**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 :)

{
    auto txn = local_context->getCurrentTransaction();
    DataPartsVector res;
    {
        auto lock = lockParts();
        for (const auto & partition_id : partition_ids)
        {
            DataPartStateAndPartitionID active_parts{MergeTreeDataPartState::Active, partition_id};
            insertAtEnd(
                res,
                DataPartsVector(
                    data_parts_by_state_and_info.lower_bound(active_parts),
                    data_parts_by_state_and_info.upper_bound(active_parts)));
            if (txn)
            {
                DataPartStateAndPartitionID outdated_parts{MergeTreeDataPartState::Active, partition_id};
                insertAtEnd(
                    res,
                    DataPartsVector(
                        data_parts_by_state_and_info.lower_bound(outdated_parts),
                        data_parts_by_state_and_info.upper_bound(outdated_parts)));
            }
        }
    }
    if (txn)
        filterVisibleDataParts(res, txn->getSnapshot(), txn->tid);
    return res;
}
MergeTreeData::DataPartPtr MergeTreeData::getPartIfExists(const MergeTreePartInfo & part_info, const MergeTreeData::DataPartStates & valid_states)