Hi, First, I'm not sure this mail should go to this mailing list. As it refers to source code (mainly src/backend/postmaster/bgwriter.c and src/backend/access/transam/xlog.c), I sent it here. I apologize if I'm wrong.
I'm a bit puzzled by the different informations I can read on the documentation, on the source files and other README files from the source package. I'm actually using 8.2.5 source files. I try to answer a simple question : what happens when I do a simple "INSERT" on a just started PostgreSQL server. # pg_ctl start # psql foobase [...] foobase=# CREATE TABLE footable (i int4); CREATE TABLE foobase=# INSERT INTO footable (i) VALUES (1); INSERT 0 1 >From what I understand with the INSERT statement, here is what happens : * backend loads first (and only) block from footable file into a shared buffer * it modifies this block on the shared buffer, and sets it as dirty After checkpoint_timeout seconds or after a manual CHECKPOINT (there's also some other ways to have a checkpoint but in my simple example, these two seem to be the most probable ones) : * bgwriter runs CreateCheckPoint * it will, beyond other things, flush this block from shared buffer to disk, fsync, sets the block as non-dirty... ... and here is my first question : flushes to the current XLOG file or to footable data file ? I think it is to XLOG file but I don't find where in the code the content of WAL files is flushed to data files. If you know where I can look to get this information, I'm really interested. >From other parts of the documentation, it seems checkpoints are also in charge of flushing WAL files to data files. I've read something like this: "a checkpoint writes dirty shared buffers, sync dirty kernel buffers, recycle WAL files." Also, because of the checkpoint_segments GUC. And I don't find where wal_buffers are used in all this process. One last thing (wrt this mail), I think I understand what full_page_writes does but I wonder what this sentence means : "Writes full pages to WAL when first modified after a checkpoint." ? (it's the last part that I don't understand) Thanks for any pointers. Regards. -- Guillaume. http://www.postgresqlfr.org http://dalibo.com ---------------------------(end of broadcast)--------------------------- TIP 5: don't forget to increase your free space map settings
