User Guide

SAP AG November 2002
Availability
To ensure high availability of the database system, bear in mind the security requirements
[Page 75].
See also:
Directory Structure of he Database System for Open Source [Page
74]
Directory Structure of the Database System for SAP Systems [Page 67]
Security Requirements
Using the appropriate hardware, operating system, or database features can improve the
availability [Page 38] of a database instance [Page 132].
• Log volumes [Page
144]
In the production system, the log area [Page 142] should always be mirrored for
security.
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.
Restartability
If the database fails (for example, as the result of a power failure), and the volumes [Page
153] were fully functioning, the database system carries out a restart [Page 146]. This means
that the effects that completed transactions had on the data volumes [Page 130] are
reproduced (rolled forward), and the effects that uncompleted transactions would have had
are cancelled out (rolled back).
If a data volume fails (physical disk error), the last, complete data backup [Page 131] must be
imported after the problem has been rectified. However, the last consistent database state
can only be recovered if all the required redo log entries [Page 44] can be imported from the
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