**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 :)

{
            auto flushed_part_path = part_in_memory->getRelativePathForPrefix("tmp_freeze");
            data_part_storage = part_in_memory->flushToDisk(flushed_part_path, metadata_snapshot);
        }
        auto callback = [this, &part, &backup_part_path](const DiskPtr & disk)
        {
            /
            /
            createAndStoreFreezeMetadata(disk, part, fs::path(backup_part_path) / part->data_part_storage->getPartDirectory());
        };
        auto new_storage = data_part_storage->freeze(
            backup_part_path,
            part->data_part_storage->getPartDirectory(),
            / true,
            callback,
            / false);
        part->is_frozen.store(true, std::memory_order_relaxed);
        result.push_back(PartitionCommandResultInfo{
            .partition_id = part->info.partition_id,
            .part_name = part->name,
            .backup_path = new_storage->getFullRootPath(),
            .part_backup_path = new_storage->getFullPath(),
            .backup_name = backup_name,
        });
        ++parts_processed;
    }
    LOG_DEBUG(log, "Freezed {} parts", parts_processed);
    return result;
}
void MergeTreeData::createAndStoreFreezeMetadata(DiskPtr, DataPartPtr, String) const