Quantum 3.5 manual Real-time I/O

Appendix D Quality of Service Guide

Real-time I/O

tab of the SNFS control panel on Windows platforms. The default is sixty (60) seconds.

This means that after sixty seconds without non-real-time I/O on a stripe group, the non-real-time token for that stripe group is released. The parameter should be specified in five (5) second increments. If it is not, it will be silently rounded up to the next five-second boundary. If the syslog level is set to debug, the file system dumps out its mount parameters so the value can be seen.

Real-time I/O

A process requests real-time (ungated) I/O by using the SNFS External API SetRtio call (F_SETRIO ioctl). A library function is included in the External API sample source code that provides all the required cross- platform handling.

As an example, assume that a video playback application requires a constant rate of 186 MB/sec to correctly display images without dropping any frames. The application gates itself; that is, it requests I/O at a rate to satisfy the requirements of correctly displaying an image. QOS provides a mechanism so other I/O requests do not perturb the real-time display.

In the following example, assume the I/O subsystem has been qualified at 216 MB/sec. The file system block size is 4k. The disk subsystem is actually a large RAID array that internally maps many drives to a single LUN. There are four LUNs in the stripe group; each LUN is optimized for a 1.5 MB transfer. This corresponds to the following in the fsm configuration file:

[StripeGroup MyStripeGroup]

StripeBreadth 384

Node CvfsDisk0 0

Node CvfsDisk1 1

Node CvfsDisk2 2

Node CvfsDisk3 3

Rtmb 216

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