gc cr disk read

You might encounter RAC wait event ‘gc cr disk read’ in 11.2 while tuning your applications in RAC environment. Let’s probe this wait event to understand why a session would wait for this wait event.

Understanding the wait event

Let’s say that a foreground process running in node 1, is trying to access a block using a SELECT statement and that block is not in the local cache. To maintain the read consistency, foreground process will require the block consistent with the query SCN. Then the sequence of operation is(simplified):

1. Foreground session calculates the master node of the block; Requests a LMS process running in the master node to access the block.
2. Let’s assume that block is resident in the master node’s buffer cache. If the block is in a consistent state (meaning block version SCN is lower (or equal?) to query SCN), then LMS process can send the block to the foreground process immediately. Life is not that simple, so, let’s assume that requested block has an uncommitted transaction.
3. Since the block has uncommitted changes, LMS process can not send the block immediately. LMS process must create a CR (Consistent Read) version of the block: clones the buffer, applies undo records to the cloned buffer rolling back the block to the SCN consistent with the requested query SCN.
4. Then the CR block is sent to the foreground process.

LMS is a light weight process

Global cache operations must complete quickly, in the order of milli-seconds, to maintain the overall performance of RAC database. LMS is a critical process and does not do heavy lifting tasks such as disk I/O etc. If LMS process has to initiate I/O, instead of initiating I/O, LMS will downgrade the block mode and send the block to the requesting foreground process (this is known as Light Works rule). Foreground process will apply undo records to the block to construct CR version of the block.

RMOUG 2012 – Hello Denver!

On February 14-16, I’ll be at the Colorado Convention Center in Denver, Colorado for RMOUG’s Training Days Conference. This is the largest regional Oracle User Conference in North America and attracts presenters from all around the country and the globe. I’ll be presenting:

Presentation Name: Troubleshooting RAC Background Process

Abstract: RAC background process performance is critical to keep the application performance. This session will demo techniques to review the performance of RAC background processes such as LMS, LMD, LMON, etc. using various statistics and UNIX tools. The presentation will also discuss why certain background processes must run in higher priority to maintain the application performance in RAC.

Presentation Name: A Kind and Gentle Introduction to RAC

Abstract: This session will introduce basic concepts such as cache fusion, conversion to RAC, protocols for interconnect, general architectural overview, GES layer locks, clusterware, etc. The session will also discuss the srvctl command and demo a few of these commands to improve the understanding.

Presentation Name: Parallel Execution in RAC

Abstract: This presentation will start to discuss and demo parallel server allocation, intra, and inter node parallelism aspects. The session will discuss the new parallelism features such as parallel statement queuing, parallel auto dop, and discuss the interaction of those features with RAC. The session will probe a few critical parameters to improve PQ performance in RAC.

Click here for more information or to register for RMOUG’s Training Days.

Oracle Open World 2011 – My presentation on RAC topic

I will be talking about “Administering Parallel Execution in RAC” with demos on Sunday morning 9AM-10AM (session id 28060). This is part of IOUG RAC SIG presentation series. You would enjoy the content and demos I have prepared.

I know, it is too early, but hoping to see you there!
BTW, if you have attended my RAC Advanced Troubleshooting class series, please don’t hesitate to introduce yourself when we meet.

Session details:
Session ID: 28060
Session Title: IOUG: Administering Parallel Execution in Oracle RAC
Venue / Room: Moscone West- 2005
Date and Time: 10/2/11, 9:00 – 10:00

Update: I just completed this session in IOUG. Thank you for coming, if you were in the room.
You can download the pdf file from
PX execution in RAC

RAC hack session – Tuesday – July 11 2011

I will be conducting a 1-hour deep dive session about RAC LMS process (and about LGWR processes too if time permits) using advanced UNIX utilities. Read Tanel’s blog entry for details:
RAC hack session

See you there!

1-Day Expert Oracle-centric Conference at MIT – 07/07/11 -virtual or physical

You probably aware of 1 day event happening on July 7th Thursday 9AM-5PM EDT (virtually and physically). I will be talking about advanced UNIX tools to debug issues. You can find details of 1-day event here

Here is the outline of my presentation:

Advanced tools and techniques in Unix environment – Riyaj Shamsudeen – 07/07/2011, 3:15pm – 4:00pm EDT

Unix environments provides rich set of tools to debug performance issues. Even if the issue is complex to understand, if we use right tool for the job, we can identify the root cause of an issue quickly. In this presentation, the speaker will demo tools such as truss/strace, pfiles, pmap, prstat, vmstat, mpstat etc to show how you can debug complex issues. The speaker will also introduce Solaris/dtrace to understand performance issues.

Why not join us?

Advanced RAC Training

I will be delivering an intense, advanced 2-week RAC training seminar in Aug 22-26 and Sep 19-23 with numerous demos, dumps and scripts. We will meet 4 hours per day, in a virtual world, 8AM-12 Noon Pacific time for those two weeks. Tanel has written has a great blog post about this seminar series Advanced RAC training.

If you like my presentations about RAC and performance tuning, you will love my seminar series. You will gain better understanding about RAC internals, and you will be able to advance your debugging and performance tuning skills attending my seminar series.

Join me and Let’s explore RAC further.

Presenting in IOUG Webinar: RAC Performance tuning – Private inter connect

I will be presenting in IOUG Webinar on Thursday June 30, 2011 11AM-12Noon CDT. Hope you can join me.
Register at Private interconnect .
I know, I have not been blogging more actively, but I have been working on books and few presentations lately. Hopefully, you will see more blog entries soon.
Thanks for reading my blog.

Books and presentations

As you probably know that my first co-authored book Expert Oracle Practices was released in 2009. I have co-authored one more book Pro Oracle SQL with my esteemed colleagues. This books covers many aspects of better SQL development. Have fun reading 🙂

I also will be presenting in few conferences in the upcoming months: RMOUG Training days 2011 , Hotsos symposium ’11, and IOUG Collob . Hopefully, I will see you in one of the conference (or all of the conferences)

Hotsos 2011

I will be talking about advanced RAC troubleshooting in Hotsos symposium ’11. Hotsos Symposium, conducted every March in Dallas, TX, is an intensive seminar series probing the deep waters of Performance related to Oracle Database.

There are many great speakers in this conference. It gives me a great pleasure to meet many folks that I have known for years, exchange ideas, and learn from industry leaders.

Hope to see you there!

DOUG 2010 Presentation

You can find pdf version of the presentation here: Performance tuning – for developers . Feel free to ask questions in the comments.
Thank you Mary Elizabeth Mcneely for the opportunity to talk in the Dallas Oracle User Group technology meetup.