VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

VERITAS Volume Manager Prerelease 8 September 2005, 8:54am
28 VERITAS Storage Foundation for Oracle Administrator’s Guide
For databases that support online transaction processing (OLTP) workloads, we recommend either
mirrored-stripe or striped-mirror volumes to improve database performance and reliability. For
highest availability, we recommend striped-mirror volumes (RAID 1+0).
RAID-5 (Striping with Parity)
RAID-5 provides data redundancy through the use of parity (a calculated value that the system uses
to reconstruct data after a failure). While data is written to a RAID-5 volume, parity is also
calculated by performing an exclusive OR (XOR) procedure on data. The resulting parity is then
written to another part of the volume. If a portion of a RAID-5 volume fails, the data that was on
that portion of the failed volume can be recreated from the remaining data and the parity.
RAID-5 offers data redundancy similar to mirroring, while requiring less disk space. RAID-5 read
performance is similar to that of striping but with relatively 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.
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.
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