User Guide

SAP AG November 2002
Ideally, you should use hardware-based options for this mirroring.
However, you can also use the DUAL log mode [Page 143] of the SAP DB database
system.
You should not use RAID-5 configurations for the log volumes. RAID-5 systems do
not allow full mirroring. Because the database instance writes log entries sequentially,
this can lead to a loss in performance.
In a production system, logging should never be deactivated, even if you use RAID
systems.
See also: Log Concept [Page
43]
• Data volumes [Page
130]
If you wish to ensure a high standard of availability, we recommend using RAID-5 or
RAID-1 configurations for the data area [Page 131]. A disk crash and change will then
not affect the running of the database, if the RAID system is able to carry out a
recovery.
Every volume category should be stored on a different disk.
When using fault-tolerant hardware, it is best to only use the same type of hardware when
you want to extend the capacity. For example, RAID-5 systems should only be extended
using RAID-5 systems and mirroring disks with mirroring disks.
UNIX: In a production system, data volumes and log volumes should be used in conjunction
with raw devices. In the event of a system crash, raw devices are extremely secure.
Performance Requirements
For performance reasons, each of the different types of volume [Page 153] should be stored
on a different disk. Therefore, create data volumes [Page 130] and log volumes [Page 144] on
different disks. Because all changes to the database instance are logged in the log areas
[Page 142] , it is the log volumes for a database instance [Page 132] that see the most write
activity.
When RAID-5 systems are used, the database instance should be configured with several
data volumes. Performance will be better with many data volumes than with a single one,
because some parallel mechanisms used by the database system depend on the number of
configured data volumes.
For performance reasons, the log volumes must not be created on RAID-5 systems but only
on dedicated disks or RAID-1 systems.
If swap or paging areas and log entries are kept on the same disk, performance will be
negatively affected.
UNIX: Raw devices should be used for the data volumes and log volumes, because
accessing to data in raw devices is generally quicker than accessing data in files.
User Manual: SAP DB 76