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ENQ: KO - FAST OBJECT CHECKPOINT tips

Oracle Database Tips by Burleson Consulting

May 21, 2016

Question: What does the wait event ENQ: KO - FAST OBJECT CHECKPOINT mean?  Can you show an example of the ENQ: KO - FAST OBJECT CHECKPOINT wait event?  I get this event:

WAITING: enq: KO - fast object checkpoint 

Answer:  You see this enq:  ko - fast object checkpoint event because an object level checkpoint that happens when you run direct path reads on a table.

When you run full table scan (or a parallel query full-table scan), you will see direct path reads wait event.  But in the beginning you will also see the enq:  ko - fast object checkpoint wait event. This will checkpoint any blocks that belong to the object you are doing direct path read so that latest change goes into the datafile.

Waiting for the enq: ko - fast object checkpoint means that your session wants to run a full-table scan and has sent the CKPT process a message with instruction to do an object level checkpoint. The CKPT process, in turn, then directs the database writer process (DBWR) to perform the checkpoint.

Deepak Bhatnagar notes a test to see the enq: ko - fast object checkpoint:

I updated only 1 row of the table TEST_A. When I executed SELECT query,  it performed direct path read because table size is big as compared to buffer cache size.

Before direct path read, a wait event ENQ: KO - FAST OBJECT CHECKPOINT was posted.

This is because, before reading data blocks from data file directly to PGA,  object level checkpoint was occurred and dirty blocks of the table TEST_A were written from buffer cache to data file.  

Rampant author Mladen Gogala writes this about the enq: ko - fast object checkpoint

Prior to Oracle Database 10g administrators could specify the expected crash recovery time (MTTR) by setting the value of a checkpoint-related initialization parameter (FAST_START_MTTR_TARGET). They could do so by using the MTTR advisory, which helps predict the number of physical writes that would arise with different MTTR target values.

With Oracle Database 10g, the database can self-tune checkpoints activity to achieve good recovery times with low impact on normal throughput.

With automatic checkpoint tuning, Oracle Database takes advantage of periods of low I/O usage to write out data modified in memory to the data files without adverse impact on the throughput. Consequently, a reasonable crash recovery time can be achieved even if the administrator does not set any checkpoint-related parameter or if this parameter is set to a very large value.

Another enhancement done in the second release of Oracle Database 10g dramatically improves the performance of object-checkpoint requests issued for objects accessed through direct path reads, a situation that can occur with parallel query.

Before an object can be accessed through direct path reads, dirty buffers of the object must be written to data files on disk via an object-checkpoint request. Prior to Oracle Database 10g Release 2, the checkpoint request is handled by issuing a checkpoint for the tablespace the object belongs to, writing out all the dirty buffers for the entire tablespace.

Since a large number of objects may reside in the same tablespace, this implementation may cause large number of unnecessary disk writes. With the new release, a checkpoint request for a target object will only write out the dirty buffers of that object, without incurring any additional writes for the dirty buffers of other objects"

Of course, this is a very important new performance feature. Many people have noticed and inquired about the "KO locks", queried v$lock_type and didn't investigate any further, but this dramatically changes the way the database functions.

It also dramatically impacts performance consideration as a big buffer cache in which large parts of a big table can be cached can cause a serious I/O contention and a lock contention. I'm looking for a mechanism to turn off this new behavior, at least until the next patch version, if not until the next major version.

Reference:  MOSC bug 9881076


   
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