Hey dude,
Thought this might be interesting to someone.
I had a BW on HANA system table (RSBERRORLOG) which was 6bn rows, and those don't get partitioned by the BW migration tools. So I partitioned it by HASH(KEY1, KEY2 etc.) with 10 partitions, and was able to complete the migration.
Then I cleared down RSBERRORLOG using RSBM_ERRORLOG_DELETE. After that, I ran a merge partitions to set the system back as it should be. This merge took a long time and saved a lot of memory.
John