42Understanding Veritas Volume Manager

Volume layouts in VxVM

Mirroring (RAID-1)

Note: You need a full license to use this feature with disks other than the root disk.

Mirroring uses multiple mirrors (plexes) to duplicate the information contained in a volume. In the event of a physical disk failure, the plex on the failed disk becomes unavailable, but the system continues to operate using the unaffected mirrors.

Note: Although a volume can have a single plex, at least two plexes are required to provide redundancy of data. Each of these plexes must contain disk space from different disks to achieve redundancy.

When striping or spanning across a large number of disks, failure of any one of those disks can make the entire plex unusable. Because the likelihood of one out of several disks failing is reasonably high, you should consider mirroring to improve the reliability (and availability) of a striped or spanned volume.

See “Creating a mirrored volume” on page 249 for information on how to create a mirrored volume.

Disk duplexing, in which each mirror exists on a separate controller, is also supported. See “Mirroring across targets, controllers or enclosures” on page 255 for details.

Striping plus mirroring (mirrored-stripe or RAID-0+1)

Note: You need a full license to use this feature.

VxVM supports the combination of mirroring above striping. The combined layout is called a mirrored-stripelayout. A mirrored-stripe layout offers the dual benefits of striping to spread data across multiple disks, while mirroring provides redundancy of data.

For mirroring above striping to be effective, the striped plex and its mirrors must be allocated from separate disks.

Figure 1-17shows an example where two plexes, each striped across three disks, are attached as mirrors to the same volume to create a mirrored-stripe volume.