**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 :)

{
    MutationCommands commands = getFirstAlterMutationCommandsForPart(part);
    AlterConversions result{};
    for (const auto & command : commands)
        /
        /
        /
        if (command.type == MutationCommand::Type::RENAME_COLUMN)
            result.rename_map[command.rename_to] = command.column_name;
    return result;
}
MergeTreeData::WriteAheadLogPtr MergeTreeData::getWriteAheadLog()
{
    std::lock_guard lock(write_ahead_log_mutex);
    if (!write_ahead_log)
    {
        auto reservation = reserveSpace(getSettings()->write_ahead_log_max_bytes);
        write_ahead_log = std::make_shared<MergeTreeWriteAheadLog>(*this, reservation->getDisk());
    }
    return write_ahead_log;
}
NamesAndTypesList MergeTreeData::getVirtuals() const