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Status Future consideration
Created by Guest
Created on Dec 11, 2023

Check and Remove existing IR bitmap when Preparing for Secondary Migration to avoid full copy during resync to new secondary

The CSM secondary migration feature uses the IR bitmap on the primary system to perform an incremental resync to a new secondary system. Actually, when CSM perform the Prepare for Secondary Migration step, it will request the DS8000 to set the IR bitmap with the NoInit option. However, there is no error when a IR bitmap already exists and there is no way to query how many OOS tracks are in the IR bitmap. That means existing IR bitmaps on the primaries are simply picked up by the DS8000 during the CSM request and may result in full copy.

Management solutions such as GDPS may set the IR bitmap already as part of their 3 DR site solutions, but existing IR bitmap is not validated by CSM. Furthermore CSM does not cleanup up the IR bitmap when it was set once by the Prepare for secondary Migration step. So if the CSM Secondary Migration sequence is aborted after the Preparation step, the IR bitmap will remain set all the time until a real migration is being performed, therefore accumulating OOS tracks until the next migration attempt is started and potentially resyncing much more data than required during the migration step.

While GDPS has a script to remove the IR bitmap, CSM has currently no option to remove the IR bitmap again from the old pairs and it is not easy to remove it with DSCLI either, since the -incrementalresync disable option is only allowed with the mkpprc command. mkpprc however is rejected when the pairs are not simplex, and only accepted when pairs are in GC mode and the mkpprc -type mmir is used to convert them to MM pairs without suspend.

CSM needs to implement a mechanism to warn for existing IR bitmaps at a minimum, in order to alert users that a full copy may occur when progressing with the secondary migration.

Optimally CSM implements a mechanism to clear existing IR bitmaps when desired (or enforces that the HW re-initializes them as empty IR bitmaps), since existing (filled) IR bitmaps interfere with the secondary migration solution independently why they were setup previously. 

Idea priority High