Guaranteed-Rate I/O Basics

Guaranteed-Rate I/O Basics

Guaranteed-rate I/O (GRIO) is applied on a file basis. The file must have these characteristics for any guarantee to be granted:

The file must be managed by XFS. EFS, the older IRIX file system, does not support GRIO.
The file must be contained in the real-time subvolume of a logical volume created by XLV.
The real-time subvolume of an XLV volume can span multiple disk partitions, and can be striped. The real-time subvolume differs from the more common data subvolume in that it contains only data, no file system management data such as directories or inodes.

Note: Real-time subvolumes cannot include RAID partitions.
The predictive failure analysis feature and the thermal recalibration feature of the drive firmware must be disabled, as these can make device access times unpredictable.
A guaranteed-rate stream must be available. Unless extra-cost options are installed, a maximum of four streams can be in use at one time.

You can request either of two types of guarantee. A hard guarantee asks XFS and IRIX to subordinate all other considerations, including data integrity, to meet the guaranteed rate. A soft guarantee asks IRIX to make its best effort at the rate, accepting that error correction might cause glitches.

You can qualify either type of guarantee as being for Video On Demand (VOD), indicating a particular, special use of a striped volume. These three types of guarantee are discussed further in the following topics.

For information about using XFS, XLV, and how to prepare a real-time subvolume for GRIO, see the IRIX Admin: Disks and Filesystems manual (see "Other Useful Books" on page xxiii). For an example of how the grio_request() function is used, see the function starting in "Guaranteed-Rate Request".