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WAL internals

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Hello,
I am currently working on a heavily stressed system and i am
having some difficulties to make the background writer work properly.

But before going any further, i would like to be sure that i understood how transaction processing, and management are handled by postgreSQL. Correct me if i'm wrong :

1. A Transaction is sent from the postgreSQL backend

2. If the result of the transaction is within the shared buffers, we get our result instantly. Else some searching is done within the database datafiles to get the result which is copied to the shared buffers memory zone. The transaction is stocked in a WAL
buffer.
Now, when the result is copied to the shared buffer, if the transaction was an update or a delete
the line is flagged to be updated/deleted in the datafiles.

transactions go on and on this way.

At some points, the WAL buffers are written in the checkpoint segments. I don't know when,
if you could just precise this point.

3. Then periodically, there are checkpoints, those will make the changes into the datafiles from
the shared buffers (meaning shared buffers are flushed into the datafiles).
The last written record in the datafiles is flagged into the checkpoint segments
that way REDOs are possible.

Now i tried to set the bgwriter_lru_percent to 100% and bgwriter_lru_maxpages to 1000 and i did not spot any difference with the disk activities, cpu occupation or anything else from the default set up which is 1% and 5 so i was wondering if commiting after every transaction would prevent me from seeing any difference ? or is there another explanation ?

by the way, i made sure all changes took effect i restarted the postmaster process.
I first asked these questions on the novice mailing list.

Thanks in advance for your help

Regards,

Fabrice Franquenk.


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