writes) both from an RMS and application viewpoint. Synchronizing this disk quiescense can be accomplished fairly easily through the use of a cluster-wide lock held CR with a blocking AST by all writers. If a process tries to raise CR to EX then all the writers will get the AST which can indicate to them that the database is to be quiesced. The writers indicate that they have quiesced by giving up the CR lock they hold. When the process that requested the EX lock gets it, the shadow set is ok to be split up. The writers should requeue for the lock CR so that they can tell when to proceed with more writes. The holder of the EX lock should give it up when the dismount has completed. People considering using the RMS Common Journalling Facility (CJF) should be aware that under some circumstances CJF will turn on deferred writes to user data files even though this option is not explicitly used by the application code. This "feature" of the CJF, at least in its initial release, will not be avoidable. Corot Reason, PROMIS Systems Corp., Toronto (416) 364-5361x428