**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::unique_lock lock(write_mutex);
        if (shutdown_called)
             return;
        if (sync_scheduled)
            sync_cv.wait(lock, [this] { return !sync_scheduled; });
        shutdown_called = true;
        out->finalize();
        out.reset();
    }
    
    sync_task->deactivate();
}
std::optional<MergeTreeWriteAheadLog::MinMaxBlockNumber>