Hardware Configuration Requirements for GRIO

• Put only real-time subvolume volume elements on a single disk (not log or data subvolume volume elements). This configuration is recommended for soft guarantees and required for hard guarantees.

• The drive firmware in each disk used in the real-time subvolume must have the predictive failure analysis and thermal recalibration features disabled. All disk drives have been shipped from Silicon Graphics this way since March 1994.

• When possible, disks used in the real-time subvolume of an XLV volume should have the RC (read continuous) bit enabled. (The RC bit is a disk drive parameter that is discussed in more detail later in this section.) This allows the disks to perform faster, but at the penalty of occasionally returning incorrect data (without giving an error).

• Disks used in the data and log subvolumes of the XLV logical volume must have their retry mechanisms enabled. The data and log subvolumes contain information critical to the filesystem and cannot afford an occasional disk error.

• Each disk used for hard guarantees must be on a controller whose disks are used exclusively for real-time subvolumes. These controllers cannot have any devices other than disks on their buses. Any other devices could prevent the disk from accessing the SCSI bus in a timely manner and cause the rate to be missed.

• For hard guarantees, the disk drive retry and error correction mechanisms must be disabled for all disks that are part of the real-time subvolume. (Disk drive retry and error correction mechanisms are controlled by drive parameters that are discussed in more detail below.) When the drive does error recovery, its performance degrades and there can be lengthy delays in completing I/O requests. However, when the drive error recovery mechanisms are disabled, occasionally invalid data is returned to the user without an error indication. Because of this, the integrity of data stored on an XLV real-time subvolume is not guaranteed when drive error recovery mechanisms are disabled.

Disk Drive Parameters for GRIO

Parameter	New Setting
Auto bad block reallocation (read)	Disabled
Auto bad block reallocation (write)	Disabled
Delay for error recovery (disabling this parameter enables the read continuous (RC) bit)	Disabled

Setting disk drive parameters can be performed on approved disk drive types only. You can use the fx command to find out the type of a disk drive. fx reports the disk drive type after the controller test on a line that begins with the words "Scsi drive type." The approved disk drives types whose parameters can be set for real-time operation are shown in Table 9-2.

Disk Drives Whose Parameters Can Be Changed

Disk Drive Types Approved for Changing Disk Parameters
SGI 0664N1D 6s61
SGI 0664N1D 4I4I

The procedure for enabling the RC bit and disabling the disk drive retry and error correction mechanisms is described in the section "Disabling Disk Error Recovery" in this chapter.