**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 :)

MergeTreeData(const StorageID & table_id_,
                  const String & relative_data_path_,
                  const StorageInMemoryMetadata & metadata_,
                  ContextMutablePtr context_,
                  const String & date_column_name,
                  const MergingParams & merging_params_,
                  std::unique_ptr<MergeTreeSettings> settings_,
                  bool require_part_metadata_,
                  bool attach,
                  BrokenPartCallback broken_part_callback_ = [](const String &){});