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Appendix D Quality of Service Guide
Real-time I/O
StorNext 3.1.3 Installation Guide 142
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
Also, assume there is only one stripe group for user data in the file
system. As recommended by Quantum, there may be other stripe groups
for metadata and journal that are not shown.