**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 :)

bool renameTempPartAndReplaceImpl(
        MutableDataPartPtr & part,
        Transaction & out_transaction,
        DataPartsLock & lock,
        DataPartStorageBuilderPtr builder,
        DataPartsVector * out_covered_parts);
    
    
    
    struct CurrentlyMovingPartsTagger
    {
        MergeTreeMovingParts parts_to_move;
        MergeTreeData & data;
        CurrentlyMovingPartsTagger(MergeTreeMovingParts && moving_parts_, MergeTreeData & data_);
        ~CurrentlyMovingPartsTagger();
    };
    using CurrentlyMovingPartsTaggerPtr = std::shared_ptr<CurrentlyMovingPartsTagger>;
    
    bool moveParts(const CurrentlyMovingPartsTaggerPtr & moving_tagger);
    
    CurrentlyMovingPartsTaggerPtr selectPartsForMove();
    
    CurrentlyMovingPartsTaggerPtr checkPartsForMove(const DataPartsVector & parts, SpacePtr space);
    bool canUsePolymorphicParts(const MergeTreeSettings & settings, String * out_reason = nullptr) const;
    std::mutex write_ahead_log_mutex;
    WriteAheadLogPtr write_ahead_log;
    virtual void startBackgroundMovesIfNeeded() = 0;
    bool allow_nullable_key{};
    void addPartContributionToDataVolume(const DataPartPtr & part);
    void removePartContributionToDataVolume(const DataPartPtr & part);