**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 :)

loaded_parts.push_back(part);
    }
    return loaded_parts;
}
namespace
{
inline ReservationPtr checkAndReturnReservation(UInt64 expected_size, ReservationPtr reservation)
{
    if (reservation)
        return reservation;
    throw Exception(fmt::format("Cannot reserve {}, not enough space", ReadableSize(expected_size)), ErrorCodes::NOT_ENOUGH_SPACE);
}
}
ReservationPtr MergeTreeData::reserveSpace(UInt64 expected_size) const
{
    expected_size = std::max(RESERVATION_MIN_ESTIMATION_SIZE, expected_size);
    return getStoragePolicy()->reserveAndCheck(expected_size);
}
ReservationPtr MergeTreeData::reserveSpace(UInt64 expected_size, SpacePtr space)
{
    expected_size = std::max(RESERVATION_MIN_ESTIMATION_SIZE, expected_size);
    auto reservation = tryReserveSpace(expected_size, space);
    return checkAndReturnReservation(expected_size, std::move(reservation));
}
ReservationPtr MergeTreeData::reserveSpace(UInt64 expected_size, const DataPartStoragePtr & data_part_storage)
{
    expected_size = std::max(RESERVATION_MIN_ESTIMATION_SIZE, expected_size);
    return data_part_storage->reserve(expected_size);
}
ReservationPtr MergeTreeData::reserveSpace(UInt64 expected_size, const DataPartStorageBuilderPtr & data_part_storage_builder)