**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 :)

{
        if (min_format_version == MERGE_TREE_DATA_MIN_FORMAT_VERSION_WITH_CUSTOM_PARTITIONING.toUnderType())
            throw Exception(
                "MergeTree data format version on disk doesn't support custom partitioning",
                ErrorCodes::METADATA_MISMATCH);
    }
    String reason;
    if (!canUsePolymorphicParts(*settings, &reason) && !reason.empty())
        LOG_WARNING(log, "{} Settings 'min_rows_for_wide_part', 'min_bytes_for_wide_part', "
            "'min_rows_for_compact_part' and 'min_bytes_for_compact_part' will be ignored.", reason);
#if !USE_ROCKSDB
    if (use_metadata_cache)
        throw Exception(ErrorCodes::LOGICAL_ERROR, "Can't use merge tree metadata cache if clickhouse was compiled without rocksdb");
#endif
    common_assignee_trigger = [this] (bool delay) noexcept
    {
        if (delay)
            background_operations_assignee.postpone();
        else
            background_operations_assignee.trigger();
    };
    moves_assignee_trigger = [this] (bool delay) noexcept
    {
        if (delay)
            background_moves_assignee.postpone();
        else
            background_moves_assignee.trigger();
    };
}
StoragePolicyPtr MergeTreeData::getStoragePolicy() const