Exadata

The hang manager

Recently I was looking in the trace directory in the diag dest of my (12.2.0.1) instance, and found some trace files which were different from the others:

$ ls | grep dia0
test_dia0_9711_base_1.trc
test_dia0_9711_base_1.trm
test_dia0_9711_lws_1.trc
test_dia0_9711_lws_1.trm
test_dia0_9711.trc
test_dia0_9711.trm

The dia0 ‘test_dia0_9711.trc’ file is fully expected. But what are these ‘..lws_1.trc’ and ‘..base_1.trc’ files? And ‘base’ is something that I understand, but what would ‘lws’ mean? Lunatics Without Skateboards?

First, let’s look into the normally named trace file of the dia0 process:

Oracle 12.1 big table caching IO code path

Recently I was triggered about the ‘automatic big table caching’ feature introduced in Oracle version 12.1.0.2 with Roger Macnicol’s blogpost about Oracle database IO and caching or not caching (https://blogs.oracle.com/smartscan-deep-dive/when-bloggers-get-it-wrong-part-1 https://blogs.oracle.com/smartscan-deep-dive/when-bloggers-get-it-wrong-part-2). If you want to read something about the feature in general, search for the feature name, you’ll find several blogposts about it.

Oracle C functions annotations

Warning! This is a post about Oracle database internals for internals lovers and researchers. For normal, functional administration, this post serves no function. The post shows a little tool I created which consists of a small database I compiled with Oracle database C function names and a script to query it. The reason that keeping such a database makes sense in the first place, is because the Oracle C functions for the Oracle database are setup in an hierarchy based on the function name. This means you can deduct what part of the execution you are in by looking at the function name; for example ‘kslgetl’ means kernel service lock layer, get latch.

To use this, clone git repository at https://gitlab.com/FritsHoogland/ora_functions.git

The full table scan direct path read decision for version 12.2

This post is about the decision the Oracle database engine makes when it is using a full segment scan approach. The choices the engine has is to store the blocks that are physically read in the buffercache, or read the blocks into the process’ PGA. The first choice is what I refer to as a ‘buffered read’, which places the block in the database buffercache so the process itself and other processes can bypass the physical read and use the block from the cache, until the block is evicted from the cache. The second choice is what is commonly referred to as ‘direct path read’, which places the blocks physically read into the process’ PGA, which means the read blocks are stored for only a short duration and is not shared with other processes.

Exadata Capacity on Demand and Elastic Rack

Since X4 we can do Capacity on Demand on Exadata: disable some CPU cores to lower cost of Oracle Database licenses. Depending on the models, and the configuration, there are different minimums and here is a recap table about those.

Here is the summary of Capacity on Demand minimum, maximum and increment. Those numbers come from the configuration file of OEDA, the Oracle Exadata Deployment Assistant (es.properties) and you can see that it already has an option for Exadata X7-2

Linux memory usage

One of the principal important configuration settings for running an Oracle database is making appropriate use of memory. Sizing the memory regions too small leads to increased IO, sizing the memory regions too big leads to inefficient use of memory and an increase in memory latency most notably because of swapping.

On Linux, there is a fair amount of memory information available, however it is not obvious how to use that information, which frequently leads to inefficient use of memory, especially in today’s world of consolidation.

The information about linux server database usage is available in /proc/meminfo, and looks like this:

Taming XML data in the v$cell% family of Exadata views

While researching Exadata 12.2 features that have been introduced with Oracle 12.2.1.1.0 I ran into an old problem of mine, querying the v$cell% views that are new with Exadata 12.1.1.1.0 and Oracle RDBSM 12c Release 1 in a meaningful way. In case you haven’t seen them yet, these views expose cell metrics and other information in the database without having to actually connect to the cells and execute cellcli commands. They are very useful, and form the basis of the latter part of the Exadata AWR reports.

On our 12.2 Exadata system (using cellos 12.2.1.1.0 and RDBMS 12.2.0.1.0) the following views are exposed to the curious researcher:

New workshop "Exadata For Developers"

Just a short note that I've developed a new two day course that covers all relevant features that a database application developer should know when dealing with the Oracle Exadata Database Machine platform.

It covers in detail Smart Scans, the Exadata Flash Cache, Hybrid Columnar Compression and all surrounding features like Storage Indexes, (serial) direct path reads etc. etc.. Of course it also includes features that were added in 12c, like Attribute Clustering and Zone Maps.

All features are presented with live demo scripts, and there will be enough time to discuss your specific questions and analyse existing applications if desired.

For more information and details, check the corresponding pages:

German: Exadata für Anwendungsentwickler

DOAG Exaday 2017: a preview

I am quite excited to have been accepted to speak at the upcoming DOAG Exaday taking place June 20 in Frankfurt. It is the third time I have been lucky enough to present at the event, having spoken both in Frankfurt in Hamburg in previous years.

As you can probably imagine by looking at this weblog, I am a great supporter of the Oracle Engineered Systems community. My interest is not limited to Germany: I still remember the many great meetings in London in the context of UKOUG’s Exadays. I am also very lucky to work for @enkitec where we have been sharing real-world experience at E4 for years and will do so again in June this year.