Posted by Riyaj Shamsudeen on April 19, 2012
Last week (March 2012), I was conducting Advanced RAC Training online. During the class, I was recreating a ‘gc buffer busy’ waits to explain the concepts and methods to troubleshoot the issue.
Let’s define these events first. Event ‘gc buffer busy’ event means that a session is trying to access a buffer,but there is an open request for Global cache lock for that block already, and so, the session must wait for the GC lock request to complete before proceeding. This wait is instrumented as ‘gc buffer busy’ event.
From 11g onwards, this wait event is split in to ‘gc buffer busy acquire’ and ‘gc buffer busy release’. An attendee asked me to show the differentiation between these two wait events. Fortunately, we had a problem with LGWR writes and we were able to inspect the waits with much clarity during the class.
Remember that Global cache enqueues are considered to be owned by an instance. From 11g onwards, gc buffer busy event differentiated between two cases:
- If existing GC open request originated from the local instance, then current session will wait for ‘gc buffer busy acquire’. Essentially, current process is waiting for another process in the local instance to acquire GC lock, on behalf of the local instance. Once GC lock is acquired, current process can access that buffer without additional GC processing (if the lock is acquired in a compatible mode).
- If existing GC open request originated from a remote instance, then current session will wait for ‘gc buffer busy release’ event. In this case session is waiting for another remote session (hence another instance) to release the GC lock, so that local instance can acquire buffer.
Following output should show the differentiation with much clarity.
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Posted in 11g, Oracle database internals, Performance tuning, RAC | Tagged: gc buffer busy, gc buffer busy acquire, gc buffer busy release, oracle performance, RAC performance | 11 Comments »
Posted by Riyaj Shamsudeen on February 27, 2012
Just a quick note, I will be conducting next 2-week advanced RAC Training, online class in March 26-30 and April 9-13.
You can find agenda and other details here .
I received few emails about the training outline. You can find the outline below:
Yes, I do accept Purchase Orders and can invoice you :)
Posted in Presentations, RAC | Tagged: RAC, RAC training, training online | 1 Comment »
Posted by Riyaj Shamsudeen on February 13, 2012
Temporary tablespaces are shared objects and they are associated to an user or whole database (using default temporary tablespace). So, in RAC, temporary tablespaces are shared between the instances. Many temporary tablespaces can be created in a database, but all of those temporary tablespaces are shared between the instances. Hence, temporary tablespaces must be allocated in shared storage or ASM. We will explore the space allocation in temporary tablespace in RAC, in this blog entry.
In contrast, UNDO tablespaces are owned by an instance and all transactions from that instance is exclusively allocated in that UNDO tablespace. Remember that other instances can read blocks from remote undo tablespace, and so, undo tablespaces also must be allocated from shared storage or ASM.
Space allocation in TEMP tablespace
TEMP tablespaces are divided in to extents (In 11.2, extent size is 1M, not sure whether the size of an extent is controllable or not). These extent maps are cached in local SGA, essentially, soft reserving those extents for the use of sessions connecting to that instance. But, note that, extents in a temporary tablespace are not cached at instance startup, instead instance caches the extents as the need arises. We will explore this with a small example:
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Posted in 11g, Oracle database internals, Performance tuning, RAC | Tagged: CI enqueue, DFS lock handle, oracle performance, RAC performance, SS enqueue, temporary tablesapce, temporary tablespace, temporary tablespace groups | 16 Comments »
Posted by Riyaj Shamsudeen on February 10, 2012
I just uploaded my presentation materials for ‘Truss, pstack etc’ for HOTSOS 2012 symposium , a performance intensive conference, happening right here in my home town Dallas, TX.
I can’t believe, it is been ten years from the start of this annual conference! This is the tenth annual symposium and I have been presenting in this symposium for almost all years except few early years. Quality of presentations and quality of audience is very high in this symposium and many of the audience are repeat audience, almost this feels like an annual pilgrimage to “sanctum of performance”. If you are interested in learning the techniques and methods to debug and resolve performance issues in a correct way, you should definitely consider attending this symposium. To top it off, Jonathan Lewis is conducting Training Day this year.
There are many great authors talking in this symposium.
Let me take this opportunity to welcome you to Dallas and encourage you to attend this symposium !
PS: Mark Bobak has been presenting or attending all ten years of this symposium, kudos Mark! And, Yes, that’s the same Mark Bobak who is the list admin for that most famous oracle-l mailing list.
PPS: Even though I live in Texas, no, I do not ride an horse to commute, instead, I drive a Ford Mustang car.(Incidentally, Mustang means “a small breed of horse, often wild or half wild, found in the southwestern US” as defined by dictionary.com, So, after all, I am driving a string of wild horse).
Posted in Performance tuning, Presentations, RAC | Tagged: hotsos symposium 2012, oracle performance, RAC performance | 1 Comment »
Posted by Riyaj Shamsudeen on February 10, 2012
I will be leaving to Denver in few days to talk about the following presentations in RMOUG 2012. Stop by and say hello to me if you intend to attend RMOUG training days.
My sessions in RMOUG 2012 are
- Room 402:Session 2: Parallel Execution in RAC – Wednesday 10:45 AM to 11:45AM
- Room 4f: Session 10: Troubleshooting RAC background processes – Thursday 1:30PM to 2:30PM
- Room 4f: Session 11: A kind and Gentle introduction to RAC – Thursday 2:45 PM to 3:45 PM
Hope to see you there.
Posted in Performance tuning, Presentations, RAC | Tagged: oracle performance, pstack, RAC presentations, rmoug | 2 Comments »
Posted by Riyaj Shamsudeen on January 25, 2012
It is probably easy to calculate hourly redo rate or daily redo rate using AWR data. For example, my script awr_redo_size.sql can be used to calculate daily redo rate, and awr_redo_size_history.sql can be used to calculate hourly redo rate. Hourly redo rate is especially useful since you can export to an excel spreadsheet, graph it to see redo rate trend.
Update: I added another script to calculate redo rate if you don’t have AWR license. redo_size_archived_log.sql.
Introduction to Direct Mode Writes
Direct mode operations write directly in to the database file skipping buffer cache. Minimal redo(aka invalidation redo) is generated, if the database is not in force logging mode. Keeping the database in no force logging mode is peachy as long as you don’t use Data guard, Streams, or Golden Gate.
Suddenly, business decide to use one of these log mining based replication products. This means that you must turn on Force logging at the database level so that replication tools can capture (just replay in the case of Data guard) the redo information correctly and consistently.
But, what if your application performs high amount of direct mode operation, such as insert /*+ append */ operations? Now, you need to estimate the redo size to identify the effect of FORCE LOGGING mode That estimation gets little tricky.
But wait, there’s more!
Posted in Oracle database internals, Performance tuning, RAC | Tagged: golden gate redo size, oracle performance, redo nologging size, redo size script, streams redo size | 8 Comments »
Posted by Riyaj Shamsudeen on January 20, 2012
This video was created circa July 2011. Click the Read More link to review the video. Version Oracle Database 220.127.116.11
Synopsis: Essentially, we probe the importance of LMS processes using DTrace. Explain why LMS should run in elevated priority. How to review deep statistics about LMS processes and much more.
Posted in Oracle database internals, Performance tuning, Presentations, RAC, video | Tagged: LMS tuning, oracle performance, RAC performance, RAC training, video RAC training | 7 Comments »
Posted by Riyaj Shamsudeen on January 19, 2012
In this blog entry, we will explore the wonderful world of SCNs and how Oracle database uses SCN internally. We will also explore few new bugs and clarify few misconceptions about SCN itself.
What is SCN?
SCN (System Change Number) is a primary mechanism to maintain data consistency in Oracle database. SCN is used primarily in the following areas, of course, this is not a complete list:
- Every redo record has an SCN version of the redo record in the redo header (and redo records can have non-unique SCN). Given redo records from two threads (as in the case of RAC), Recovery will order them in SCN order, essentially maintaining a strict sequential order. As explained in my paper, every redo record has multiple change vectors too.
- Every data block also has block SCN (aka block version). In addition to that, a change vector in a redo record also has expected block SCN. This means that a change vector can be applied to one and only version of the block. Code checks if the target SCN in a change vector is matching with the block SCN before applying the redo record. If there is a mismatch, corruption errors are thrown.
- Read consistency also uses SCN. Every query has query environment which includes an SCN at the start of the query. A session can see the transactional changes only if that transaction commit SCN is lower then the query environment SCN.
- Commit. Every commit will generate SCN, aka commit SCN, that marks a transaction boundary. Group commits are possible too.
SCN is a huge number with two components to it: Base and wrap. Wrap is a 16 bit number and base is a 32 bit number. It is of the format wrap.base. When the base exceeds 4 billion, then the wrap is incremented by 1. Essentially, wrap counts the number of times base wrapped around 4 billion. Few simple SQL script will enumerate this better:
But wait, there’s more!
Posted in 11g, corruption, Oracle database internals, Performance tuning, RAC, recovery | Tagged: get_system_change_number, hot backup scn growth, kcmgas calls, kcvblg, ORA-600 , oracle performance, SCN bug, tracefile_name | 30 Comments »
Posted by Riyaj Shamsudeen on January 13, 2012
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):
- Foreground session calculates the master node of the block; Requests a LMS process running in the master node to access the block.
- 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.
- 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.
- 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.
Posted in 11g, Oracle database internals, Performance tuning, Presentations, RAC | Tagged: gc cr disk read, RAC performance | 6 Comments »