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==Introduction ==

By default, AIX is tuned for a mixed workload, and will grow its [[VMM]] file cache up to 80% of physical RAM. While this may be great for an NFS server, SMTP relay or web server, it is very poor for running any application which does its own cache management. This includes most databases (Oracle, DB2, Sybase, PostgreSQL, MySQL using InnoDB tables, TSM) and some other software (eg. the Squid web cache).

Common symptoms include high paging (high <tt>pgspin</tt> and <tt>pgspout</tt> in <tt>[[topas]]</tt>), high system CPU time, the [[lrud kernel thread]] using CPU, slow overall system throughput, slow backups and slow process startup.

For most database systems, the ideal solution is to use [[raw logical volumes]]. If this is not acceptable, then [[direct I/O]] and [[concurrent I/O]] should be used. If for some reason this is not possible, then the last solution is to tune the [[AIX]] file caches to be less aggressive.

== Parameters ==

The four main parameters that should be tuned are the three controlling the size of the persistent file cache (<tt>minperm%</tt> and <tt>maxperm%</tt>) used for JFS filesystems, and the client file cache (<tt>maxclient%</tt>) used by NFS, CDRFS and JFS2 filesystems, and also the <tt>lru_file_repage</tt> parameter, which influences what pages the [[VMM]] page stealing algorithm will steal (present in AIX 5.2 ML4+ and AIX 5.3 ML1+).

; numperm% : Defines the current size of the persistent file cache.
; minperm% : Defines the minimum amount of RAM the persistent file cache may occupy. If <tt>numperm%</tt> is less than or equal to <tt>minperm%</tt>, file pages will not be stolen when RAM is required.
; maxperm% : Defines the maximum amount of RAM the persistent file cache may occupy before it is used as the sole source of new pages by the page stealing algorithm. By default, <tt>numperm%</tt> may exceed <tt>maxperm%</tt> if there is free memory available. The setting <tt>strict_maxperm</tt> may be set to one to change <tt>maxperm%</tt> into a hard limit, guaranteeing <tt>numperm%</tt> will never exceed <tt>maxperm%</tt>.
; strict_maxperm : As above, if set to 1, changes <tt>maxperm%</tt> into a hard limit.
; numclient% : Defines the current size of the client file cache.
; maxclient% : Defines the hard maximum size of the client file cache.
; strict_maxclient : Introduced in 5.2 ML4, allows the changing of <tt>maxclient%</tt> into a soft limit, similar to <tt>strict_maxperm</tt>.
; lru_file_repage : Introduced in AIX 5.2 ML4 and AIX 5.3 ML1, this influences the [[VMM]] page stealing algorithm. If set to 0, the algorithm will strongly prefer stealing file pages to satisfy memory requests.

Note that <tt>maxclient%</tt> may never exceed <tt>maxperm%</tt>. In later versions of vmtune, this is enforced by changing both parameters if necessary.

== Tuning for AIX 5.1 and Earlier ==

The tool to use is <tt>/usr/samples/kernel/vmtune</tt>, installed as part of the <tt>bos.adt.samples</tt> fileset. If run without options, it will display the currently configured tuneable values, and some of the current runtime values.

'''Note:''' vmtume may be used to set the current runtime parameters only. To have changes take effect on reboot, vmtune must be initiated as part of the system startups.

An example of a tuning command used on a system running Oracle may be:

 # /usr/samples/kernel/vmtune -p 3 -P 5 -h 1 -t 5

Which sets <tt>minperm%</tt> to 3%, <tt>maxperm%</tt> and <tt>maxclient%</tt> to 5%, and enables <tt>strict_maxperm</tt>.

== Tuning for AIX 5.2 and Later ==

'''Note:''' AIX 5.2 includes a compatibility version of <tt>vmtune</tt>. It is probably most wise to become familiar with the new tools, instead of relying on the backwards compatibility commands.

The main tool to use is <tt>/usr/sbin/vmo</tt>, installed as part of the <tt>bos.perf.tune</tt> fileset. To display current cache sizes (<tt>numperm%</tt> and <tt>numclient%</tt>) use <tt>vmstat -v</tt>.

<tt>vmo</tt> can change both persistent (reboot) values as well as runtime values, and so does not need to be present in the startups. It stores the persistent values in the <tt>/etc/tunables/nextboot</tt> file.

Current values and characteristics may be displayed using:

 # vmo -L
 NAME                      CUR    DEF    BOOT   MIN    MAX    UNIT           TYPE
      DEPENDENCIES
 --------------------------------------------------------------------------------
 memory_frames             512K          512K                 4KB pages         S
 --------------------------------------------------------------------------------
 pinnable_frames           427718        427718               4KB pages         S
 --------------------------------------------------------------------------------
 maxfree                   128    128    128    16     200K   4KB pages         D
      minfree
      memory_frames
 ...

A similar example to the <tt>vmtune</tt> example above using <tt>vmo</tt> may be:

 # vmo -p -o minperm%=3 -o maxperm%=5 -o strict_maxperm=1 -o maxclient%=5

And if making use of <tt>lru_file_repage</tt>:

 # vmo -p -o minperm%=3 -o maxperm%=90 -o strict_maxperm=1 -o maxclient%=90 -o lru_file_repage=0

To check the current size of the persistent file cache and the client file cache, see the <tt>numperm</tt> and <tt>numclient</tt> values reported by <tt>vmstat&nbsp;-v</tt>:

 $ vmstat -v
                524288 memory pages
                474939 lruable pages
 ...
                  10.0 minperm percentage
                  20.0 maxperm percentage
                  44.5 numperm percentage
                211365 file pages
 ...
                  19.7 numclient percentage
                  20.0 maxclient percentage
                 94027 client pages

== See Also ==

=== Internal ===

* [[direct I/O]]
* [[concurrent I/O]]
* [[lrud kernel thread]]

=== External ===

* [http://www-941.ibm.com/collaboration/wiki/display/WikiPtype/Performance+Monitoring+Documentation AIX Wiki Performance Monitoring], links to "VMM Tuning Tip: Protecting Computational Memory" and "Understanding DIO & CIO".
* [http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/WP100556 Oracle 9i & 10g on IBM AIX5L: Tips & Considerations] White Paper.
* [http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/WP100657 Oracle Architecture and Performance Tuning on AIX] White Paper.
* [http://www-03.ibm.com/support/techdocs/atsmastr.nsf/WebIndex/WP100377 Tuning SAP R/3 with Oracle on pSeries] White Paper.
* [http://www.redbooks.ibm.com/abstracts/redp9122.html?Open JFS2/DIO Sequential Input/Output Performance on IBM pSeries 690] Redpaper. IBM Form Number REDP-9122-00.
* [http://service.sap.com/~form/handler?_NNUM=822896 SAP Note #822896]: Increased Repaging Rates in AIX 5.2 and above with JFS2
* [http://service.sap.com/~form/handler?_NNUM=750205 SAP Note #750205]: High memory usage with AIX5.2 and Oracle9.2
* [http://service.sap.com/~form/handler?_NNUM=103747 SAP Note #103747]: Performance: Parameter recommendations for Rel. 4.0 and high
* [http://service.sap.com/~form/handler?_NNUM=78498 SAP Note #78498]: High paging rate on AIX servers, in part. database

[[Category:AIX]]