Disk and Filesystems Management in Solaris

A disk device has physical components and logical components. The physical components include disk platters and read/write heads. The logical components include disk slices, cylinders, tracks, and sectors.

A disk is physically composed of a series of flat, magnetically coated platters that are stacked on a spindle. The spindle turns while the read/write heads move as a single unit radially, reading and writing data on the platters. A disk platter is divided into sectors, tracks, and cylinders.

• Sector The smallest addressable unit on a platter. One sector usually hold 512 bytes of data. In Solaris sectors are also often called disk blocks.
• Track  A series of sectors positioned end-to-end in a circular path. The number of sectors per track varies with the radius of a track on the platter. The outer tracks are larger and can hold more sectors than the inner tracks. Disks present a fixed number of sectors per track to the Solaris OE even though, physically, the number of sectors per track varies by track location.
• Cylinder A stack of concentric tracks spanning several physical disk platters.

Because a disk spins continuously and the read/write heads move as a single unit, the most efficient seeking occurs when the sectors to be read from or written to are located in a single cylinder.

Disks are logically divided into individual partitions known as disk slices. Disk slices are groupings of cylinders that are commonly used to organize data by function. For example, one slice can store critical system files and programs while another slice on the same disk can store user-created files.

Note – Grouping cylinders into slices is done to organize data, facilitate backups, and provide swap space.

By convention, Slice 2 in Solaris represents the entire disk. Slice 2 maintains important data about the entire disk, such as the size of the actual disk and the total number of cylinders available for the storage of files and directories.

A starting cylinder and an ending cylinder define each slice. These cylinder boundaries determine the size of a slice.

A typical partitioning for Solaris before Solaris 10 was:

• 0 / The root directory’s system files

• 1 swap Swap area

• 2 Entire disk

• 5 /opt Optional software

• 6 /usr System executables and programs

• 7 /export/home User files and directories

An eight-character string typically represents the full name of a slice. The string includes the controller number, the target number, the disk number, and the slice number.

The embedded SCSI configuration and the integrated device electronics (IDE) configuration represent the disk slice naming conventions across two different architectures. The disk number is always set to d0 with SCSI disks.

• Controller number Identifies the host bus adapter (HBA), which controls communications between the system and disk unit. The HBA takes care of sending and receiving both commands and data to the device. The controller number is assigned in sequential order, such as c0, c1, c2, and so on.

• Target number Target numbers, such as t0, t1, t2, and t3, correspond to a unique hardware address that is assigned to each disk, tape, or CD-ROM. Some external disk drives have an address switch located on the rear panel. SCSI disks have address pins to assign that disk’s target number.

• Disk number The disk number is also known as the logical unit number (LUN). This number reflects the number of disks at the target location. For  SCSI disks it is always zero.

• Slice number A slice number ranging from 0 to 7.

c#t#d#s#

• c# Controller number

• t# Target number -- actually disk number

• d# Disk number -- always d0 for SCSI disks.

• s# Slice number

Recommended Links

You too can understand device numbers and mapping in Solaris ...

High Availability: Configuring Boot, Root and Swap (PDF)

Scrubbing Disk Using the Solaris Operating Environment Format Program (June 2000)
-by Rob Snevely
Rob explains how to effectively scrub disks on a Solaris Operating Environment system, using the format utility.