Re: Fast object checkpoint

  • From: Mladen Gogala <mgogala@xxxxxxxxxxx>
  • To: oracle-l <oracle-l@xxxxxxxxxxxxx>
  • Date: Sat, 23 Sep 2006 13:19:52 -0400

On 09/22/2006 08:02:52 PM, Mladen Gogala wrote:
> What is a "fast object checkpoint"? I have script which has to update ~60M 
> rows.
> I had to kill it and re-start it. The new incarnation did not wait for a 
> bunch of
> TX locks, it was waiting for the following event:
> ENQ: KO - fast object checkpoint
> Surprisingly enough, this lock was held by the CKPT process, not PMON, as I've
> expected. Very few undo blocks were actually consumed in v$undostat. My 
> question
> is what is "fast object checkpoint" and is there a new method of reclaiming 
> resources
> held by murdered transactions? Is it documented anywhere?
> -- 
> Mladen Gogala

I found the description of this new feature in the following document:

"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 
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 
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 
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 
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 
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 
any further, but this dramatically changes the way the database functions. It 
dramatically impacts performance consideration as a big buffer cache in which 
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.

Mladen Gogala


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