**Describe the bug**

Since we upgraded to 20.10.3.30, one of our clickhouse cluster takes much more time to start. (Up to 50 minutes, and before it took ~3 minutes)
The log file is filled with
>  <Debug> DiskLocal: Reserving 1.00 MiB on disk `default`, having unreserved 16.90 TiB

Additionally, the memory consumption increases, linearly, a lot (up to arround 230 G for us) and then drop back to normal values once clickhouse finished starting up. During that loading phase, a single CPU core is used at 100%.
If that memory is not available for the machine, clickhouse gets OOMed by the kernel.


**How to reproduce**
* Issue appeared after we upgraded to ClickHouse server version 20.10.3.30

* How could I find wich of our tables may be responsible for that ?


**Expected behavior**
A fast startup :)

/
    /
    const auto time_now = std::chrono::system_clock::now();
    part_log_elem.event_time = time_in_seconds(time_now);
    part_log_elem.event_time_microseconds = time_in_microseconds(time_now);
    /
    part_log_elem.duration_ms = elapsed_ns / 1000000;
    part_log_elem.database_name = table_id.database_name;
    part_log_elem.table_name = table_id.table_name;
    part_log_elem.partition_id = MergeTreePartInfo::fromPartName(new_part_name, format_version).partition_id;
    part_log_elem.part_name = new_part_name;
    if (result_part)
    {
        part_log_elem.disk_name = result_part->data_part_storage->getDiskName();
        part_log_elem.path_on_disk = result_part->data_part_storage->getFullPath();
        part_log_elem.bytes_compressed_on_disk = result_part->getBytesOnDisk();
        part_log_elem.rows = result_part->rows_count;
        part_log_elem.part_type = result_part->getType();
    }
    part_log_elem.source_part_names.reserve(source_parts.size());
    for (const auto & source_part : source_parts)
        part_log_elem.source_part_names.push_back(source_part->name);
    if (merge_entry)
    {
        part_log_elem.rows_read = (*merge_entry)->rows_read;
        part_log_elem.bytes_read_uncompressed = (*merge_entry)->bytes_read_uncompressed;
        part_log_elem.rows = (*merge_entry)->rows_written;
        part_log_elem.bytes_uncompressed = (*merge_entry)->bytes_written_uncompressed;
        part_log_elem.peak_memory_usage = (*merge_entry)->memory_tracker.getPeak();
    }