Physical & Logical Device Paths - What are They & How are They Built [ID 1001526.1]

Modified 18-DEC-2008 Type HOWTO Migrated ID 202090 Status PUBLISHED

Description

Physical & Logical Device Paths - What are They & How are They Built

Steps to Follow

Solaris[TM] uses two types of device paths for accessing disks, and tape drives to perform data reading and writing. The two device path types are the physical, and logical device paths.

Physical Device Paths

When a SPARC(R) based system is initially powered on, the Power On Self Test (POST) is run. POST probes the devices and buses required to access the boot device. POST then saves this information for the operating

system's automatic reconfiguration (boot -r). The Open Boot PROM (OBP), builds the rest of the device paths for various other hardware attached to the system. Once POST has completed, the Solaris kernel boots and

initializes device drivers based on the physical device paths that have been constructed.

Instance Numbers

The kernel talks directly to hardware devices using the long physical device paths constructed by POST and the OBP. The kernel accesses these physical device paths through a "shortcut" known as an instance number. An

instance number is simply a pointer that references the long device path for each device. The mapping of instance numbers to the long physical device path is stored in the /etc/path_to_inst file.

The following is an example of a disk entry in the /etc/_path_to_inst file:

"/sbus@1f,0/SUNW,fas@e,8800000/sd@0,0" 0 "sd"

/sbus@1f,0/SUNW,fas@e,8800000/sd@0,0 - This part of the entry represents the physical device path.

The 0 after the physical device path is the instance number for the device sd@0,0.

The sd, at the end of the instance line, defines the type of device that is associated with the instance, in this case, sd represents a SCSI disk.

The Solaris kernel creates the second type of device path known as the logical device path. The logical device path is created for devices that may require system administrator intervention for configuration. In the case of disk

drives, there are two types of logical paths for each drive. The first logical path is a pointer to the unformatted partitions for each disk. This unformatted partition pointer is known as the raw device. The second type of logical

device path for disk drives points to the disk partitions in a "formatted" state. This formatted state is referred to as the block device. Generally speaking, a formatted state reflects that there is some type of underlying structure

that data is built on top of.

All of the native Solaris logical device paths for raw disk partitions are stored in the /dev/rdsk directory. Below is an example of a logical device path for an unformatted disk partition.

/dev/rdsk

%ls -l

c0t0d0s0 -> ../../devices/sbus@1f,0/SUNW,fas@e,8800000/sd@0,0:a,raw%

Notice that the logical device above is actually a link to the physical device path of the device. When the kernel builds its logical device paths, it uses the /devices directory as the root for all the devices.

The logical name for this particular disk is c0t0d0s0. The disk wasfound on the first controller (c0), at target 0 (t0), its logical unit number (disk)is d0, and the slice represented is 0 (s0), thus the first part of the entry c0t0d0s0.

A disk partition and slice are synonymous. The entry above from the /dev/rdsk directory points to the physical device entry for the first raw disk partition which is referenced by the letter a.

For native Solaris file system layouts, the logical paths to formatted partitions are stored in the /dev/dsk directory. Below is an example of the logical path to a formatted disk partition:

%pwd

/dev/dsk

%ls -l

c0t0d0s0 -> ../../devices/sbus@1f,0/SUNW,fas@e,8800000/sd@0,0:a

%

Notice that the only difference between the raw and formatted path is the word `raw' at the end of the physical device path, and the directory in which the different paths are stored.

The `sd@0,0:a' references the first partition on disk drive sd@0,0. The first partition is referenced by the `:a'. The letter after the colon indicates which individual partition is referenced from that device path, `a' references partition

1, `b' is partition 2, `c' is partition 3, `d' is partition 4, `e' is partition 5, `f' is partition 6, and `g' is partition 7.

Product

Solaris 9 Operating System

Solaris 8 Operating System

Related

Products

Sun Microsystems > Operating Systems > Solaris Operating System > Solaris SPARC Operating System

Keywords

SOLARIS

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