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Throttling IO with Linux

Why?

I guess the first question which comes to mind when reading this title is ‘Why’? For a database, but I guess for any IO depended application, we want IO’s to be faster, not throttle them, alias make them slower. Well, the ‘why’ is: if you want to investigate IO’s, you sometimes want them to slow down, so it’s easier to see them. Also, (not so) recent improvements in the Oracle database made great progress in being able to use the available bandwidth by doing IO in parallel, which could strip away much of the ability to see them in Oracle’s popular SQL trace.

ORA-00600: internal error code, arguments: [kmgs_component_init_3], [60], [65], [17], [], [], [], [], [], [], [], []

Recently I patched an 11.2.0.2 grid infrastructure to an higher version. After the patching I started the grid infrastructure on that host, and ASM was unable to start. Looking in the alert.log file of the ASM instance it turned out that upon starting ASM, even before the contents of the pfile/spfile was displayed, the ASM crashed with the ORA-00600 error:

Using udev on RHEL 6 / OL 6 to change disk permissions for ASM

When you use Oracle ASM (Automatic Storage Management) for your database, the permissions on the block devices on the operating system layer which are used by ASM need to be changed. To be more precise, the owner and group need to be set to ‘oracle’ and ‘dba’ (Oracle documentation) in my case.

I used to do this in a very lazy way, using a simple ‘/bin/chown oracle.dba /dev/sdb’ in /etc/rc.local. This worked for me with RHEL/OL version 5. This has changed with RHEL/OL 6, because the system V startup system has changed to ‘upstart’. Also, the disk devices change ownership back in OL6 if you set it by hand to oracle.dba.

Rename Oracle Managed File (OMF) datafiles in ASM

(Version edited after comments -> rman backup as copy)
(Version edited to include delete leftover datafile from rman)

Recently I was asked to rename a tablespace. The environment was Oracle version 11.2.0.3 (both database and clusterware/ASM).

This is the test case I build to understand how that works:
(I couldn’t find a clean, straightforward description how to do that, which is why I blog it here)

I created an empty tablespace ‘test1′ for test purposes:

SYS@v11203 AS SYSDBA> create bigfile tablespace test1 datafile size 10m;

(I use bigfile tablespaces only with ASM. Adding datafiles is such a labour intensive work, bigfile tablespaces elimenate that, when auto extent is correctly set)

A tablespace can be easily renamed with the alter tablespace rename command:

The linux ‘perf’ utility with EL 6

Some of you might have experimented with, or used Oprofile, ltrace/strace, Systemtap, and wished you could look further into the userland process, like dtrace does, because dtrace allows you to profile and look into a process’ function calls.

If you’ve used oprofile/strace/systemtap/etc., you probably found out you can see all the kernel functions which are processed, but it does not get into userspace, or with very limited information. Perhaps the only tool which is different is gdb, which enables you to see userspace function names, but gdb is not a profiler, it’s a debugger. And it works best with (I assume it made for) debugging symbols in the executable. Most (all I presume) commercial executables, like the oracle database executable, do not have debugging symbols.

Getting to know Oracle wait events in Linux.

May 4th: some updates after discussion with Jeff Holt, Cary Millsap and Ron Christo of Method-R.

There’s all the documentation, and there all the terrific blogs about performance and Oracle wait events. But if you more or less start with this stuff, or encounter a wait event that is not (extensive enough) documented, or an event turns up and gives a response time you don’t expect, you need to understand what that wait event means. If you don’t know, it’s easy to get stuck at this point.

If you are familiar with waits, and just want to dig further, progress to “Get to know wait events”, if you want to get up speed with waits, read on.

What is a wait event?

This is the definition of the performance tuning guide in the Oracle documentation:

Using R and Oracle tracefiles

If you are not familiar with R, this is the description from the R site: R is a language and environment for statistical computing and graphics. I encountered R at the Erasmus university, where I am working on a project with DNA data which is put in an Oracle database (see: http://huvariome.erasmusmc.nl/ (It’s down at the moment)).

The Oracle Exadata IO Resource Manager Limit Clause, part 2

In my previous post I described how the IO resource manager (IORM) in the exadata storage can be used both to guarantee a minimum amount of IO a database can get (which is what is covered in most material available), but also to set a maximum amount of IO. This is what Oracle calls an inter-database resource manager plan. This is set and configured at the cell level using the cellcli with ‘alter iormplan dbplan’.

Control file inconsistencies during startup when using ASM (ORA-000214)

During some testing I encountered an ORA-000214 during startup of an Oracle 11.2.0.2 database instance:

ORA-00214: control file '+RECO_XXXX/test/controlfile/current.334.755391511'
version 268 inconsistent with file
'+DATA_XXXX/test/controlfile/current.299.755390399' version 265

This is a RAC instance on Exadata, but all techniques in this article will work on any Oracle 11.2.x database using ASM.

This message means the database found two controlfiles which have a different version. If this message appears when the database is open, the database will crash. If an instance is startup after this message, the same error appears, and the database remains in nomount state. Further diagnosis: the control file version in the recovery area is more recent than the version in the data diskgroup (version 268 versus version 265).

6th Planboard DBA Symposium

The 17th of may 2011 Planboard is organising the 6th version of the Planboard DBA Symposium. The symposium is a whole day event, in Amstelveen near Amsterdam.

There is a wide range of Oracle database topics, including Apex, Oracle GoldenGate, Oracle E-Business suite and Oracle Exadata (presented by me :) !).

If you are interested, take a look at the full agenda!