Disk Devices

When performing disk administration, it is important to understand the difference between a disk name and a device name.

When a disk is placed under VxVM control, a VM disk is assigned to it. You can define a symbolic disk name (also known as a disk media name ) to refer to a VM disk for the purposes of administration. A disk name can be up to 31 characters long. If you do not assign a disk name, it defaults to disk## if the disk is being added to rootdg (where ## is a sequence number). Otherwise, the default disk name is groupname##, where groupname is the name of the disk group to which the disk is added. Your system may use a device name that differs from the examples.

The device name (sometimes referred to as devname or disk access name) defines where the disk is located in a system.




	NOTE: The full pathname of a device is /dev/vx/[r]dsk/devicename. In this document, only the device name is listed and /dev/vx/[r]dsk is assumed.

Disk Device Naming in VxVM

Prior to VxVM 3.2, all disks were named according to the c#t#d# format. Fabric mode disks were not supported by VxVM. From VxVM 3.2 onward, there are two different methods of naming disk devices:

c#t#d# Based Naming Scheme

In this naming scheme, all disk devices except fabric mode disks are named using the c#t#d# format.

The syntax of a device name is c#t#d#, where c# represents a controller on a host bus adapter, t# is the target controller ID, and d# identifies a disk on the target controller.

Fabric mode disk devices are named as follows:

Disk in supported disk arrays are named using the enclosure name_# format. For example, disks in the supported disk array name FirstFloor are named FirstFloor_0, FirstFloor_1, FirstFloor_2 and so on. (You can use the vxdmpadm command to administer enclosure names.)
Disks in the DISKS category (formerly know as JBOD disks) are named using the Disk_# format.
Disks in the OTHER_DISKS category are named using the fabric_# format

Enclosure Based Naming Scheme

The enclosure-based naming scheme operates as follows:

All fabric or non-fabric disks in supported disk arrays are named using the enclosure_name_# format. For example, disks in the supported disk array, enggdept are named enggdept_0, enggdept_1, enggdept_2 and so on. (You can use the vxdmpadm command to administer enclosure names. See “Administering DMP Using vxdmpadm” and the vxdmpadm(1M) manual page for more information.)
Disks in the DISKS category (formerly known as JBOD disks) are named using the Disk_# format.
Disks in the OTHER_DISKS category are named as follows:
- Non-fabric disks are named using the c#t#d# format.
- Fabric disks are named using the fabric_# format.

See “Changing the Disk-Naming Scheme” for details of how to switch between the two naming schemes.

To display the native OS device names of a VM disk (such as disk01), use the following command:

# vxdisk list diskname

For information on how to rename an enclosure, see “Renaming an Enclosure”.

Private and Public Disk Regions

A VM disk usually has two regions:

private region—a small area where configuration information is stored. A disk header label, configuration records for VxVM objects (such as volumes, plexes and subdisks), and an intent log for the configuration database are stored here. The default private region size is 2048 blocks (2048 kilobytes), which is large enough to record the details of about 4000 VxVM objects in a disk group.

Under most circumstances, the default private region size should be sufficient. For administrative purposes, it is usually much simpler to create more disk groups that contain fewer volumes, or to split large disk groups into several smaller ones (as described in “Splitting Disk Groups”). If required, the value for the private region size may be overridden at installation time by choosing the Custom Installation path, or when you add or replace a disk using the vxdiskadm command.




	NOTE: Each disk that has a private region holds an entire copy of the configuration database for the disk group. The size of the configuration database for a disk group is limited by the size of the smallest copy of the configuration database on any of its member disks.

public region—an area that covers the remainder of the disk and is used to store subdisks (and allocate storage space).

The following basic disk types are used by VxVM:

simple—the public and private regions are on the same disk area (with the public area following the private area). Typically, most or all disks on your system are configured as this disk type.
nopriv—there is no private region (only a public region for allocating subdisks).
This is the simplest disk type consisting only of space for allocating subdisks. Nopriv disks are most useful for defining special devices (such as RAM disks, if supported) on which private region data would not persist between reboots. They can also be used to encapsulate disks where there is insufficient room for a private region. Such disks cannot store configuration and log copies, and they do not support the use of the vxdisk addregion command to define reserved regions. VxVM cannot track the movement of nopriv disks on a SCSI chain or between controllers.

On some systems, VxVM asks the operating system for a list of known disk device addresses. These device addresses are auto-configured into the rootdg disk group when vxconfigd is started. Auto-configured disks are always of type simple, with default attributes.

For more information about disk types and their configuration, see the vxdisk(1M) manual page.

Metadevices

Two classes of disk device files can be used with VxVM: standard devices, and special devices known as metadevices. Metadevices are only used with operating systems that support dynamic multipathing (DMP). Such devices represent the physical disks that a system can access via more than one physical path. The available access paths depend on whether the disk is a single disk, or part of a multiported disk array that is connected to a system.

You can use the vxdisk utility to display the paths subsumed by a metadevice, and to display the status of each path (for example, whether it is enabled or disabled). For more information, see Chapter 3 “Administering Dynamic Multipathing (DMP)”.

Disk Devices

Technical documentation

» Table of Contents

» Glossary

» Index

Disk Device Naming in VxVM

c#t#d# Based Naming Scheme

Enclosure Based Naming Scheme

Private and Public Disk Regions

Metadevices