Re: Index Scans become Seq Scans after VACUUM ANALYSE

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.

> 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.

cjs
--
Curt Sampson <cjs(at)cynic(dot)net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC