Storage Management

Due to the requirements imposed by the NA48 experiment, the main goals in designing our storage management system are:

• Real-time behaviour and guaranteed throughput: It must be guaranteed that the incoming raw-data and the results of the online reconstruction can be written to disk and tape. Write accesses have priority over any user request.
• Reliability and fault tolerance
• User transparency: The user has to consider ``files'' or even abstract ``data objects'' and not worry about the media and how to access them.

The SMS is intended to be used on the CS-2, but it is easily portable to any other hardware platform on which the required I/O performance is guaranteed.

  
Figure 1: Logical Structure of the Storage Management System

Figure 1 gives a rough overview of the proposed system. The depicted structure is only a logical one. It does not make any assumptions about assigning the processes or their interactions to physical hardware.

There is a clear border-line between the SMS and the rest of the Data Recording System. The only interactions between the two parts are data access to the PFS and user requests to the active instances of the Storage Server and the Directory Server. All these user requests must be handled through the Client API, which provides a clearly defined user interface, independent of the implementation. The Input Manager interacts with the SMS like any other user process, but having higher priority then all others. Some other user processes which run in real-time (online reconstruction) are granted a high priority level too, because their output data must also be written within a given amount of time.