**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 :)

friend class MergeTreeDataWriter;
    friend class MergeTask;
    friend class IPartMetadataManager;
    friend class IMergedBlockOutputStream; 
    bool require_part_metadata;
    
    
    String relative_data_path;
    
    ColumnSizeByName column_sizes;
    
    IndexSizeByName secondary_index_sizes;
    
    BrokenPartCallback broken_part_callback;
    
    
    std::shared_ptr<String> log_name;
    std::atomic<Poco::Logger *> log;
    
    
    MultiVersion<MergeTreeSettings> storage_settings;
    
    mutable std::shared_mutex pinned_part_uuids_mutex;
    PinnedPartUUIDsPtr pinned_part_uuids;
    
    mutable std::atomic_bool transactions_enabled = false;
    
    struct TagByInfo{};
    struct TagByStateAndInfo{};
    static const MergeTreePartInfo & dataPartPtrToInfo(const DataPartPtr & part)