**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 :)

{
    ReservationPtr reserved_space;
    auto min_bytes_to_rebalance_partition_over_jbod = getSettings()->min_bytes_to_rebalance_partition_over_jbod;
    if (tagger_ptr && min_bytes_to_rebalance_partition_over_jbod > 0 && part_size >= min_bytes_to_rebalance_partition_over_jbod)
    {
        try
        {
            const auto & disks = getStoragePolicy()->getVolume(max_volume_index)->getDisks();
            std::map<String, size_t> disk_occupation;
            std::map<String, std::vector<String>> disk_parts_for_logging;
            for (const auto & disk : disks)
                disk_occupation.emplace(disk->getName(), 0);
            std::set<String> committed_big_parts_from_partition;
            std::set<String> submerging_big_parts_from_partition;
            std::lock_guard lock(currently_submerging_emerging_mutex);
            for (const auto & part : currently_submerging_big_parts)
            {
                if (part_info.partition_id == part->info.partition_id)
                    submerging_big_parts_from_partition.insert(part->name);
            }
            {
                auto lock_parts = lockParts();
                if (covered_parts.empty())
                {
                    /
                    MergeTreeData::DataPartPtr covering_part;
                    covered_parts = getActivePartsToReplace(part_info, part_name, covering_part, lock_parts);
                }
                /
                std::erase_if(covered_parts,