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Tuning Informix Engine Parameters v1.7

William D. Burton
source: http://www.bamph.com/printable.htm


Tuning engines is one of my all time favorite activities be they internal combustion or database.

Rare is the database engine that cannot be tuned for another 20%. Several orders of magnitude improvement is not uncommon for a well organized tuning session. This is not due to any mysterious talents of mine. Rather, that systems go out of tune rather quickly or were never tuned in the first place.

Below is a list of suggested settings for Informix Engine Configuration parameters I have gathered over the years. I keep a copy in my organizer for easy field reference.

I have collected many books that discuss tuning Informix databases. It has been my observation that they conflict more than they agree. In my mind there are several reasons for this:

  1. Changes to underlying engine architecture obsoletes recommendations with tremendous frequency.
  2. Unix implementations differ wildly at granularities important to the tuner.
  3. The rate of hardware advances quickly overtakes information learned.

The first item above was demonstrated in an early release of version 7. The Informix engine disk IO subsystem changed so markedly that all previous NUMAIOVPS recommendations were incorrect.

A wonderful example of item 2 above is the efficiency of the implementation of the Unix process manager. A series of tests I performed at Informix's benchmark labs in Menlo Park demonstrated configuring twice as many CPUVPS as hardware CPUs was optimal for HP/UX. The version of Solaris that we were testing preferred the recommended 1:1 relationship for this.

My favorite conflicting pronouncements are on page 6-134 of INFORMIX-OnLine Dynamic Server Performance Tuning Training Manual (2/97). When discussing configuring NUMAIOVPS for systems that do not use kernel IO the page states a 1:1 relationship with controllers while the third sentence following states one per disk.

Note that these examples discuss wide variability in what are arguably the two most significant tuning parameters.

This should leave the reader with three important conclusions.

  • It is dangerous to speak of such things in absolute terms.
  • Each important end user environment must be measured and tuned individually i.e. cookie cutter approaches will cause trouble for the DBSA.
  • More importantly, due to the fluidity of this most fundamental knowledge set DBSA should continue to be well compensated.

What follows is my collection of notes for each onconfig parameter. Most are direct out of a book. Comments in blue/italics are from my own experience.

Where more than one suggestion is listed they are in chronological order, oldest first.

The assumption is made that the system under test (SUT) is a dedicated DB server. If it isn't, it should be. Informix should be thought of as an Operating System unto itself. All it really wants from Unix is slices of CPU time. If it must share it's sandbox with applications, other database engines, or even other Informix instances it is operating out of the bounds of it's target design.

Acknowledgement that this level of isolation is not always possible in the real world has been addressed recently with new onconfig parameters. Important adjustments for tuning in a "mixed environment" are marked "  ".

Any mission critical DB should be on a dedicated machine.

"-" implies the onstat command.

Sometimes the DSBA does not have the luxury of an extended tuning session. He must make a quick change to a production box to get it back within acceptable parameters. I have noted parameters that allow for a " big bang for the buck " with a * .

These parameters are grouped logically.

Sometimes the DBA does not have the luxury of an extended tuning session. He must make a quick change to a production box to get it back within acceptable parameters. I have noted parameters that allow for a "big bang for the buck" with a * .



  • Logical Logs + [Phys logs] + [Temp Tables] + [Data] + [On-Arc.catalogs] + crtl info (res. pages ). This is a basic size calculation formula. For systems of any import most of these logical units should of course be on separate spindles.


  • unique for each engine on CPU.



  • On a dedicated server turn this on (1). The parameter is ignored if it is not supported.
  • In a mixed environment turning this on may make Informix "not play well with it's freinds"


  • #HWCPUs - 1 if #HWCPUs > 3
  • #scan thrds (frag issue) SB a multiple or factor of NUMCPUVPS determine w/ glo ath rea.
  • For data loading = #HWCPUs unless HPL used, then = HWCPUs - CONVERTVPS (onpload config param)
  • I have seen systems that enjoy 1:1 VCPUS to HW CPUs relationship and I have seen some that prefer 2:1:1



  • Both of these turn on different housekeeping mechanics. If NUMCPUVPS is set to one it is very important to turn both of these off, or 0 and 1 respectively (yech). You will see no benefit to configuring one CPUVP unless these are both off. This is overlooked more often than not!
  • I would suggest that these should really be considered as one parameter together as if one is adjusted the other should be adjusted likewise.


  • 1 if supported this parameter turns off Unix nicing (this is good). Nicing is the Unix mechanism that lowers a processes priority over time to ensure equality in a mixed environment.  As Informix just wants machine cycles a dedicated server should have this enabled.
  • Tuning for a mixed environment is not so black and white. It may be best to have this off initially and turn it on if it is desired to give Informix more CPU time.


  • CPU number to start binding to. mpstat will provide number of CPUs and their associated number. This numbering system seems whimsical.


  • Number of hardware CPUs to bind to.
  • This can be very beneficial for systems that are not dedicated to the DB.
  • Other non-Informix processes may be allowed to run on the HW CPUs identified here, but the CPUVPS will be restricted to those identified.


  • 0 internal timer is faster. I have never seen OS timing used.

-g glo There should be a 10:1 ratio between time spent in usr vs. sys for CPUVPS. (This has become perhaps 3:1 with KAIO). If sys is too high on a system with just a few HW CPUs try using just 1 CPUVPS.

-g rea If this shows seven or more threads waiting adding CPUVPS can bring it down.



  • Very marginal tool. load average is a combination of system resource measurements. I have seen slow systems with a la of 2 and I have seen systems that seemed find at double digits.
  • Useful only relative to an earlier measurement on the same machine.


BUFFERS * (pages)

  • start w/20% RAM, may be up to 50% RAM. On a dedicated machine why not start at 50%?
  • increase size till increase in cache hits insignificant or excess system paging occurs, use sar or vmstat to determine excess paging.
  • OLTP Target 95% read, 85% write cache hits
  • buffers smaller than largest table for DSS will force light scans. Use -g lsc to measure light scans
  • Maximize for data loading (50% or more) (except HPL express mode)
  • More buffers can mean longer checkpoints

NUMAIOVPS * - Suggestions for this seem to change with the weather.

  • 1 per db disk + 1 for ea. chunk accessed freq.
  • If KAIO is used allocate 1 + 2 for each cooked chunk (get rid of any cooked chunks)
  • For KAIO systems 2 for OnLine 1 per controller containing cooked chunks
  • For systems w/o KAIO 2 for OnLine + 1 for each controller then add as indicated.
  • 1 per dbspace
  • 1 per disk
  • 1 per mirrored pair
  • 1 per chunk.
  • '-g ioq' to monitor IO Qs
  • DSA spawns one read thread per dbspace (AIO or KAIO)
  • My suggestion is to get a system that supports KAIO and then set this to 2.


  • most machines limit at 30, all limit at 32.
  • Dig: For systems that do not perform light scans, do not set RA_PAGES higher than 32.
  • higher for typically sequential DSS
  • if too high will lower %cached reads
  • if bufwaits unusually high RA_PAGES may be too high, or difference between RA_PAGES and RA_THRESHOLD may be too small.


  • Set close to RA_PAGES e.g. RA_PAGES 32 and RA_THRESHOLD 30, if bufwaits (-p) increase reduce RA_THRESHOLD. If most machines limit at 30 won't the RA_THRESHOLD remain in a constant TRUE state?
  • Ideally RA-pgsused = (ixda-RA + idx-RA + da-RA)


  • at least two each on a different drive, more if building large indices
  • DSS environments should use HW striping a small number of TEMPDBS across multiple disks.
  • Max space required for index build is: non-fragmented tbls (key_size+4) * num_recs *2, fragmented (key_size+8) * num_recs *2

FILLFACTOR (indices)

  • 90 is typical, 100 for SELECT/DELETE only tables
  • forces initially very compact indices & efficient caching.
  • 50-70% for tbls with high INSERTS to delay need for node splitting


  • always mirror.
  • A few years ago the fellow that tests this at Informix posited on USENET to use HW mirroring over Informix every time . This make sense as who will have a more intimate knowledge of the devices? HW solutions are always faster than SW. In order of preference I would suggest HW, OS and then Informix mirroring.
  • For machines where availability is paramount one can mirror across controllers and even arrays.



  • when set to "1" this undocumented parameter will generate read and write timings in the syschktab SMI table. See Appendix C of DSA Performance Tuning Training Manual

Increasing the Unix priority of AIO processes can improve the performance of data returned from disk.

Monitor IO with -g ioq (iof and iov are also worthy). When AIOs used gfd len SB < 10, maxlen <25. Maxlen often breaks 25 during engine initialization when it is unimportant so make this distinction . -D will show hotspots at disk level -g ppf at partition level.

When building an important Data Warehouse for my current employer the Sun Hotshot suggested placing all data in only the middle 2GB sectors of each 4GB disk, leaving the remaining unused. The highly paid Informix representative felt strongly that using only the leading 2GB of sectors would perform better. I suggested that we test and if it was within 5% that this be decided by ease of maintenance. The leading sectors proved 2% faster than the middle. I do not recall which I ended up implementing.

If your system is IO bound verify if it be controller or disk bound. The solutions are different.

Throughput = (pg_size * num_pgs_requested/max_transfer_rate) + latency

The use of clustered indices can greatly increase sequential reads.

Informix recommends using fragmentation over HW striping unless the table is a poor canditate for fragmentation. I would like to test this statement someday.

I have not been able to test how Kernal IO effects NUMCPUVPS configuration. From DSA Performance Tuning Manual (2-97) "If your system supports kernal aio, onstat -g ath will show one kio thread per CPUVP." Therefore should NUMCPUVPS be associated with the number of disks, etc or remain a function of the number of hardware CPUs?





  • 1 per disk if < 20 disks
  • 1 per 2 disks if 20 to 100
  • 4 per disk if > 100
  • UPDATE at least one per LRU queue pair. This has most recently been best for me.
  • If -f indicates that all cleaners are active allocate more.


  • ! rootdbs. Place on a separate spindle


  • = usrthreads * 5 (or size of most freq. blob) * 4


  • = usrthrds * 5 * 4
  • UNLESS tblspace blobs in DB w/o logging then usrthrds * size of freq blob pg * 4


  • > 3
  • In all my manuals I can find no good reference for performance tuning this parameter.
  • I have no experience adjusting this save for eliminating anomalous behavior.


  • > 200kb
  • small for greatest recovery, if tape slow, or blobpages volatile
  • (connects * maxrows (in one trans)) * 512


  • size of 3 LL buffers in RAM
  • determines freq of flushing to disk


  • LOGFILES + 3


  • adjust interval of checkpoints, see LRU.
  • For data loading (except HPL express mode) and parallel index builds 3000
  • a large interval will allow size of physlog (i.e. amount of work accomplished) to determine when chkpts occure.
  • -F will show if writes are LRU driven or CHKPTINTVL driven
  • -l and -m will determine if checkpoint interval is driven by Physlog = 75% full or this param


  • max(4, NUMCPUVPS)
  • if -R shows #dirty pages > LRU_MAX add LRU Qs. If no change increase CLEANERS
  • UPDATE: 4 per CPUVP (this relationship has most recently been best for me.)
  • 1 per 500-750 buffers, up to 128 (max)
  • More LRUs better support large number of users by reducing buffwaits

To monitor -g spi shows contention for individual LRU queues

LRU_MAX[MIN]_DIRTY - %buffers assigned to modified queue

  • lower to decrease checkpoint duration
  • For data loading (except HPL express mode) & parallel index builds 70 & 80%, allow to almost fill before flushing

Monitor: -R queue length, -F writes forced by this parameter by CLEANER thread


  • Set to /dev/null Informix will just marked the LL as backed up, not even going through the motions. This should be done only on systems where logical log recovery is not required and the user has been made aware of the implications.


  • 50


  • 60

LBU_PRESERVE - Preserve last log for log backup

  • If physlog frequently fills decrease CKPINTVL
  • phys logging - buffsize/pages IO SB >75%, if near 100% increase physbuff size
  • physical and logical log buffers should be about 75% full when flushed.
  • Huge bang for the buck makes Checkpoints another early thing to look at.




  • 0



  • 1 if CPU, additional poll threads assign to NETVPs
  • 300 single HWCPU 350 if more
  • For data loading one per CPUVP. Each poll thread should be on a CPU class VPS (running a poll thread in-line )
  • Do not increase user connects as this will increase work for the poll thread




  • max # consumed by any query * # concurrent users




SHMVIRTSIZE * (kilobytes)

  • OLTP - Big Resident (buffers), small Virtual (SHMVIRTSIZE)
  • DSS - Small Resident, big Virtual
  • DSS may be up to 75% RAM if paging is not induced

SHMADD (kilobytes) -

  • 10 - 20% of SHMVIRTSIZE


  • 0 unlimited
  • can be used to make Informix more polite reserving resources for other applications. I have also had to use this when a failing malloc panics oninit

Monitoring: -g seg - One of the first things I check as consolidating shared memory segments can be huge bang for the buck.

Note that onmode -F (used to free memory segments) can cause system failures on an active system and should be avoided.


Sar -g 3 3 - will show paging activity



To calc shared memory segments corresponding to a DB instance shmid - 52564801*.0001 = SERVERNUM



  • if > 0, will enhance parallelization of index builds (which after 7.2 are always parallel to some degree)


  • 100



  • DSS should be 90% of SHMVIRTSIZE


  • 1048576


Each sort thread gets quantum/#sort threads memory

A users effective priority = (pdqpriority/100) * ( MAX_PDQPRIORITY/100) where pdqpriority is set by the environment variable or the SET PDQPRIORITY statement.



  • 10


  • 1



  • 0 As many systems that I have seen fail due to Informix filling /tmp the "out of the box" default should be off.


  • 0


  • 0


  • 1


  • off


  • 0



  • 0


  • 30


  • 30


  • /usr/informix/etc/dr.lostfound


Oncheck -pr can be used to replace lost onconfig file

Resident - Buffer Pool (Pool is for residents use only - I needed a mnemonic for this.)

Virtual - Light Scan Area (Virtually no logging - this too.)

Storage overhead

  • 28 bytes per data page is used by the engine
  • 4 byte slot entry for each row

Scans and Sorts for index builds are always parallel with 7.2. Idx builds on fragmented tables add parallel B-(sub)tree builders

UPDATE STATISTICS - single most important SQL statement for Q perf.


  • lead columns in each index
  • all columns queried with equality filters (=)
  • all join columns
  • 1 st col to uniquely distinguish a composite idx from another on same table and all cols preceding


  • all other columns


  • all idx cols. Not run through on HIGH

To speed up update statistics set PSORT_NPROCS to 2, use DBSPACETEMP (duh) do NOT set DBUPSPACE (limits RAM for US)

The following came from a class handout. I have no idea the original author.

Database Tuning the Informix Way.

  1. Establish performance objectives.
  2. Measure database activity and use of resources.
  3. Identify performance problems such as excessive use of CPU, memory, or disks.
  4. Tune the operating system.
  5. Tune the Online Dynamic Server
  6. Optimize the placement of logs, sort space, and temporary space.
  7. Optimize table placement, sizes of extents, and fragmentation
  8. Make sure the indices are appropriate.
  9. Optimize background activities such as logging, checkpoints, and page cleaning.
  10. Schedule backups and batch jobs for off-peak hours.
  11. Review application programs to make sure appropriate access methods are used to retrieve data and algorithms are efficient.
  12. Repeat steps 2 through 11.

My additions would be:

  • .5 and 2.5 Set users expectations
  • 3 & 4 Find a seasoned Systems Administrator for assistance.
  • 11 could be performed much earlier in the cycle as more often than not, the largest performance gains come from applications tuning.
  • Set aside as much time as possible to dedicate to this task. Three days is a minimum. In a development environment ensure that two weeks are dedicated to this at the end of the project. This will invariably get squeezed to three days which, as above, is the absolute minimum to perform a thorough job.

Calculating maximum number of extents for a particular table:

Max#extents <= (pagesize - ((4 * number of columns in a table) + (8 * number of BLOB and VARCHAR columns + 136) + (12 * number of indices) + (4 * number of columns in the indices) + 84))

Loading Hints:

Fragment large tables by round robin

Find hot tables:

SELECT t.tabname table, p.pf_dskreads + p.pf_dskwrites totalops,p.pf_dskreads diskreads , p.pf_dskwrites diskwrites

FROM sysptntab p, systabnames t

WHERE t.partnum = p.partnum

ORDER BY totalops DESC


INFORMIX-OnLine Dynamic Server Performance Tuning Training Manual - Informix part number 502-5-403-1-9999999-1 - Liz Suto, Course Designer

Tuning Informix Dynamic Server and Your System for Optimum Performance - Art S. Kagel

More to come...

Please forward any comments to informix@bamph.com

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