**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 :)

original_active_part->force_keep_shared_data = true;
            }
            modifyPartState(original_active_part, DataPartState::DeleteOnDestroy);
            data_parts_indexes.erase(active_part_it);
            auto part_it = data_parts_indexes.insert(part_copy).first;
            modifyPartState(part_it, DataPartState::Active);
            ssize_t diff_bytes = part_copy->getBytesOnDisk() - original_active_part->getBytesOnDisk();
            ssize_t diff_rows = part_copy->rows_count - original_active_part->rows_count;
            increaseDataVolume(diff_bytes, diff_rows,  0);
            
            
            lockSharedData(*part_copy);
            original_active_part->data_part_storage->writeDeleteOnDestroyMarker(log);
            return;
        }
    }
    throw Exception("Cannot swap part '" + part_copy->name + "', no such active part.", ErrorCodes::NO_SUCH_DATA_PART);
}
MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const MergeTreePartInfo & part_info) const
{
    auto lock = lockParts();
    return getActiveContainingPart(part_info, DataPartState::Active, lock);
}
MergeTreeData::DataPartPtr MergeTreeData::getActiveContainingPart(const String & part_name) const
{
    auto part_info = MergeTreePartInfo::fromPartName(part_name, format_version);
    return getActiveContainingPart(part_info);
}
MergeTreeData::DataPartsVector MergeTreeData::getVisibleDataPartsVectorInPartition(ContextPtr local_context, const String & partition_id) const