**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 :)

{
            pool.scheduleOrThrowOnError([&, thread]
            {
                while (true)
                {
                    std::pair<String, DiskPtr> thread_part;
                    {
                        const std::lock_guard lock{part_select_mutex};
                        if (remaining_thread_parts.empty())
                            return;
                        
                        auto thread_idx = thread;
                        if (threads_parts[thread].empty())
                        {
                            
                            std::uniform_int_distribution<size_t> distribution(0, remaining_thread_parts.size() - 1);
                            auto it = remaining_thread_parts.begin();
                            std::advance(it, distribution(thread_local_rng));
                            thread_idx = *it;
                        }
                        auto & thread_parts = threads_parts[thread_idx];
                        thread_part = thread_parts.back();
                        thread_parts.pop_back();
                        if (thread_parts.empty())
                            remaining_thread_parts.erase(thread_idx);
                    }
                    load_part(thread_part.first, thread_part.second);
                }
            });
        }