Symptoms: all threads in BackgroundScheduledPool busy, lot of tasks in replication queue, none of the tasks is executing. 

![Screenshot from 2021-04-16 16-21-44_](https://user-images.githubusercontent.com/1549571/115193796-e767d600-a0ec-11eb-887c-5c9c2412e50c.png)

Related to #16895

Possible workarounds (poor options): increasing the background_schedule_pool_size to huge values, or restart server every time when that happens etc. - 

}
            }
            return false;
        }), parts.end());
    if (parts.empty())
    {
        String no_parts_to_move_message;
        if (moving_part)
            no_parts_to_move_message = "Part '" + partition_id + "' is already on volume '" + volume->getName() + "'";
        else
            no_parts_to_move_message = "All parts of partition '" + partition_id + "' are already on volume '" + volume->getName() + "'";
        throw Exception(no_parts_to_move_message, ErrorCodes::UNKNOWN_DISK);
    }
    if (!movePartsToSpace(parts, std::static_pointer_cast<Space>(volume)))
        throw Exception("Cannot move parts because moves are manually disabled", ErrorCodes::ABORTED);
}
void MergeTreeData::movePartitionToShard(const ASTPtr & / bool / const String & / ContextPtr /
{
    throw Exception(ErrorCodes::NOT_IMPLEMENTED, "MOVE PARTITION TO SHARD is not supported by storage {}", getName());
}
void MergeTreeData::fetchPartition(
    const ASTPtr & /
    const StorageMetadataPtr & ,
    const String & /
    bool /
    ContextPtr /
{
    throw Exception(ErrorCodes::NOT_IMPLEMENTED, "FETCH PARTITION is not supported by storage {}", getName());
}
Pipe MergeTreeData::alterPartition(