> Yesterday I had an interesting discussion with my colleague about shared > buffers size for our new server. This machine (is dedicated for db) has got > 512GB of RAM and database size is about 80GB, so he assumes that db will > never have to read from disk, so there is no point to adjust read ahead > setting, because every block gonna be read from RAM. As I've red in Greg > Smith book, once a block is changed it will be written to a disk and buffers > page is marked as clean, which would mean than changes occur in the same > page as before? What if dirty page doesn't have enough space for another row > and row has to be written to another page? Is it still occurs in RAM? If > that's true all updates of FSM occurs in RAM as well? > > What about buffers_clean and pg_clog then? Are those maintained completely > in RAM as well without direct read from disk at all? As long as they are on shared buffers, they are read from the buffer, not from disk. > To be precise, does the path to update and read updated row looks like a or > b?: > a). clean page (shared buffers) -> dirty page (shared buffers) -> to disk -> > read from disk -> shared buffers -> query > b). clean page (shared buffers) -> dirty page (shared buffers) -> to disk > & dirty page (shared buffers) -> clean page (shared buffers) -> query I'm not sure what you exactly mean by a) or b) but both look incorrect for me. A reader can read a page from shared bufferes even if it's dirty. So: clean page (shared buffers) -> dirty page (shared buffers) -> query will be closer to the reality. Note that dirty page will be written by bgwriter process at different timing. Also note that I completely ignore lock or buffer replacement algorithm. Please read src/backend/storage/buffer/README for more precise information. Best regards, -- Tatsuo Ishii SRA OSS, Inc. Japan English: http://www.sraoss.co.jp/index_en.php Japanese:http://www.sraoss.co.jp -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
