The systems running IRIX 6.2 support SCSI hard disk drives on SCSI or VME (Jaguar) controllers. Figure 1-1 shows how disk drives and other peripheral devices are connected to controllers in systems.

Figure 1-1. Controllers and Disk Drives

Each disk drive is managed by a controller. Each type of controller can support a fixed number of drives. Your workstation can support a fixed number of controllers. (For the number and type of controllers supported by your model of workstation, see your hardware owner's guide.) SCSI controllers support up to seven disks per controller or up to 15 disks per controller (depending upon the SCSI controller type), and VME controllers support up to 14 disks per controller.

Each disk is assigned a drive address (called the unit number in output from the hinv command and also known as a SCSI ID). This address is set by a switch, a dial, or jumpers on the disk, or by the physical location of the disk. See the hardware owner's guide for the system for information on setting the drive address of a disk.

Some SCSI devices, such as RAIDs (an array of disks with built-in redundancy), have an additional identifying number called a logical unit number or lun. It is used to address disks within the device.

Figure 1-2 shows the physical structure of a disk. A disk is composed of circular plates called platters. Each platter has an upper and lower oxide-coated surface. Recording heads, at least one per surface, are mounted on arms that can be moved to various radial distances from the center of the platters. The heads float very close to the surfaces of the platters, never actually touching them, and read and record data as the platters spin around.

Figure 1-2. Physical Disk Structure

When the recording heads are at a particular position, the portions of the disk that can be read or written are called a cylinder. As shown in Figure 1-2, a cylinder is made up of rings on the upper and lower surfaces of all of the platters. The ring on one surface is called a track. Each track is divided into disk blocks (sometimes called sectors, these physical blocks on a disk are different from filesystem blocks). On SCSI disks, the number of disk blocks per cylinder may vary; outer cylinders may have more disk blocks than inner cylinders.

Formatting a disk divides the disk into tracks and disk blocks that can be addressed by the disk controller, writes timing marks, and identifies bad areas on the disk (called bad blocks). SCSI disk drives are shipped preformatted. They do not require formatting at any time. Bad block handling is performed automatically by SCSI disks. Bad blocks are areas of a disk that cannot reliably store data. Bad block handling maps bad blocks to substitute blocks that are in a reserved area of disk that is inaccessible by normal IRIX commands.

Disks are divided into logical units called partitions. An example of a partitioned disk is shown in Figure 1-3. Partitions divide the disk into fixed-size portions which can be used by IRIX or by users for different purposes. Partition sizes are measured in 512-byte disk blocks. On SCSI disks, partitions merely need to be integral numbers of disk blocks. They can be an integral number of cylinders or a fractional number of cylinders.

Figure 1-3. Disk Partitions

Each disk block can belong to any number of partitions, including no partition (in which case the disk space of the cylinder is unused or wasted). This means that partitions can overlap. For example, a disk can be divided into several non-overlapping partitions and have an additional partition defined that is the entire disk.

Each partition on a disk has a number from 0 through 15. By convention, some of these partition numbers have a particular function and a name. These numbers, names, and functions are listed in Table 1-1.

System disks contain the IRIX operating system. Specifically, they must contain a volume header that includes sash (see the section “Volume Headers” in this chapter), the Root filesystem, a swap partition, and possibly a Usr filesystem. Each workstation or server has one system disk; IRIX is booted from this disk when the system is brought up. On workstations, the system disk is on controller number 0 and drive address 1 by default. On some servers, the default controller and drive address for the system disk is controller 1 and drive address 1. The location of the system disk is reported by the nvram command; it is the value of OSLoadPartition.

All other disks on the system other than the system disk are known as option disks.

Disks are shipped from Silicon Graphics with one of several “standard” partition layouts. You can list the partitions of a disk with the prtvtoc command (see the section “Displaying a Disk's Partitions With prtvtoc” in Chapter 2). The standard partition layouts are described and illustrated below.

Figure 1-4 and Figure 1-5 show the two common layouts of a system disk with separate partitions for the Root and Usr filesystems. The layout in Figure 1-4 is used for EFS filesystems and for XFS filesystems when the XFS log doesn't have its own partition (it is an internal XFS log). Figure 1-5 shows the partition layout when an XFS log partition is included (an external log).

Figure 1-4. Partition Layout of System Disks With Separate Root and Usr

Figure 1-5. Partition Layout of System Disks With Separate Root and Usr and an XFS Log Partition

Separate root and usr partitions were standard on older systems and are still used on servers. In the original UNIX design, only the Root filesystem needed to be mounted to boot UNIX. This is not true for IRIX anymore—both filesystems must be mounted, so there is no longer the concept of the Root filesystem being a minimal subset of operating system software.

Figure 1-6 shows the layout of a system disk with a single partition for a combined Root and Usr filesystem and a swap partition. This arrangement is standard on most newer systems and applies to both EFS and XFS filesystems. However, restrictions on making the root partition part of a logical volume may make separate root and usr partitions a better choice than a single combined partition (see Chapter 6, “Logical Volume Concepts,” for information about logical volume restrictions).

Figure 1-6. Partition Layout of System Disks With Combined Root and Usr

Figure 1-7 shows the standard layout of an option disk that doesn't have an XFS log partition. It has a single partition for data.

Figure 1-7. Partition Layout of Option Disks

Figure 1-8 shows the layout of an option disk with two partitions, one for data and one for an XFS log.

Figure 1-8. Partition Layouts of Options Disks With XLV Log Subvolumes

The default partition layouts are generic in nature and should be evaluated by the system administrator. After your system has been in operation for a few months, you may decide that a different arrangement would better serve your users' needs. Some points to consider in choosing partition layouts are:

The fx command is used for changing disk partitions (called repartitioning a disk). It knows about standard partition layouts or can be used to create custom partition layouts. Additional information about using fx to repartition disks is provided in the section “Repartitioning a Disk With fx” in Chapter 2.

Once disks have been partitioned, these partitions may be used as filesystems, as parts of a logical volume, or as raw disk space. Filesystems are described in Chapter 3, “Filesystem Concepts.” Logical volumes are described in Chapter 6, “Logical Volume Concepts.”

Each partition has a type that is displayed by fx and prtvtoc. Table 1-2 lists the partition types, their uses, and the partition numbers that can be assigned to those types. (Partition 9 isn't listed in this table; remember that it is reserved.) Partition types, except for xlv, are assigned by fx. The type xlv is automatically assigned by several XLV logical volume commands.

The partitions listed as standard partitions in Table 1-2 are created when you use the fx repartition commands rootdrive, usrrootdrive, and optiondrive. Prompts ask you whether you want partition type efs or xfs, and, if you specify xfs for usrrootdrive or optiondrive, if you want an xfslog partition. To use an xfslog partition (an external XFS log), you must configure the xfslog partition as an XLV log subvolume. (See Chapter 7, “Creating and Administering XLV Logical Volumes,” for more information about XLV.) If you do not use an xfslog partition, the XFS log is stored in an xfs partition (and called an internal log).

To assign a partition type to a partition number listed as a custom partition in Table 1-2, you must use the expert mode of fx (fx –x) to create the partition and assign the type. (See the fx(1M) reference page for more information about the expert mode of fx.)

A partition called the volume header is stored on the partition that begins at disk block 0. (For proper system operation, the volume header must begin at disk block 0). It contains a minimal filesystem with a few files that contain information about the device parameters, the partition layout, the version number of the most recently used version of fx, and logical volume information. It also may contain some standalone programs.

The files and standalone programs that may be in a volume header are:

The fx command can be used to display and modify the device parameters and the partition layout. See the fx(1M) reference page and the section “Repartitioning a Disk With fx” in Chapter 2. Using fx has the side effect of creating the file sgilabel in the volume header.

The command prtvtoc is also used to display partition layout information. See the section “Displaying a Disk's Partitions With prtvtoc” in Chapter 2 for instructions.

The dvhtool command can be used to add and delete standalone programs from the volume header. dvhtool can also be used to delete logical volume labels from the volume header. See the sections “Adding Files to the Volume Header With dvhtool,” and “Removing Files in the Volume Header With dvhtool” in Chapter 2 for more information.

The volume header is consulted (and therefore any mistakes made creating or modifying the volume header become apparent) only at these times:

IRIX programs communicate with hardware devices through two types of files, called special files. The two types are character device files (also called raw device files) and block device files.

Device files are in the /dev directory of the Root filesystem. Since every entry in a directory is a file (see Table 3-2), conceptually a disk device is treated as if it were a file. In practice, there are differences between regular files and device files, so the latter are referred to as special files.

Device files are created automatically when system software is installed and, if necessary, at system boot up by the command MAKEDEV. The device files created by MAKEDEV are based on the hardware configuration of the system; however, not all possible device files are created. Disk device files are created only for partitions 0, 1, 6, 7, 15, vh, and vol (vh stands for volume header and is partition 8, vol is the entire volume and is the same as partition 10). You can run MAKEDEV manually if you added a supported device, or, to create a specific device file, you can use the mknod command. For more information about MAKEDEV, see the section “Creating Device Files With MAKEDEV” in Chapter 2. For more information about mknod, see the section “Creating Device Files With mknod” in Chapter 2.

The following examples of output are the results of the ls -l command invoked on a user's regular file and on the /dev directory. They show the difference in structure between regular and device files. This is a regular file:

-rw-r----- 1 ralph raccoons 1050 Apr 23 08:14 scheme.notes 

Regular files are indicated by a dash (–) in the first column. The remainder of the output is explained in the guide IRIX Admin: System Configuration and Operation .

These are device files:

brw------- 2 root sys 128,16 Apr 15 10:59 /dev/dsk/dks0d1s1 
brw------- 2 root sys 128,16 Apr 15 10:59 /dev/root 
brw------- 2 root sys 128,22 Apr 12 13:51 /dev/dsk/dks0d1s6 
brw------- 2 root sys 128,22 Apr 12 13:51 /dev/usr 
crw------- 2 root sys 128,16 Apr 15 10:58 /dev/rdsk/dks0d1s0 
crw------- 2 root sys 128,16 Apr 15 10:58 /dev/rroot 
crw------- 2 root sys 128,22 Apr 12 13:51 /dev/rdsk/dks0d1s6 
crw------- 2 root sys 128,22 Apr 12 13:51 /dev/rusr 

The device file listing has some similar information to the listing of the regular file, but also contains additional information. The device files shown have the following characteristics:

The following sections explain each of these characteristics of device files.