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Oracle EE

Direct path read and fast full index scans

This is yet another blogpost on Oracle’s direct path read feature which was introduced for non-parallel query processes in Oracle version 11.

For full table scans, a direct path read is done (according to my tests and current knowledge) when:

- The segment is bigger than 5 * _small_table_threshold.
- Less than 50% of the blocks of the table is already in the buffercache.
- Less than 25% of the blocks in the buffercache are dirty.

When does an Oracle process know it’s on Exadata?

When an Oracle process starts executing a query and needs to do a full segment scan, it needs to make a decision if it’s going to use ‘blockmode’, which is the normal way of working on non-Exadata Oracle databases, where blocks are read from disk and processed by the Oracle foreground process, either “cached” (read from disk and put in the database buffercache) or “direct” (read from disk and put in the process’ PGA), or ‘offloaded mode’, where part of the execution is done by the cell server.

The code layer where the Oracle database process initiates the offloading is ‘kcfis’; an educated guess is Kernel Cache File Intelligent Storage. Does a “normal” alias non-Exadata database ever use the ‘kcfis’ layer? My first guess would be ‘no’, but we all know guessing takes you nowhere (right?). Let’s see if a “normal” database uses the ‘kcfis’ functions on a Linux x64 (OL 6.3) system with Oracle 11.2.0.3 64 bit using ASM.

Exadata and the db_block_checksum parameter.

With Exadata version 11.2.3.2.0 came the Unbreakable Linux Kernel for Exadata, which had been the stock EL5 redhat kernel prior to this version (2.6.18). With the unbreakable kernel came the opportunity to run the perf utility. This utility has the opportunity to see which functions are active inside an executable when there’s a symbol table. And the oracle database executable has a symbol table! One reason to do this, is to get a more granular overview of what the Oracle database is doing than the wait interface, especially to get a more detailed overview of what the database is doing in what is visible in the wait interface as ‘on cpu’.

Oracle 11.2 and the direct path read event

In my previous post I touched the topic of a “new” codepath (codepath means “way of utilising the operating system”) for of full segment scans. Full segment scan means scanning a segment (I use “segment” because it could be a partition of a table or index) which is visible commonly visible in an explain plan (but not restricted to) as the rowsources “TABLE ACCESS FULL”, “FAST FULL INDEX SCAN” and “BITMAP FULL SCAN”.

Look at my presentation About multiblock reads to see how and when direct path reads kick in, and what the difference between the two is. Most notably, Oracle has released very little information about asynchronous direct path reads.

Oracle 11.2.0.1 and the ‘kfk: async disk IO’ wait event

Recently I was discussing some IO related waits with some friends. The wait I was discussing was ‘kfk: async disk IO’. This wait was always visible in Oracle version 11.2.0.1 and seems to be gone in version 11.2.0.2 and above. Here is the result of some investigation into that.

First: the wait is not gone with version 11.2.0.2 and above, which is very simple to prove (this is a database version 11.2.0.3):

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.