**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 :)

{
public:
    
    enum class ActionType : UInt8
    {
        ADD_PART = 0,
        DROP_PART = 1,
    };
    struct ActionMetadata
    {
        
        
        
        
        UInt8 min_compatible_version = 0;
        
        UUID part_uuid = UUIDHelpers::Nil;
        void write(WriteBuffer & meta_out) const;
        void read(ReadBuffer & meta_in);
    private:
        static constexpr auto JSON_KEY_PART_UUID = "part_uuid";
        String toJSON() const;
        void fromJSON(const String & buf);
    };
    constexpr static UInt8 WAL_VERSION = 1;
    constexpr static auto WAL_FILE_NAME = "wal";
    constexpr static auto WAL_FILE_EXTENSION = ".bin";
    constexpr static auto DEFAULT_WAL_FILE_NAME = "wal.bin";
    MergeTreeWriteAheadLog(MergeTreeData & storage_, const DiskPtr & disk_,
        const String & name = DEFAULT_WAL_FILE_NAME);