**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 :)

{
    std::lock_guard lock(storage.currently_submerging_emerging_mutex);
    for (const auto & part : submerging_parts)
    {
        if (!storage.currently_submerging_big_parts.contains(part))
        {
            LOG_ERROR(log, "currently_submerging_big_parts doesn't contain part {} to erase. This is a bug", part->name);
            assert(false);
        }
        else
            storage.currently_submerging_big_parts.erase(part);
    }
    storage.currently_emerging_big_parts.erase(emerging_part_name);
}
}