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. - 

throw Exception("Part " + part->name + " fetched on wrong disk " + part->volume->getDisk()->getName(), ErrorCodes::LOGICAL_ERROR);
        replaced_disk->removeFileIfExists(replaced_part_path);
        replaced_disk->moveDirectory(part->getFullRelativePath(), replaced_part_path);
    }
    catch (const Exception & e)
    {
        
        
        if (e.code() == ErrorCodes::DIRECTORY_ALREADY_EXISTS)
            return false;
        throw;
    }
    catch (...)
    {
        write_part_log(ExecutionStatus::fromCurrentException());
        throw;
    }
    ProfileEvents::increment(ProfileEvents::ReplicatedPartFetches);
    LOG_DEBUG(log, "Fetched part {} from {}", part_name, source_replica_path);
    return true;
}
void StorageReplicatedMergeTree::startup()
{
    if (is_readonly)
        return;
    try
    {
        queue.initialize(getDataParts());
        InterserverIOEndpointPtr data_parts_exchange_ptr = std::make_shared<DataPartsExchange::Service>(*this);
        [[maybe_unused]] auto prev_ptr = std::atomic_exchange(&data_parts_exchange_endpoint, data_parts_exchange_ptr);