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End of Support Dates

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IBM Informix Webcast

Best Practices for Informix Database Performance: A Field Study
June 18, 2002

Questions and Answers Log -- by Marty Lurie, Presenter

This is a very good set of questions - hope I've done them justice.


Ernie Knox Asked: How can I get a copy of this slide show emailed to me?

Please see: http://www7b.software.ibm.com/dmdd/zones/informix/library/techarticle/lurie/0201lurie.html


Gerardo Rojas Asked: How can I detect a performance trouble between hardware and IDS?

My favorite tools are vmstat and iostat. Run vmstat 3 and watch the pipo columns which show paging. If the machine is paging you need to fix that before you can do any database tuning. For disk i/o run iostat. For processor utilization try the "top" utility. You may have to download this since it is not part of all distributions.


Brent Kwasnica Asked: What are the party highlights during the conference?

Awesome party, don't miss it ;-)


Vineet Mehrotra Asked: How to tune the VPs. and what is the recommended load for them in top list

1 cpu vp per physical processor. If the application is resident on the database server then reduce the number of CPU vps.


Brent Kwasnica Asked: What are the party highlights during the conference?

Answered Privately: Please see http://www-3.ibm.com/services/learning/conf/us/dm/index.html for information regarding the conference logistics.


Zafar A. Mohammed Asked: We have 3.2 billions rows in a single table

There are a number of large systems that were not in the survey - I hope next time around you'll participate.


Vineet Mehrotra Asked: how to tune the update stats in OLTP and whats the best practice..when to run low,medium and high

Please see http://www.informix.com/answers/english/docs/93ids/8344.pdf, the performance tuning guide, starting at page 13-13


The group provided some great numbers for database uptime, very impressive!


Peter Diaz de Leon Asked: How do I reduce extents in system tables

You can rebuild sysmaster, but I'd like you to have support work with you to do this. Unless the systables are a real mess the effort involved probably isn't worth it.


Peter Diaz de Leon Asked: How do I reduce extents in system tables

You can rebuild sysmaster, but I'd like you to have support work with you to do this. Unless the systables are a real mess the effort involved probably isn't worth it.


Deborah DeCorrevont Asked: we have a query that is running hourly, it runs in 2 minutes, but after ulpdate stats low, it went to 8 minutes. we captrued times through a script. what's up w/ that?

Stats low doesn't gather distributions. Set explain on and capture the query plans, and that will show what the database is doing differently. Please take a look at: http://www.informix.com/answers/english/docs/93ids/8344.pdf, the performance tuning guide, starting at page 13-13


Patrick Edison Asked: Where can we get more info on Nested Loop Joins and Hash Joins? What they are; what causes them, etc.?

This is well covered in the Managing and Optimizing class. A nested loop join may be very good for what you're trying to accomplish, so don't write them off. The optimizer will look at the statistics and make a choice about nl join or hash join to get the answer as fast as possible.


John Bejarano Asked: Re: extents. I have many tables with attached pk indexes, that form their own extents. I always thought attached indexes shared extents with their parent table. Am I wrong?

According to the "Internals and Advanced Architecture" course guide for a fragmented table "there is no case where a table fragment will have data pages and index pages intermingled."


Gerardo Rojas Asked: Is it restrictive de use of CPUVPS in relation with physical CPUs?

For DSS configure 1 cpuvp per physical cpu. For OLTP configure cpuvps=(physical cpus - 1). If you are using multiple residency or the application is residnet on the same server as the database, all bets are off.


Mike Varney Asked: How much value is there in fragmenting your table when you have a RAID array? Is the benefit of having multiple scan threads on multiple DBspaces negated by the RAID array distributing read/write load across it's member disks?

If you don't fragment you only get one scan thread. This is slow. I recommend you fragment for a query environment. Fewer fragments are needed for OLTP. When you fragment work with your RAID provider to make sure you don't end up with one "hot" disk in the RAID array.


Don Wolford Asked: How can you correctly build statistics for a transitory table (usually empty, sometimes huge)?

Build this into your SQL, for example:
select * from customer into temp foo;
-- use "resolution 5" on a large table to speed the update
update statistics medium for table foo resolution 5;


dbrieck Asked: What performance impact does ovbuff from onstat -p indicate?

From the admin guide:
The onstat -p output contains a statistic (ovbuff) that indicates the number of times the database server attempted to exceed the maximum number of shared buffers specified by the BUFFERS parameter in the ONCONFIG file.
If your buffer hit rates are good ( 95%+ for read, 85%+ for writes), you're probably ok, otherwise you probably need more buffers, and more RAM on the server if adding buffers results in iopaging (use vmstat to check for paging).


Corrado Asked: why I have slow performance on IDS 7.31 when I scan a table of 1.000.000. of rows ?

I assume this is 1 million rows. I'd check 1)table fragmentation and get 3 scan threads for each CPU, 2)vmstat for paging 3)iostat for disk bottlenecks.


Vineet Mehrotra Asked: how to tune the VPs (CPU) if in top command the oninits are taking too much values

Oninit using all the processor resource is good, assuming there is a heavy workload. If you have other processies that need cpu cycles then reduce the number of oninits.


Susan McGlasson Asked: fragmentation - can you fragment based on i/o. We have some medium size tables but they get the most i/o on our system. Would it help to fragment these?

Yes - you can distribute the i/o load over mutliple disks which will give better performance.


James Chhay Asked: Is it true that large buffer on OLTP system causes delayed check point?

Large buffer pools can cause longer checkpoints. Don't over configure the buffers. To minimize checkpoint time keep maxdirty and mindirty at low numbers, for example 2 and 1.


Clifton Bean Asked: Where is the survey.sh located?

It was on the IIUG site, Ron may be able to repost it.


Vineet Mehrotra Asked: How to tune the network part in ids

Please see the Admin Guide, Network Virtual Processors 10-32


James Asked: How does one determine the best size for your logical logs? There are a lot of different ideas out there.

Yes there are a lot of suggestions. Here are the averages:
OLTP
Average Number of logs 85
Median Number of logs: 38
Average Log size: 44 meg
Median Log size: 10 meg
DSS ? uses both logged and non-logged!
Average Number of logs 56
Median Number of logs: 20
Average Log size: 337 meg
Median Log size: 144 meg


Tom Puch Asked: What is there a client tool that can be used by the DBA similar to TOAD for Oracle?

Not familiar with TOAD for Oracle, but I now understand why Oracle has many warts (humor intended).


Deborah DeCorrevont Asked: does setting DS_TOTAL_MEM turn on PDQPRIORITTY?

No, you must do it in the SQL, where XX is a number 0 to 100: SET PDQPRIORITY XX


Rakesh Shah Asked: how do you know ip for user which is giving listener thread error in online log?

Start with onstat -g ses... sorry I can't research this one right now.


MILIND Asked: we have some large tables which are spread across multiple chunks which are on different disks but on one dbspace, will this create one scan threads or multiple ?

Only ONE scan thread per DBSPACE. So many chunks in one DBSPACE only get one thread.


Bryan Osborne Asked: With multiple instances on a box, does the vp/cpu rule still apply?

Don't allocate more CPUVPs in total across all the instances than you have physical CPUs. If there are more CPUVPs they will just fight each other for a time slice.


Elena Korol Asked: whould you recomend informix in Lunix?

Yes! See http://www7b.boulder.ibm.com/dmdd/library/techarticle/lurie/0111lurie.html


Clifton Bean Asked: Are you available for IIUG Local Meetings?

I can try - what are the details of the event party? (humor intended). Please contact me directly.


Darrell Murphy Asked: We use EMC disk - they claim that their cache eliminates need for worrying about what spindles the data resides on. What's your take?

EMC has a software tool to manage hot disks within the array. So there are times when you will break the cache and need to worry about disk layout - EMC is very good a fixing this one it is proved there is a problem.


Clifton Bean Asked: OS Mirroring vs Informix Mirroring: Which is Better for Recovery Purposes?

If you have OS mirroring use it. Informix mirroring was included before many OS's had mirroring. Informix works fine, but the recent OS mirror technology - hot swap of a dead disk - is a great thing to have.


James Chhay Asked: Is the maximum extent size limited to 2GB?

Not for long - watch for future 9.X products. XPS already has large chunk support.


John Bejarano Asked: The smaller (~25 Gb) of our two databases has been up 231 days at Shutterfly. Our larger database (which was just bounced) is ~220Gb (mostly OLTP with a little DSS).

Well done!


Vineet Mehrotra Asked: where I an get the information for best practices if you are using clarion disks.

Talk with Clarion and also talk to other customers using Clarion. Sorry I can't be of much help here.


Vineet Mehrotra Asked: what are the tools available to measure performance graphically as onperf doesn't work on 64 bit unix

Informix Server Administrator ISA. No real time GUI graphs, but a nice web admin interface.


janis cichon Asked: Is there any issues with configuring more than one bar-worker thread per cosserver for backups/recoveries? logical recoveries fail when more than one is defined per cosever.

This sounds like a bug - please call support.


Peter Kannar Asked: How applicable are the comments you have made about v7 and v9 to v5?

You can try and apply some of the rules of thumb, but v5 is a very different architecture.


david Asked: Enterprise Replication reccomends 1 hour processing per log

Thank you for this info!


Clifton Bean Asked: Use Multiple Logdbspaces on different logs: Reads take place on one drive while backup is on another: good or not good?

Spreading out the logs to balance the I/O across different disks is a good idea. But - this is too much work if you're getting good performance without this effort.


CCC Asked: Would checkpoints ever write data more than LRU_MAX_DIRTY? Should checkpoints be longer than LRU writes?

If the cleaners can't keep up then the checkpoint would have more than the lrumax% to write. The LRU writes are ongoing - the checkpoint is a flush of all changed pages.


Susan McGlasson Asked: is fragmentation elimination important for fragmentation expressions? In our case a modal expression would be best for the expression but it appears this will not use fragmentation elimination. Would it be better to use the modal expression or round robin be best? Thanks, great job!

Thanks. I would try to find a way to get the expressions to get you fragment elimination. This may take some more work, but it would be worth the effort. Look at the queries and see if there are other columns you could use for your fragment key. Round robin is a good quick and dirty way to get fragmented. If performance is good with round robin, use it. But once the data is in the database start preparing for when you'll need to fragment by doing the analysis now.


M.Pruet Asked: onmode -p according ot activity???

Madison makes a good suggestion to dynamically change the number of VPs of a class using the onmode -p command. This can be very useful for changing the configuration without bouncing the server.