Oracle Database 12c R1 ( is finally released!

You can download it from

And now I (and other beta testers) can finally start writing about the new cool features in it! :)

Looks like only the Linux x86-64, Solaris x86-64 + SPARC ports are available first (as usual).

(just a screenshot below, you’ll need to go to to download the files)

Screen Shot 2013-06-25 at 9.34.22 PM


Update: Also the Oracle 12c Documentation is now available in the OTN website.


Posted in Announcement, Oracle | 9 Comments

Getting the Most Out of ASH online seminar

Just a little reminder – next week (10-11th June) I’ll be delivering my last training session before autumn – a short 1-day (2 x 0.5 days actually) seminar about Getting the Most Out of Oracle’s Active Session History. In the future it will act as sort of a prequel (or preparation) for my Advanced Oracle Troubleshooting class, as the latter one deliberately goes very deep. The ASH seminar’s 1st half is actually mostly about the GUI way of troubleshooting the usual performance problems (EM/Grid Control) and the 2nd half is about all my ASH scripts for diagnosing more complex stuff.

P.S. I’ll also have multiple very cool news in a few months ;-)

Posted in Announcement, Oracle | Leave a comment

Forcing Smart Scans on Exadata – is the _serial_direct_read parameter safe to use in production?

One of the most common Exadata performance problems I see is that the direct path reads (and thus also Smart Scans) don’t sometimes kick in when running full scans in serial sessions. This is because in Oracle 11g onwards, the serial full segment scan IO path decision is done dynamically, at runtime, for every SQL execution – and for every segment (partition) separately. Whether you get a direct path read & smart scan, depends on the current buffer cache size, how big segment you’re about to scan and how much of that segment is actually cached at the moment. Note that the automatic IO path decision for index fast full scans is slightly different from table scans.

This dynamic decision unfortunately can cause unexpected surprises and variance in your report/batch job runtimes. Additionally, it looks like the SELECT part of your UPDATE/DELETE statements (the select part finds the rows to update/delete) does not ever automatically get direct path read/smart scan chosen – by design! So, when your SELECT statement may use smart scan and be really fast, the same select operation in an INSERT SELECT (or UPDATE/DELETE) context will not end up using smart scans by default. There’s even a bug explaining that – closed as “not a bug” (Exadata Smartscan Is Not Being Used On Insert As Select[Article ID 1348116.1]).

To work around these problems and force a direct path read/smart scan, you can either:

  1. Run your query in parallel as parallel full segment scans will use direct path reads, unless your parallel_degree_policy = AUTO, then you may still get buffered reads thanks to the dynamic in-memory parallel execution decision of Oracle 11.2
  2. Run your query in serial, but force the serial direct path reads by setting _serial_direct_read = TRUE (or ALWAYS in

Here are the valid options for this parameter in

Continue reading

Posted in Exadata, Oracle | 1 Comment

Debugger Dangers – Part 2

About 5 years ago I wrote about the risks that connecting to Oracle processes via debuggers may cause and what are (in my opinion) the safer and less safer options for taking stack samples from running Oracle processes.

In the end of that article I listed different options for getting a stack traces and whether they were safe or not.

For example, ORADEBUG-based process stack traces (DUMP ERRORSTACK, SHORT_STACK and event the process/system state dumps (at level 256 or higher) are not 100% safe – because they alter the execution path of the process they attached to. Your process may crash or get some error if you hit a bug (of course once you patch/fix the bug, you’ll be fine again – until you may hit the next bug).

An example bug is this:


LGWR may hang when SIGUSR interrupts libaio (that’s what LGWR uses for asynchronous IO). And if LGWR is hung – your whole instance will be hung pretty soon. But what does the SIGUSR signal have to do with Oracle? Well, that’s exactly the signal what ORADEBUG sends to the process it has attached to, to notify it about some debug/dump work it needs to do. And apparently receiving or handling such a signal (jumping into some stack dump function), when being in libaio codepath causes an issue.

Continue reading

Posted in Oracle | Leave a comment

List Exadata Storage Cell disk summary with cellpd.sql and cellpdx.sql scripts

In the previous post I explained how to list Exadata disk layout and topology details with the exadisktopo scripts, in this post I’ll introduce one celldisk overview script, which I use to quickly see the celldisk configuration, specs and error statuses. The cellpd.sql script (Cell Physical Disk) will show the following output:

SQL> @exadata/cellpd.sql
Show Exadata cell versions from V$CELL_CONFIG....

-------------------- -------------------- -------------------- ---------- ---------- ---------- ---------- ---------- ----------- ---------- ----------
FlashDisk           normal                      183         23          8
FlashDisk           not present                 183         23          8                     3
FlashDisk           normal                      366         23         16
FlashDisk           normal                      366         23         16
HardDisk            normal                    22352       1863         12
HardDisk            normal                    22352       1863         12
HardDisk            normal                    22352       1863         12

Above you see that some of my Flash Disks are missing and before they went completely missing (because the Oracle Ninja removed them :) these disks indicated the “poor performance” status.

The cellpdx.sql script will display extended info for each celldisk, including disk model, firmware version, serial number and various error metrics. Warning, lots of wide output coming again:

Continue reading

Posted in Exadata, Oracle | 1 Comment

List Exadata Disk Layout and Topology with the exadisktopo scripts

Here are two more Exadata scripts for listing the end-to-end ASM<->Exadata disk topology from V$ASM_ views and from V$CELL_CONFIG. These scripts see both the ASM level layout and the storage cell-level disk topology.

The exadisktopo.sql script shows all disks starting from the ASM diskgroup layer, going deeper and deeper all the way to the OS disk device level in the storage cells. It uses outer joins, so will show celldisks even if there are no corresponding grid disks allocated on them (or if there are no ASM disks using them). It also shows the Flash cards used as flash cache, thus there are no ASM disks on them usually.

The output should be self-explanatory, you just read the data from left to right (from ASM diskgroup to ASM disk to Grid Disk level etc), the more rightwards you scroll, the “deeper” you go in the Exadata IO stack layers. Warning, lots of wide output coming :)

Continue reading

Posted in Exadata, Oracle | 3 Comments

V$CELL_THREAD_HISTORY – “ASH” for Exadata Storage Cells

Did you know that there’s something like Active Session History also in the Exadata storage cells? ;-)

The V$CELL_THREAD_HISTORY view is somewhat like V$ACTIVE_SESSION_HISTORY, but it’s measuring thread activity in the Exadata Storage Cells:

SQL> @desc v$cell_thread_history
           Name                            Null?    Type
           ------------------------------- -------- ----------------------------
    1      CELL_NAME                                VARCHAR2(1024)
    2      SNAPSHOT_ID                              NUMBER
    3      SNAPSHOT_TIME                            DATE
    4      THREAD_ID                                NUMBER
    5      JOB_TYPE                                 VARCHAR2(32)
    6      WAIT_STATE                               VARCHAR2(32)
    7      WAIT_OBJECT_NAME                         VARCHAR2(32)
    8      SQL_ID                                   VARCHAR2(13)
    9      DATABASE_ID                              NUMBER
   10      INSTANCE_ID                              NUMBER
   11      SESSION_ID                               NUMBER
   12      SESSION_SERIAL_NUM                       NUMBER

It keeps about 10 minutes worth of samples of Exadata Storage Cell thread activity:

SQL> @minmax snapshot_time v$cell_thread_history
Show min/max (low/high) values in column "snapshot_time" of table v$cell_thread_history...

----------------- -----------------
20130419 14:42:15 20130419 14:52:54

Note that it’s not the V$ view or the database instance which stores this array – it’s the storage cells themselves. If you query the v$cell_thread_history view, your Oracle database session is going to “gather” this instrumentation data from all the required cells and present it to you, that’s why the “cell statistics gather” wait event shows up:

PARSING IN CURSOR #140596385017248 len=42 dep=0 uid=0 oct=3 lid=0 tim=1366404896817011 hv=4063158547 ad='19e452578' sqlid='63awy1gg
select count(*) from v$cell_thread_history
PARSE #140596385017248:c=0,e=115,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=4272803188,tim=1366404896817010
EXEC #140596385017248:c=0,e=27,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=4272803188,tim=1366404896817127
WAIT #140596385017248: nam='SQL*Net message to client' ela= 1 driver id=1413697536 #bytes=1 p3=0 obj#=40 tim=1366404896817152
WAIT #140596385017248: nam='cell statistics gather' ela= 283 cellhash#=0 p2=0 p3=0 obj#=40 tim=1366404896818846
WAIT #140596385017248: nam='cell statistics gather' ela= 352 cellhash#=0 p2=0 p3=0 obj#=40 tim=1366404896819317
WAIT #140596385017248: nam='cell statistics gather' ela= 376 cellhash#=0 p2=0 p3=0 obj#=40 tim=1366404896820929
WAIT #140596385017248: nam='cell statistics gather' ela= 326 cellhash#=0 p2=0 p3=0 obj#=40 tim=1366404896822198
WAIT #140596385017248: nam='cell statistics gather' ela= 580 cellhash#=0 p2=0 p3=0 obj#=40 tim=1366404896823620

Now, how frequently do the cells sample their “ASH” data:

SQL> SELECT DISTINCT (snapshot_time - LAG(snapshot_time) 
                      OVER (ORDER BY snapshot_time)) * 86400 lag_seconds
     FROM (SELECT distinct snapshot_time 
           FROM v$cell_thread_history WHERE cell_name = '');



Looks like the sampling is done exactly once per second!

So, great, what can we do with this data?

Here’s an Exadata performance script (cth.sql) which uses V$CELL_THREAD_HISTORY to dig into cell activity from the database layer.

Continue reading

Posted in Exadata, Oracle | 1 Comment

Oracle Database 11.2 Upgrade and Migration slides

Update: The Oracle 12c upgrade slides are available too.

Check out the extensive slide deck (over 500 slides) about upgrading techniques to Oracle 11.2, by Oracle Corp (Roy Swonger and Mike Dietrich):

It has lots of examples (from real customer upgrade cases) in it.

Thanks to Randolf Geist for telling me about this.

You can also download other interesting presentations from that page (check the right hand side download section), like the Oracle Database Behavior Changes 8.0 through 11.2 doc. Pretty cool!


Posted in Oracle, Oracle 12c | 5 Comments

How to Compose New Gmail Messages in Full Screen (instead of the tiny compose box of new Gmail)

I’m writing this (unusual) post as I am a long time Gmail user and recently I’ve seen plenty of people & articles complain about the Gmail’s new compose window (the one that shows up as a small hovering window in the bottom right of your screen):


The top google hits so far only return tips to disable the new editor completely, but I want to use the new one, just in a bigger window! There is a very easy workaround for that – and there’s no need to switch back to the old compose mode at all!

If you are using your mouse, then just:

  1. Hold down SHIFT key when clicking the Compose button to get a stand-alone new window for composing a message.
  2. Hold down CTRL (on Windows) or Command (on Mac OSX) key to get a full-screen new browser tab for composing a message.

Examples 1 & 2 below:


If you use Gmail keyboard shortcuts for productivity (they’re awesome!), then you can just:

  1. Press “c” for the new (small) compose box
  2. Press SHIFT-C (capital “C”) for the stand-alone new window for composing a message
  3. Press D for a full screen new browser tab for composing a message

Note that you can view the Gmail shortcuts reference, if this feature is enabled, just by pressing the ? (question mark) key:
So, there’s no need to switch back to the old compose mode completely, just remember SHIFT, CTRL and the “D” keys :)

Posted in Productivity | 38 Comments

Public Appearances and Exadata Performance Training

I will be doing a lot of (Exadata) talking and teaching in the coming months. Here’s a list of events where you’ll see me speaking, teaching, hacking, learning and hopefully also drinking beer:

  • 16 April 2013UKOUG Engineered Systems Summit
    • London, UK
    • I will talk about the common Exadata performance lessons learned in my “4 years of Exadata performance” talk
    • Andy Colvin and Martin Bach of Enkitec will also speak there (yes, Martin has joined Enkitec in Europe!!! :-)
  • 30 April 2013 – NYOUG Training Day
    • Manhattan, NYC
    • This is the first ever public delivery of my new Getting the Most Out of Oracle’s Active Session History, Time Model and Wait events seminar!
  • 2-3 May 2013Advanced Exadata Performance seminar – in-classroom!
    • First public delivery of this class – and we’ll be going very deep into Exadata internals and performance ;-)
    • Irving, TX (in Enkitec HQ)
  • 13-16 May 2013Advanced Exadata Performance seminar – online
    • Same as previous, but online.
  • 10-11 June 2013 – My new ASH seminar – online
    • Getting the Most Out of Oracle’s Active Session History, Time Model and Wait events
    • More details will appear in the training & seminar page soon
  • 13-14 June 2013 – Oracle Data Warehouse Global Leaders forum (by invitation-only event run by Oracle)
    • Amsterdam, Netherlands
    • I will speak about my “4 years of Exadata performance” experience and probably learn from others’ experience too
  • 5-6 Aug 2013Enkitec Extreme Exadata Expo (E4) conference
    • Irving, TX
    • I will be speaking there, haven’t set the exact topic yet, but it will include demos and hacking something I suspect ;-)
  • 22-26 September 2013Oracle OpenWorld conference
    • San Francisco, CA
    • I will attend the ACE Directors briefing before the conference, likely speak, hang out at Enkitec booth, probably hack something at Enkitec’s Conference HQ for fun and definitely drink beer there. Maybe I’ll even attend the Wednesday’s party for a change!

As you see, my great plans to not travel much are not going to work out well :)

Actually it is better, this year I’ve managed to only travel twice so far (and one of the trips was for vacation!) and I haven’t had to do too many of the crazy around-the-world in 5 days trips I did when living in back Singapore… but looks like I’ll get to 2 million km nevertheless this year:

Tanel's Tripit stats April 2013

Well, see you in some corner of the world some day!

Posted in Announcement, Exadata, Oracle | 1 Comment