Curt Sampson wrote: > On 23 Jun 2002, J. R. Nield wrote: > > > So since we have all this buffering designed especially to meet our > > needs, and since the OS buffering is in the way, can someone explain to > > me why postgresql would ever open a file without the O_DSYNC flag if the > > platform supports it? > > It's more code, if there are platforms out there that don't support > O_DYSNC. (We still have to keep the old fsync code.) On the other hand, > O_DSYNC could save us a disk arm movement over fsync() because it > appears to me that fsync is also going to force a metadata update, which > means that the inode blocks have to be written as well.
Again, see postgresql.conf: #wal_sync_method = fsync # the default varies across platforms: # # fsync, fdatasync, open_sync, or open_datasync > > > Maybe fsync would be slower with two files, but I don't see how > > fdatasync would be, and most platforms support that. > > Because, if both files are on the same disk, you still have to move > the disk arm from the cylinder at the current log file write point > to the cylinder at the current ping-pong file write point. And then back > again to the log file write point cylinder. > > In the end, having a ping-pong file as well seems to me unnecessary > complexity, especially when anyone interested in really good > performance is going to buy a disk subsystem that guarantees no > torn pages and thus will want to turn off the ping-pong file writes > entirely, anyway. Yes, I don't see writing to two files vs. one to be any win, especially when we need to fsync both of them. What I would really like is to avoid the double I/O of writing to WAL and to the data file; improving that would be a huge win. -- Bruce Momjian | http://candle.pha.pa.us [EMAIL PROTECTED] | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026 ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster
