Quantum manual StorNext 3.5 Installation Guide 157

Appendix D Quality of Service Guide

Callbacks

them that they will need a token to perform any non-real-time I/O. The first I/O after receiving the callback will then request a non-real-time I/O token from the FSM.

The FSM calculates the amount of non-real-time bandwidth using the following formula:

avail_nrtio = rtio_limit - rtio_current;

avail_nrtio /= current_num_nonrtio_clients + 1

In the above calculation, the amount of existing real-time I/O (rtio_current) has already been adjusted with the reserve parameter. As each client requests a non-real-time I/O token, the number of clients increases (current_num_nonrtio_clients in the above formula) and the amount of available non-real-time I/O decreases.

Each time there is a change in the amount of non-real-time I/O available, the FSM sends callbacks to the clients with tokens. It is important to note that unlike the initial set of callbacks where the FSM sent callbacks to all connected clients, it is now only necessary to send callbacks to those clients that have an existing token.

Once a client has a token, it can perform as much I/O per second as is allowed by that token. It does not need to contact the FSM on every I/O request. The FSM will inform the client whenever the token changes value.

StorNext 3.5 Installation Guide 157