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Oracle disk RAID Technology

The need for the DBA to manually stripe data files across multiple disk spindles has been lessened with the advent of hardware and software RAID.  The two most common levels of RAID are RAID 10 (mirroring and striping) and RAID 5.


For all systems experiencing significant updates, Oracle recommends using SAME  (Stripe and Mirror Everywhere, a.k.a., RAID 1+0 ) due to the update penalty with RAID 5 architectures.   The SAME approach is the foundation of the Oracle10g Automatic Storage Management (ASM) approach.

The Horrors of RAID 5

Many disk vendors persist in pushing RAID 5 as a viable solution for highly updated systems even though using RAID 5 for an Oracle system with a high update rate can be disastrous to performance.


In short, it is clearly troublesome for any company with high volumes of updates to use RAID 5, and the goal of Oracle10g with Automatic Storage Management (ASM) is to urge Oracle customers to use RAID 1+0 .


The Oracle DBA will nevertheless be concerned about their most critical performance area, the disk I/O sub-system, until such time that solid-state disks are cheap enough to fully cache large databases.  The main points of this section include the following:

§       The old-fashion file placement rules still apply if RAID is not being used, and Oracle data files must be manually placed across the disk spindles to relieve I/O contention.

§       RAID 5 is not recommended for high update Oracle systems. The performance penalty from the parity checking will greatly diminish Oracle performance.

§       Using a RAID 10 approach (striping and mirroring) distributes data blocks across all of the disk spindles, making hot disks a random and transient event.

§       Oracle10g continues to expand support for very large RAM data buffers and buffer monitoring with the v$db_cache_advice utility and Oracle10g Automatic Memory Management (AMM).

§       Improving disk access speed will not help if disk is not the source of the bottleneck. The top-5 STATSPACK wait events should be checked to ensure that disk I/O is the bottleneck prior to undergoing expensive changes to the disk I/O subsystem.

§       Many Oracle customers are using solid-state disk for high I/O data files such as TEMP, UNDO, and REDO files.  Solid-state disk is becoming less expensive and may soon replace traditional disk devices.

In sum, Oracle DBAs must be steadfast in attempting to understand their disk I/O sub-system and making sure that disk I/O does not impede the high performance of their systems.


RAID 5 can be deadly for high-update databases!


Even with lots of caching, the goal of almost all Oracle tuning activities is to reduce I/O.  Tuning the instance parameters, sizing the library cache, tuning SQL statements all have the common focus of reducing I/O.  Even when physical disk I/O (POI) is minimized, logical I/O (LIO) is still a major contributor to response time.  Even fully cached databases will run slowly if sub-optimal SQL statements force thousands of unnecessary LIOs against the buffer cache.


Time and time again, DBAs spend time and energy tuning a component of their database that is not a top wait event, and they are surprised to find that their change did not make a huge difference in performance. For example, a faster CPU does not help an I/O-bound system, and moving to faster disk does not help a CPU-bound system.


All Oracle databases have some physical constraint, and it is not always disk. The best way to find the constraints for the system is to examine the top five wait events on the STATSPACK report. 


The system is disk I/O bound if the majority of the wait time is spent accessing data blocks. This can be db file sequential read waits, usually index access, and db file scattered read waits, usually full-table scans:


Top 5 Timed Events

                                                  % Total

Event                              Waits          Ela Time

--------------------------- ------------ ----------- --------

db file sequential read            2,598          48.54

db file scattered read            25,519          22.04

library cache load lock              673           9.26

CPU time                              44           7.83

log file parallel write           19,157           5.68


Once the determination has been made that the Oracle database is I/O-bound, the DBA should get a full grasp on the internals of disk devices, layers of caching, and the configuration of disk controllers and physical disk spindles.


An I/O bottleneck may also manifest itself with high CPU run queue values combined with high idle times.


SERVER_NAME    date      hour   runq  pg_in pg_ot  usr  sys    idl

-------------  ------- ------   ----  ----- ----   ----- ----- ----

CSS-HP1        04/06/15   22     10    4     0     11    2     87

CSS-HP1        04/06/15   09      9    4     0     11    2     87

CSS-HP1        04/06/14   22     10    2     0     46    6     50

CSS-HP1        04/06/14   07      9    4     0     10    2     88


Normally, a high vmstat run queue, where the run queue values exceed the number of CPUs on the server, indicates CPU overload.  However, when combined with high CPU idle times, the run queue values indicate that the CPU is backed up waiting for I/O to complete.




This is an excerpt from my latest book "Oracle Tuning: The Definitive Reference". 

You can buy it direct from the publisher for 50%-off and get instant access to the code depot of Oracle tuning scripts:




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