**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 :)

{
    return total_active_size_rows.load(std::memory_order_acquire);
}
size_t MergeTreeData::getPartsCount() const
{
    return total_active_size_parts.load(std::memory_order_acquire);
}
size_t MergeTreeData::getMaxPartsCountForPartitionWithState(DataPartState state) const
{
    auto lock = lockParts();
    size_t res = 0;
    size_t cur_count = 0;
    const String * cur_partition_id = nullptr;
    for (const auto & part : getDataPartsStateRange(state))
    {
        if (cur_partition_id && part->info.partition_id == *cur_partition_id)
        {
            ++cur_count;
        }
        else
        {
            cur_partition_id = &part->info.partition_id;
            cur_count = 1;
        }
        res = std::max(res, cur_count);
    }
    return res;
}
size_t MergeTreeData::getMaxPartsCountForPartition() const