Veritas Storage Foundation™ for Oracle 5.0.1 Administrator's Guide

slow write performance. RAID-5 is useful if the database workload is read-intensive
(as in many data warehousing applications). You can snapshot a RAID-5 volume
and move a RAID-5 subdisk without losing redundancy.
About online relayout
As databases grow and usage patterns change, online relayout lets you change
volumes to a different layout, with uninterrupted data access. Relayout is
accomplished online and in place. Use online relayout to change the redundancy
or performance characteristics of the storage, such as data organization (RAID
levels), the number of columns for RAID-5 and striped volumes, and stripe unit
size.
About volume resynchronization
When storing data redundantly, using mirrored or RAID-5 volumes, Veritas
Volume Manager ensures that all copies of the data match exactly. However, if
the system crashes, small amounts of the redundant data on a volume can become
inconsistent or unsynchronized. For mirrored volumes, unsynchronized data can
cause two reads from the same region of the volume to return different results if
different mirrors are used to satisfy the read request. In the case of RAID-5
volumes, unsynchronized data can lead to parity corruption and incorrect data
reconstruction.
In the event of a system crash, Veritas Volume Manager ensures that all mirrors
contain exactly the same data and that the data and parity in RAID-5 volumes
agree. This process is called volume resynchronization. Not all volumes require
resynchronization after a system failure. VxVM notices when a volume is first
written and marks it as dirty. Only volumes that are marked dirty when the system
reboots require resynchronization.
The process of resynchronization can impact system and database performance.
However, it does not affect the availability of the database after system reboot.
You can immediately access the database after database recovery although the
performance may suffer due to resynchronization. For very large volumes or for
a very large number of volumes, the resynchronization process can take a long
time. You can significantly reduce resynchronization time by using Dirty Region
Logging (DRL) for mirrored volumes or by making sure that RAID-5 volumes have
valid RAID-5 logs. However, using logs can slightly reduce the database write
performance.
For most database configurations, we recommend using dirty region logs or the
RAID-5 logs when mirrored or RAID-5 volumes are used. It is also advisable to
evaluate the database performance requirements to determine the optimal volume
configurations for the databases.
Introducing Veritas Storage Foundation for Oracle
How Veritas Volume Manager works
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