**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::map<String, std::vector<std::pair<String, DiskPtr>>> disk_part_map;
    std::map<String, MutableDataPartsVector> disk_wal_part_map;
    ThreadPool pool(disks.size());
    std::mutex wal_init_lock;
    for (const auto & disk_ptr : disks)
    {
        if (disk_ptr->isBroken())
            continue;
        auto & disk_parts = disk_part_map[disk_ptr->getName()];
        auto & disk_wal_parts = disk_wal_part_map[disk_ptr->getName()];
        pool.scheduleOrThrowOnError([&, disk_ptr]()
        {
            for (auto it = disk_ptr->iterateDirectory(relative_data_path); it->isValid(); it->next())
            {
                
                if (startsWith(it->name(), "tmp") || it->name() == MergeTreeData::FORMAT_VERSION_FILE_NAME
                    || it->name() == MergeTreeData::DETACHED_DIR_NAME)
                    continue;
                if (!startsWith(it->name(), MergeTreeWriteAheadLog::WAL_FILE_NAME))
                    disk_parts.emplace_back(std::make_pair(it->name(), disk_ptr));
                else if (it->name() == MergeTreeWriteAheadLog::DEFAULT_WAL_FILE_NAME && settings->in_memory_parts_enable_wal)
                {
                    std::lock_guard lock(wal_init_lock);
                    if (write_ahead_log != nullptr)
                        throw Exception(