Both DefaultLoadBalancer and ChaoticLoadBalancer won't balance if there are outstanding migrations.
In this instance, we have offline tables from previous CI runs. one of these tables had outstanding migrations
Later messages show these tablets being unloaded successfully. However, since the table is offline they never get loaded on the new tablet server. This means they never leave the queue, so balancing stops.
As an added complication, this last set of migrations was added after the table was already offline. I think this is because there had been unhosted tablets which caused a bunch of contention around when balancing would finally happen.
A few needed changes:
- If the balancer isn't going to balance it needs a log message saying so. Ideally, this message should also include information about the outstanding migrations that are blocking it.
- the Migration cleanup thread should look for migrations involving offline tables and clear them (I'd prefer this to trying to have the balancer figure out if a table is offline or online)
- When we offline a table, we should probably clear migrations related to that table. This isn't strictly necessary if the cleanup thread will get them eventually, but it would speed things up.
- migration state is only stored in Master memory, failing over to a different master will force recalculation which will not include offline tables.
- if for some reason you can't handle a failure of the current master, bringing the involved table back online (which might mean all offline tables) will allow migrations to resume. the table must remain online until there are no longer migrations involving it.
- I think that if you clone the offline table and then delete the original, that will clear the outstanding migrations related to it. I did not test this, because the above two options are much better.
The latter option will cause considerably more churn, especially if the offline table isn't actually providing utility.