**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 :)

{
    auto lock = lockParts();
    for (auto original_active_part : getDataPartsStateRange(DataPartState::Active)) 
    {
        if (part_copy->name == original_active_part->name)
        {
            auto active_part_it = data_parts_by_info.find(original_active_part->info);
            if (active_part_it == data_parts_by_info.end())
                throw Exception("Cannot swap part '" + part_copy->name + "', no such active part.", ErrorCodes::NO_SUCH_DATA_PART);
            
            
            original_active_part->force_keep_shared_data = false;
            if (original_active_part->data_part_storage->supportZeroCopyReplication() &&
                part_copy->data_part_storage->supportZeroCopyReplication() &&
                original_active_part->data_part_storage->getUniqueId() == part_copy->data_part_storage->getUniqueId())
            {