On Wed, 2011-05-04 at 00:17 -0400, Tian Luo wrote: > So, "nbytes" should always be multiples of XLOG_BLCKSZ, which in the > default case, is 8192. > > My question is, if it always writes full pages no matter > "full_page_writes" is on or off, what is the difference? Most I/O systems and filesystems can end up writing part of a page (in this case, 8192 bytes) in the event of a power failure, which is called a "torn page". That can cause problems for postgresql, because the page will be a mix of old and new data, which is corrupt. The solution is "full page writes", which means that when a data page is modified for the first time after a checkpoint, it logs the entire contents of the page (except the free space) to WAL, and can use that as a starting point during recovery. This results in extra WAL data for safety, but it's unnecessary if your filesytem + IO system guarantee that there will be no torn pages (and that's the only safe time to turn it off). So, to answer your question, the difference is that full_page_writes=off means less total WAL data, which means fewer 8192-byte writes in the long run (you have to test long enough to go through a checkpoint to see this difference, however). PostgreSQL will never issue write() calls with 17 bytes, or some other odd number, regardless of the full_page_writes setting. I can see how the name is slightly misleading, but it has to do with whether to write this extra information to WAL (where "extra information" happens to be "full data pages" in this case); not whether to write the WAL itself in full pages. Regards, Jeff Davis -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
