**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:
    RestoredPartsHolder(const std::shared_ptr<MergeTreeData> & storage_, const BackupPtr & backup_, size_t num_parts_)
        : storage(storage_), backup(backup_), num_parts(num_parts_)
    {
    }
    BackupPtr getBackup() const { return backup; }
    void setNumParts(size_t num_parts_)
    {
        std::lock_guard lock{mutex};
        num_parts = num_parts_;
        attachIfAllPartsRestored();
    }
    void addPart(MutableDataPartPtr part)
    {
        std::lock_guard lock{mutex};
        parts.emplace_back(part);
        attachIfAllPartsRestored();
    }
    String getTemporaryDirectory(const DiskPtr & disk)