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. - 

if (relative_data_path.empty())
        throw Exception("MergeTree storages require data path", ErrorCodes::INCORRECT_FILE_NAME);
    
    if (!attach)
        settings->sanityCheck(getContext()->getSettingsRef());
    MergeTreeDataFormatVersion min_format_version(0);
    if (!date_column_name.empty())
    {
        try
        {
            checkPartitionKeyAndInitMinMax(metadata_.partition_key);
            setProperties(metadata_, metadata_, attach);
            if (minmax_idx_date_column_pos == -1)
                throw Exception("Could not find Date column", ErrorCodes::BAD_TYPE_OF_FIELD);
        }
        catch (Exception & e)
        {
            
            e.addMessage("(while initializing MergeTree partition key from date column " + backQuote(date_column_name) + ")");
            throw;
        }
    }
    else
    {
        is_custom_partitioned = true;
        checkPartitionKeyAndInitMinMax(metadata_.partition_key);
        min_format_version = MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING;
    }
    setProperties(metadata_, metadata_, attach);