Checkpoint ; WAL

Hi,

I am reading some text on checkpoint, WAL etc in PG. read the files xlog.c, bgwriter.c. I have some newbie related questions which I am confused with. Appreciate if someone can throw more light.

The bgwriter is associated in writing the dirty buffers to the disk very so often.
Question : Does this write happen to the ( Xlog file and the data file ) or ( to the Xlog file only ) ?

Does the bgwriter mark the "dirty" buffer ( clean,free,...) after being written to the disk so that it can be used once again to store new data?

Apart from Checkpoint is there any point at which dirty buffers are written to the data file?

Thanks,
Kenneth

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