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Appendix D Quality of Service Guide
Overview
StorNext 3.1.3 Installation Guide 136
Overview
QOS is stripe-group centric; all configuration and operations act on a
specific stripe group. This allows stripe groups with differing access
characteristics to use different real-time settings. When a client requests a
certain amount of real-time I/O, the FSM informs all connected clients of
the amount remaining and available for non-real-time I/O accesses.
This information is encapsulated in a token. Think of the token as a
capability that enables the client to perform I/O without contacting the
FSM. When the amount of real-time I/O on a stripe group changes, the
FSM informs all connected clients about the change via a callback.
In both the client and the server, QOS is implemented as a state machine.
Each state has a set of events and allowable actions. In this document, the
term state thusly refers to the internal state machine.
Active vs. Passive 4
QOS is a passive, not active implementation of real-time I/O. In an active
implementation (such as the SGI IRIX guaranteed rate I/O known as
GRIO), the scheduler is tightly coupled with the I/O subsystem. The
qualities of the disk subsystem are well known so the scheduler can
guarantee that a process will be scheduled such that it will receive the
required amount of bandwidth. Since SNFS is a cross-platform file
system that does not have hooks in the operating system scheduler, it
cannot provide such a guarantee.
In a passive implementation, a real-time process gates its I/O according
to some outside metric (such as a frame rate for specific video formats).
The file system then gates all other non-real-time I/O so they do not
interfere.
These differences cannot be over-stressed. It is a misconception to think
that QOS, despite its name, guarantees a specific amount of real-time I/O
to a process.
Supported Platforms 4
QOS has been tested on Windows XP, Linux, IRIX, and Solaris. In the
Windows world, an application gets a handle to a file to perform I/O,
usually via the
Win32 CreateFile() API. In the UNIX world, an application
receives a file descriptor (
fd) via the open(2) system call. In this document,
“handle” is synonymous with
fd.