Solstice DiskSuite 4.2.1 Reference Guide

Metadevices

A metadevice is a name for a group of physical slices that appear to the system as a single, logical device. Metadevices are actually pseudo, or virtual, devices in standard UNIX terms.

You create a metadevice by using concatenation, striping, mirroring, RAID level 5, or UFS logging. Thus, the types of metadevices you can create are concatenations, stripes, concatenated stripes, mirrors, RAID5 metadevices, and trans metadevices.

DiskSuite uses a special driver, called the metadisk driver, to coordinate I/O to and from physical devices and metadevices, enabling applications to treat a metadevice like a physical device. This type of driver is also called a logical, or pseudo, driver.

You can use either the DiskSuite Tool graphical user interface or the command line utilities to create and administer metadevices.

Table 1-3 summarizes the types of metadevices:

Table 1-3 Types of Metadevices

Metadevice 

Description 

Simple

Can be used directly, or as the basic building blocks for mirrors and trans devices. There are three types of simple metadevices: stripes, concatenations, and concatenated stripes. Simple metadevices consist only of physical slices. By themselves, simple metadevices do not provide data redundancy.  

Mirror

Replicates data by maintaining multiple copies. A mirror is composed of one or more simple metadevices called submirrors. 

RAID5

Replicates data by using parity information. In the case of missing data, the missing data can be regenerated using available data and the parity information. A RAID5 metadevice is composed of slices. One slice's worth of space is allocated to parity information, but it is distributed across all slices in the RAID5 metadevice. 

Trans

Used to log a UFS file system. A trans metadevice is composed of a master device and a logging device. Both of these devices can be a slice, simple metadevice, mirror, or RAID5 metadevice. The master device contains the UFS file system. 

How Are Metadevices Used?

You use metadevices to increase storage capacity and data availability. In some instances, metadevices can also increase I/O performance. Functionally, metadevices behave the same way as slices. Because metadevices look like slices, they are transparent to end users, applications, and file systems. Like physical devices, metadevices are accessed through block or raw device names. The metadevice name changes, depending on whether the block or raw device is used. See "Metadevice Conventions" for details about metadevice names.

You can use most file systems commands (mount(1M), umount(1M), ufsdump(1M), ufsrestore(1M),and so forth) on metadevices. You cannot use the format(1M) command, however. You can read, write, and copy files to and from a metadevice, as long as you have a file system mounted on the metadevice.

SPARC and x86 systems can create metadevices on the following disk drives:

Metadevice Conventions

Table 1-4 Example Metadevice Names

/dev/md/dsk/d0

Block metadevice d0 

/dev/md/dsk/d1

Block metadevice d1 

/dev/md/rdsk/d126

Raw metadevice d126 

/dev/md/rdsk/d127

Raw metadevice d127 

Example -- Metadevice Consisting of Two Slices

Figure 1-1 shows a metadevice "containing" two slices, one each from Disk A and Disk B. An application or UFS will treat the metadevice as if it were one physical disk. Adding more slices to the metadevice will increase its capacity.

Figure 1-1 Relationship Among a Metadevice, Physical Disks, and Slices

Graphic