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## Problem Since #9916 , the chaos code is actively fighting the optimizer: tenants tend to be attached in their preferred AZ, so most chaos migrations were moving them to a non-preferred AZ. ## Summary of changes - When picking migrations, prefer to migrate things _toward_ their preferred AZ when possible. Then pick shards to move the other way when necessary. The resulting behavior should be an alternating "back and forth" where the chaos code migrates thiings away from home, and then migrates them back on the next iteration. The side effect will be that the chaos code actively helps to push things into their home AZ. That's not contrary to its purpose though: we mainly just want it to continuously migrate things to exercise migration+notification code.