On 23 Jun 2002, J. R. Nield wrote:
> On Sat, 2002-06-22 at 19:17, Bruce Momjian wrote:
> > J. R. Nield wrote:
> > > One other point:
> > >
> > > Page pre-image logging is fundamentally the same as what Jim Grey's
> > > book[1] would call "careful writes". I don't believe they should be in
> > > the XLOG, because we never need to keep the pre-images after we're sure
> > > the buffer has made it to the disk. Instead, we should have the buffer
> > > IO routines implement ping-pong writes of some kind if we want
> > > protection from partial writes.
> >
> > Ping-pong writes to where? We have to fsync, and rather than fsync that
> > area and WAL, we just do WAL. Not sure about a win there.
Presumably the win is that, "we never need to keep the pre-images
after we're sure the buffer has made it to the disk." So the
pre-image log can be completely ditched when we shut down the
server, so a full system sync, or whatever. This keeps the log file
size down, which means faster recovery, less to back up (when we
start getting transaction logs that can be backed up), etc.
This should also allow us to disable completely the ping-pong writes
if we have a disk subsystem that we trust. (E.g., a disk array with
battery backed memory.) That would, in theory, produce a nice little
performance increase when lots of inserts and/or updates are being
committed, as we have much, much less to write to the log file.
Are there stats that track, e.g., the bandwidth of writes to the
log file? I'd be interested in knowing just what kind of savings
one might see by doing this.
> The key question is: do we have some method to ensure that the OS
> doesn't do the writes in parallel?...
> It would seem to me that if the same process does both ping-pong writes,
> then there should be a cheap way to enforce a serial order. I could be
> wrong though.
Well, whether or not there's a cheap way depends on whether you consider
fsync to be cheap. :-)
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org Don't you know, in this new Dark Age, we're
alllight. --XTC
