User's Manual

Appendix D Quality of Service Guide
Callbacks
StorNext 3.1.3 Installation Guide 145
Calculating Available
RTIO 4
When the FSM receives a request for rtio, it takes the amount reserved
into consideration. The reserve amount functions as a soft limit beyond
which the FSM will not traipse. The calculation for
rtio is as follows:
avail_rtio = rtio_limit - rtio_current
avail_rtio -= rtio_reserve
All internal calculations are done in terms of I/O/sec.
Callbacks
The cornerstones of the communications between the FSM and the client
are callbacks and tokens. A callback is an unsolicited message from the
FSM to the client requesting that the client adjust its real-time I/O
parameters. The callback contains a token that specifies the amount of
non-real-time I/O available on a stripe group.
Initially, all stripe groups in a file system are in non-real-time (ungated)
mode. When the FSM receives the initial request for real-time I/O, it first
issues callbacks to all clients informing them that the stripe group is now
in real-time mode. The token accompanying the message specifies no I/O
is available for non-real-time I/O. Clients must now obtain a non-real-
time token before they can do any non-real-time I/O.
After sending out all callbacks, the FSM sets a timer based on the
RtTokenTimeout value, which by default is set to five seconds. If all clients
respond to the callbacks within the timeout value the
rtio request
succeeds, and a response is set to the requesting client.