**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 :)

MergeTreeDataPartType choosePartType(size_t bytes_uncompressed, size_t rows_count) const;
    MergeTreeDataPartType choosePartTypeOnDisk(size_t bytes_uncompressed, size_t rows_count) const;
    
    MutableDataPartPtr createPart(const String & name,
        MergeTreeDataPartType type, const MergeTreePartInfo & part_info,
        const DataPartStoragePtr & data_part_storage, const IMergeTreeDataPart * parent_part = nullptr) const;
    
    
    MutableDataPartPtr createPart(const String & name,
        const DataPartStoragePtr & data_part_storage, const IMergeTreeDataPart * parent_part = nullptr) const;
    MutableDataPartPtr createPart(const String & name, const MergeTreePartInfo & part_info,
        const DataPartStoragePtr & data_part_storage, const IMergeTreeDataPart * parent_part = nullptr) const;
    
    
    
    
    
    class Transaction : private boost::noncopyable
    {
    public:
        Transaction(MergeTreeData & data_, MergeTreeTransaction * txn_);
        DataPartsVector commit(MergeTreeData::DataPartsLock * acquired_parts_lock = nullptr);
        void addPart(MutableDataPartPtr & part, DataPartStorageBuilderPtr builder);
        void rollback();
        
        
        void rollbackPartsToTemporaryState();
        size_t size() const { return precommitted_parts.size(); }