**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 :)

const MergeTreeData & storage_,
            const String & source_dir_)
            : storage(storage_)
            , source_dir(source_dir_)
        {
        }
        
        void addPart(const String & old_name, const String & new_name, const DiskPtr & disk);
        
        void tryRenameAll();
        
        ~PartsTemporaryRename();
        struct RenameInfo
        {
            String old_name;
            String new_name;
            
            DiskPtr disk;
        };
        const MergeTreeData & storage;
        const String source_dir;
        std::vector<RenameInfo> old_and_new_names;
        bool renamed = false;
    };
    
    struct MergingParams
    {
        
        enum Mode