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Oracle Memory Troubleshooting, Part 4: Drilling down into PGA memory usage with V$PROCESS_MEMORY_DETAIL

If you haven’t read them – here are the previous articles in Oracle memory troubleshooting series: Part 1Part 2,

Slides of my previous presentations

Here are the slides of some of my previous presentations (that I haven’t made public yet, other than delivering these at conferences and training sessions):

Scripts and Tools That Make Your Life Easier and Help to Troubleshoot Better:

  • I delivered this presentation at the Hotsos Symposium Training Day in year 2010:

 

Troubleshooting Complex Performance Issues – Part1:

 

Troubleshooting Complex Performance Issues – Part2

 

Where does the Exadata storage() predicate come from?

On Exadata (or when setting cell_offload_plan_display = always on non-Exadata) you may see the storage() predicate in addition to the usual access() and filter() predicates in an execution plan:

SQL> SELECT * FROM dual WHERE dummy = 'X';

D
-
X

Check the plan:

Hotsos Symposium 2014

After missing last year’s Hotsos Symposium (trying to cut my travel as you know :), I will present at and deliver the full-day Training Day at this year’s Hotsos Symposium! It will be my 10th time to attend (and speak at) this awesome conference. So I guess this means more beer than usual. Or maybe less, as I’m getting old. Let’s make it as usual, then :0)

Oracle X$ tables – Part 1 – Where do they get their data from?

It’s long-time public knowledge that X$ fixed tables in Oracle are just “windows” into Oracle’s memory. So whenever you query an X$ table, the FIXED TABLE rowsource function in your SQL execution plan will just read some memory structure, parse its output and show you the results in tabular form. This is correct, but not the whole truth.

Check this example. Let’s query the X$KSUSE table, which is used by V$SESSION:

cell flash cache read hits vs. cell writes to flash cache statistics on Exadata

When the Smart Flash Cache was introduced in Exadata, it was caching reads only. So there were only read “optimization” statistics like cell flash cache read hits and physical read requests/bytes optimized in V$SESSTAT and V$SYSSTAT (the former accounted for the read IO requests that got its data from the flash cache and the latter ones accounted the disk IOs avoided both thanks to the flash cache and storage indexes). So if you wanted to measure the benefit of flash cache only, you’d have to use the cell flash cache read hits metric.

Hard Drive Predictive Failures on Exadata

This post also applies to non-Exadata systems as hard drives work the same way in other storage arrays too – just the commands you would use for extracting the disk-level metrics would be different.

I just noticed that one of our Exadatas had a disk put into “predictive failure” mode and thought to show how to measure why the disk is in that mode (as opposed to just replacing it without really understanding the issue ;-)

When do Oracle Parallel Execution Slaves issue buffered physical reads – Part 2?

In the previous post about in-memory parallel execution I described in which cases the in-mem PX can kick in for your parallel queries.

Diagnosing buffer busy waits with the ash_wait_chains.sql script (v0.2)

In my previous post ( Advanced Oracle Troubleshooting Guide – Part 11: Complex Wait Chain Signature Analysis with ash_wait_chains.sql ) I introduced an experimental script for analysing performance from “top ASH wait chains” perspective.

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