**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 :)

}
            break;
            case PartitionCommand::UNFREEZE_ALL_PARTITIONS:
            {
                auto lock = lockForShare(query_context->getCurrentQueryId(), query_context->getSettingsRef().lock_acquire_timeout);
                current_command_results = unfreezeAll(command.with_name, query_context, lock);
            }
            break;
            default:
                throw Exception("Uninitialized partition command", ErrorCodes::LOGICAL_ERROR);
        }
        for (auto & command_result : current_command_results)
            command_result.command_type = command.typeToString();
        result.insert(result.end(), current_command_results.begin(), current_command_results.end());
    }
    if (query_context->getSettingsRef().alter_partition_verbose_result)
        return convertCommandsResultToSource(result);
    return {};
}
void MergeTreeData::backupData(BackupEntriesCollector & backup_entries_collector, const String & data_path_in_backup, const std::optional<ASTs> & partitions)
{
    auto local_context = backup_entries_collector.getContext();
    DataPartsVector data_parts;
    if (partitions)
        data_parts = getVisibleDataPartsVectorInPartitions(local_context, getPartitionIDsFromQuery(*partitions, local_context));
    else
        data_parts = getVisibleDataPartsVector(local_context);
    backup_entries_collector.addBackupEntries(backupParts(data_parts, data_path_in_backup));
}
BackupEntries MergeTreeData::backupParts(const DataPartsVector & data_parts, const String & data_path_in_backup)