**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 isEmpty() const { return precommitted_parts.empty(); }
        ~Transaction()
        {
            try
            {
                rollback();
            }
            catch (...)
            {
                tryLogCurrentException("~MergeTreeData::Transaction");
            }
        }
    private:
        friend class MergeTreeData;
        MergeTreeData & data;
        MergeTreeTransaction * txn;
        DataParts precommitted_parts;
        std::vector<DataPartStorageBuilderPtr> part_builders;
        DataParts locked_parts;
        void clear() { precommitted_parts.clear(); }
    };
    using TransactionUniquePtr = std::unique_ptr<Transaction>;
    using PathWithDisk = std::pair<String, DiskPtr>;
    struct PartsTemporaryRename : private boost::noncopyable
    {
        PartsTemporaryRename(