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Thread: Can checkpoint creation be parallel?

Can checkpoint creation be parallel?

From
高健
Date:

Hello:

Sorry for disturbing:

I have one question about checkponint . That is : can checkpoint be parallel?

It is said that checkpoint will be activated according to either conditions:

1)After last checkpoint, checkpoint_timeout seconds passed.

2)When shared_buffers memory above checkpoint_segments size  is filled with data.

My experience is that :

There is only one checkpoint process there and works, and even when there are many data created during transactions,

There will also be only one checkpoint process deal with it

(maybe background writer or some other server process will deal it)?

 

And also, I think when the above 1) or 2) activated checkpoint ,

after the checkpoint data creation, then the 1) and 2) value will be to zero and calculated again.   

So I think that checkpoint will not be created parallel.

Is my understanding right?

And if there are some official documentation from www.postgresql.org or somewhere else, I'll be very appreciative.

Thanks!

Jian Gao

Re: Can checkpoint creation be parallel?

From
Albe Laurenz
Date:
高健 wrote:
> I have one question about checkponint . That is : can checkpoint be parallel?
> 
> It is said that checkpoint will be activated according to either conditions:
> 
> 1)After last checkpoint, checkpoint_timeout seconds passed.
> 
> 2)When shared_buffers memory above checkpoint_segments size  is filled with data.
> 
> My experience is that :
> 
> There is only one checkpoint process there and works, and even when there are many data created during
> transactions,
> 
> There will also be only one checkpoint process deal with it
> 
> (maybe background writer or some other server process will deal it)?

It would not make sense to have more than one checkpoint going on
at the same time; it is a point at which recovery can begin.

> And also, I think when the above 1) or 2) activated checkpoint ,
> 
> after the checkpoint data creation, then the 1) and 2) value will be to zero and calculated again.
> 
> So I think that checkpoint will not be created parallel.
> 
> Is my understanding right?

Yes.

> And if there are some official documentation from www.postgresql.org or somewhere else, I'll be very
> appreciative.

http://www.postgresql.org/docs/9.3/static/wal-configuration.html

 "A checkpoint is begun every checkpoint_segments log segments,
  or every checkpoint_timeout seconds, whichever comes first."

Yours,
Laurenz Albe

Re: Can checkpoint creation be parallel?

From
Jeff Janes
Date:
On Tue, Oct 8, 2013 at 1:54 AM, 高健 <luckyjackgao@gmail.com> wrote:

Hello:

Sorry for disturbing:

I have one question about checkponint . That is : can checkpoint be parallel?


PostgreSQL does not currently implement it that way.  And it is hard to see what the benefit would be of doing so.  But it should be theoretically possible.
 

It is said that checkpoint will be activated according to either conditions:

1)After last checkpoint, checkpoint_timeout seconds passed.


After the *start* of the last checkpoint.  Provided that the last checkpoint has finished.
 

2)When shared_buffers memory above checkpoint_segments size  is filled with data.


It is not shared_buffers that is measured, but the WAL files themselves (which effectively means wal_buffers usage) that is measured.
 

My experience is that :

There is only one checkpoint process there and works, and even when there are many data created during transactions,

There will also be only one checkpoint process deal with it

(maybe background writer or some other server process will deal it)?


The background writer and ordinary backends might write data (for their own reasons) that the checkpointer would have otherwise needed to write anyway.
 
Cheers,

Jeff

Re: Can checkpoint creation be parallel?

From
高健
Date:
Thanks a lot for all your kindly replying!

>The background writer and ordinary backends might write data (for their own rea>sons) that the checkpointer would have otherwise needed to write anyway.

And does the ordinary backends refer to 
the ones created when a client make a connection to PG?

For exampel:
When staring PG:
[root@pg200 ~]# ps -ef | grep post
root      3039  3015  0 11:02 pts/1    00:00:00 su - postgres
postgres  3040  3039  0 11:02 pts/1    00:00:00 -bash
postgres  3093     1  0 11:03 pts/1    00:00:00 /usr/local/pgsql/bin/postgres -D ./data
postgres  3095  3093  0 11:03 ?        00:00:00 postgres: checkpointer process         
postgres  3096  3093  0 11:03 ?        00:00:00 postgres: writer process               
postgres  3097  3093  0 11:03 ?        00:00:00 postgres: wal writer process           
postgres  3098  3093  0 11:03 ?        00:00:00 postgres: autovacuum launcher process   
postgres  3099  3093  0 11:03 ?        00:00:00 postgres: stats collector process      
root      3126  3103  0 11:03 pts/2    00:00:00 grep post

After using psql to connect:
[root@pg200 ~]# ps -ef | grep post
root      3039  3015  0 11:02 pts/1    00:00:00 su - postgres
postgres  3040  3039  0 11:02 pts/1    00:00:00 -bash
postgres  3093     1  0 11:03 pts/1    00:00:00 /usr/local/pgsql/bin/postgres -D ./data
postgres  3095  3093  0 11:03 ?        00:00:00 postgres: checkpointer process         
postgres  3096  3093  0 11:03 ?        00:00:00 postgres: writer process               
postgres  3097  3093  0 11:03 ?        00:00:00 postgres: wal writer process           
postgres  3098  3093  0 11:03 ?        00:00:00 postgres: autovacuum launcher process   
postgres  3099  3093  0 11:03 ?        00:00:00 postgres: stats collector process      
root      3150  3130  0 11:03 pts/3    00:00:00 su - postgres
postgres  3151  3150  0 11:03 pts/3    00:00:00 -bash
postgres  3183  3151  0 11:04 pts/3    00:00:00 ./psql
postgres  3184  3093  0 11:04 ?        00:00:00 postgres: postgres postgres [local] idle
root      3186  3103  0 11:04 pts/2    00:00:00 grep post
[root@pg200 ~]# 

Does the above ordinary backends mean to the process  3184?


Best Regards

jian gao


2013/10/9 Jeff Janes <jeff.janes@gmail.com>
On Tue, Oct 8, 2013 at 1:54 AM, 高健 <luckyjackgao@gmail.com> wrote:

Hello:

Sorry for disturbing:

I have one question about checkponint . That is : can checkpoint be parallel?


PostgreSQL does not currently implement it that way.  And it is hard to see what the benefit would be of doing so.  But it should be theoretically possible.
 

It is said that checkpoint will be activated according to either conditions:

1)After last checkpoint, checkpoint_timeout seconds passed.


After the *start* of the last checkpoint.  Provided that the last checkpoint has finished.
 

2)When shared_buffers memory above checkpoint_segments size  is filled with data.


It is not shared_buffers that is measured, but the WAL files themselves (which effectively means wal_buffers usage) that is measured.
 

My experience is that :

There is only one checkpoint process there and works, and even when there are many data created during transactions,

There will also be only one checkpoint process deal with it

(maybe background writer or some other server process will deal it)?


The background writer and ordinary backends might write data (for their own reasons) that the checkpointer would have otherwise needed to write anyway.
 
Cheers,

Jeff

Re: Can checkpoint creation be parallel?

From
Albe Laurenz
Date:
高健 wrote:
>> The background writer and ordinary backends might write data
>> (for their own rea>sons) that the
>> checkpointer would have otherwise needed to write anyway.
> 
> And does the ordinary backends refer to
> the ones created when a client make a connection to PG?

Yes.

Yours,
Laurenz Albe