**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 :)

const MergeTreePartInfo & new_part_info,
    const String & new_part_name,
    DataPartPtr & out_covering_part,
    DataPartsLock & ) const
{
    
    auto it_middle = data_parts_by_state_and_info.lower_bound(DataPartStateAndInfo{DataPartState::Active, new_part_info});
    auto committed_parts_range = getDataPartsStateRange(DataPartState::Active);
    
    DataPartIteratorByStateAndInfo begin = it_middle;
    while (begin != committed_parts_range.begin())
    {
        auto prev = std::prev(begin);
        if (!new_part_info.contains((*prev)->info))
        {
            if ((*prev)->info.contains(new_part_info))
            {
                out_covering_part = *prev;
                return {};
            }