**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 :)

part_out.finalizePart(part, false);
            min_block_number = std::min(min_block_number, part->info.min_block);
            max_block_number = std::max(max_block_number, part->info.max_block);
            parts.push_back(std::move(part));
        }
    }
    MergeTreeData::MutableDataPartsVector result;
    std::copy_if(parts.begin(), parts.end(), std::back_inserter(result),
        [&dropped_parts](const auto & part) { return dropped_parts.count(part->name) == 0; });
    return result;
}
void MergeTreeWriteAheadLog::sync(std::unique_lock<std::mutex> & lock)
{
    size_t bytes_to_sync = storage.getSettings()->write_ahead_log_bytes_to_fsync;
    time_t time_to_sync = storage.getSettings()->write_ahead_log_interval_ms_to_fsync;
    size_t current_bytes = out->count();
    if (bytes_to_sync && current_bytes - bytes_at_last_sync > bytes_to_sync)
    {
        sync_task->schedule();
        bytes_at_last_sync = current_bytes;
    }
    else if (time_to_sync && !sync_scheduled)
    {
        sync_task->scheduleAfter(time_to_sync);
        sync_scheduled = true;
    }
    if (storage.getSettings()->in_memory_parts_insert_sync)
        sync_cv.wait(lock, [this] { return !sync_scheduled; });
}
void MergeTreeWriteAheadLog::shutdown()