**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 :)

for (const auto & part : parts)
    {
        if ((part_values.empty() || part_values.find(part->name) != part_values.end()) && !partition_pruner.canBePruned(*part))
            res += part->rows_count;
    }
    return res;
}
String MergeTreeData::MergingParams::getModeName() const
{
    switch (mode)
    {
        case Ordinary:      return "";
        case Collapsing:    return "Collapsing";
        case Summing:       return "Summing";
        case Aggregating:   return "Aggregating";
        case Replacing:     return "Replacing";
        case Graphite:      return "Graphite";
        case VersionedCollapsing: return "VersionedCollapsing";
    }
    __builtin_unreachable();
}
Int64 MergeTreeData::getMaxBlockNumber() const
{
    auto lock = lockParts();
    Int64 max_block_num = 0;
    for (const DataPartPtr & part : data_parts_by_info)
        max_block_num = std::max({max_block_num, part->info.max_block, part->info.mutation});
    return max_block_num;
}
void MergeTreeData::loadDataPartsFromDisk(