VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 1, Understanding VERITAS Volume Manager
Volume Layouts in VxVM
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Mirroring (RAID-1)
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 theentireplex unusable.Because thelikelihood ofone outof severaldisks
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 170 for information on how to create a
mirrored volume.
Striping Plus Mirroring (Mirrored-Stripe or RAID-0+1)
Note You may need an additional license to use this feature.
VxVM supports the combination of mirroring above striping. The combined layout is
called a mirrored-stripe layout. 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.
The figure, “Mirrored-StripeVolume Laidout on Six Disks” shows an example where two
plexes, each striped across three disks, are attached as mirrors to the same volume to
create a mirrored-stripe volume.