Veritas Storage Foundation™ for Oracle 5.0.1 Administrator's Guide

in the first subdisk from beginning to end. Data is then accessed in the remaining
subdisks sequentially from beginning to end, until the end of the last subdisk.
You can use concatenation with multiple subdisks when there is insufficient
contiguous space for the plex on any one disk. This form of concatenation can be
used for load balancing between disks, and for head movement optimization on
a particular disk.
Concatenation using subdisks that reside on more than one VxVM disk is called
spanning.
Warning: Spanning a plex across multiple disks increases the chance that a disk
failure results in failure of the assigned volume. Use mirroring (RAID-1) or striping
with parity (RAID-5) to reduce the risk that a single disk failure results in a volume
failure.
Spanning is useful when you need to read or write data sequentially (for example,
reading from or writing to database redo logs) and there is not sufficient contiguous
space.
How striping (RAID-0) works
Striping is a technique of mapping data so that the data is interleaved among
multiple physical disks. Data is allocated in equal-sized units (called stripe units)
that are interleaved between the disks. Each stripe unit is a set of contiguous
blocks on a disk. A stripe consists of the set of stripe units at the same position
across all columns. A column is a set of one or more subdisks within a striped
plex.
Striping is useful if you need large amounts of data written to or read from physical
disks, and performance is important. Striping is also helpful in balancing the I/O
load from multi-user applications across multiple disks. By using parallel data
transfer to and from multiple disks, striping significantly improves data-access
performance.
When striping across multiple disks, failure of any one disk will make the entire
volume unusable.
How mirroring (RAID-1) works
Mirroring is a technique of using multiple copies of the data, or mirrors, to
duplicate the information contained in a volume. In the event of a physical disk
failure, the mirror on the failed disk becomes unavailable, but the system continues
to operate using the unaffected mirrors. For this reason, mirroring increases
system reliability and availability. A volume requires at least two mirrors to
Introducing Veritas Storage Foundation for Oracle
How Veritas Volume Manager works
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