**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 :)

prev_it = curr_it;
                ++curr_it;
            }
            else if ((*prev_it)->contains(**curr_it))
            {
                auto next = std::next(curr_it);
                deactivate_part(curr_it);
                curr_it = next;
            }
            else
            {
                ++prev_it;
                ++curr_it;
            }
        }
    }
    resetObjectColumnsFromActiveParts(part_lock);
    calculateColumnAndSecondaryIndexSizesImpl();
    LOG_DEBUG(log, "Loaded data parts ({} items)", data_parts_indexes.size());
}
static bool isOldPartDirectory(const DiskPtr & disk, const String & directory_path, time_t threshold)
{
    if (disk->getLastModified(directory_path).epochTime() >= threshold)
        return false;
    for (auto it = disk->iterateDirectory(directory_path); it->isValid(); it->next())
        if (disk->getLastModified(it->path()).epochTime() >= threshold)
            return false;
    return true;
}
size_t MergeTreeData::clearOldTemporaryDirectories(size_t custom_directories_lifetime_seconds, const NameSet & valid_prefixes)