**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 copy = getDefaultSettings();
                copy->applyChanges(new_changes);
                String reason;
                if (!canUsePolymorphicParts(*copy, &reason) && !reason.empty())
                    throw Exception("Can't change settings. Reason: " + reason, ErrorCodes::NOT_IMPLEMENTED);
            }
        }
    }
    for (const auto & part : getDataPartsVectorForInternalUsage())
    {
        bool at_least_one_column_rest = false;
        for (const auto & column : part->getColumns())
        {
            if (!dropped_columns.contains(column.name))
            {
                at_least_one_column_rest = true;
                break;
            }
        }
        if (!at_least_one_column_rest)
        {
            std::string postfix;
            if (dropped_columns.size() > 1)
                postfix = "s";
            throw Exception(ErrorCodes::BAD_ARGUMENTS,
                "Cannot drop or clear column{} '{}', because all columns in part '{}' will be removed from disk. Empty parts are not allowed", postfix, boost::algorithm::join(dropped_columns, ", "), part->name);
        }
    }
}
void MergeTreeData::checkMutationIsPossible(const MutationCommands & , const Settings & ) const