HCC compression

In my free time this week I have been playing on the exadata.  The first thing I wanted to determine is the compression ratios for my data, and the select statistics.

One of the first observations I made was how HCC saves you on selects. I took a 158g table from another system, and loaded it up in HCC query mode (I loaded it direct).. the table is now 6g in size.. WOW, but that didn't impress me as much as my select.

First I selected from the table with storage indexes off.  2.02 seconds.. 2.02 seconds ?  1.7 Billion rows that was 158g of data.  wow.

Next I select from the table with storage indexes on. it takes 9.02..  The extra 7 seconds was the exadata building the storage indexes.   Then I select again.  .30 seconds.  No indexes.

I am going to do some more testing.  but this certainly is impressive !!

pin x wait on s wait event

Chances are you've stumbled onto my website because you are searching for the keywords "pin x wait on s" "wait" "event" and "parsing".

Well I have been fighting a fire for the last 2 weeks in our performance enviroment on just these "keywords".

We have been doing our testing by sending through a set workload with very similar processing needs.  This workload is date sensitive, so it has been a challenge to keep moving dates and retesting. Recently, we have begin utilzing "flashback database" for our testing.. This has great promise for getting consistent testing done!
You run through your test case scenario, and capture your performance data.  You flashback to the start point, make the change, then run through the same exact scenario. Pretty cool huh ?

Well all this was going as planned.  We ran our processing, saved AWR data, and reran.  Suddenly we started seeing these "pin x wait on S" events holding up processing for minutes at a time. Where did these come from ? was it the release ??  We had to dig in and find out.

Well here is some background.

• Our queries are very, very complex.  They often take ~ 1000ms to parse (1 second), and 4ms to execute.
• flashback will flush your cache, kind of obvious, but pertinent to my situation.
• Our processing is very structured, and sends multiple processing streams through the same steps in the same order

So what was happening ?  Well it turns out that this is caused by hard parsing multiple duplicate sql (probably 50+ in our case) simultaneously.  This wait event is Oracle trying to parse the first sql statement, and having all the others wait.  This parse queing doesn't do well with sql of this complexity, and it throws off the parsing causing these wait events.

Just wanted to pass this on, that if you see this event, and you are doing similar testing, look at the complexity of the queries, and consider hard simultaneous parsing of the same sql as the cuplrit.

Exadata class

This week I attended "exadata and database machine admintration" class.  It was a good class, but they usually are from OU.

This class covers a lot of material in 3 days and it is a good intro to what it's going to be like managing an exadara.  Take heed to the prerecs.  They aren't manditory, but knowing about RAC and grid is necessary to understand how Exadata implements those pieces.

For those who might attend, my advice is do more with dbcontrol,  The class doesn't say much about dbcontrol or grid control.

Well to start at the beginning the class is in a virual environment, Each student has their own virtual 1/4 rack with 1 cpu per node,  This works out well because everyone has their own isolated environment.  This environment has a database with the sh default schema.

In order to get the most out of the class
1) I changed database password for sys,system,dbsnmp,and sysman.
2) I recreated the password file.
3) I installed dbcontrol (dbca)
4) I set up ssh keys for celladmin, and cellmonitor to go to each cell from the oracle account.  Be aware that to set up ssh for cellmonitor, copy the keys as root and change permissions.  The cellmonitor account has very little privileges,
5) I created my own account in asm and granted sysasm to it.

After doing all this I was able to start up dbcontrol and connect to my virtual environment,  This gave me a great view into how the disks were configured, and I was able to add the exadata cells to dbcontrol.

I think having dbcontrol set up made the class much more useful so I could visually see all the items newly available on the exadata.