Robert Haas wrote: > On Thu, Nov 11, 2010 at 10:13 PM, Bruce Momjian <br...@momjian.us> wrote: > > Robert Haas wrote: > >> On Thu, Oct 28, 2010 at 1:13 AM, Josh Berkus <j...@agliodbs.com> wrote: > >> > > >> >> I sort of agree with you that the current checkpoint_segments > >> >> parameter is a bit hard to tune, at least if your goal is to control > >> >> the amount of disk space that will be used by WAL files. ?But I'm not > >> >> sure your proposal is better. ?Instead of having a complicated formula > >> >> for predicting how much disk space would get used by a given value for > >> >> checkpoint_segments, we'd have a complicated formula for the amount of > >> >> WAL that would force a checkpoint based on max_wal_size. > >> > > >> > Yes, but the complicated formula would then be *in our code* instead of > >> > being inflicted on the user, as it now is. > >> > >> I don't think so - I think it will just be inflicted on the user in a > >> different way. ?We'd still have to document what the formula is, > >> because people will want to understand how often a checkpoint is going > >> to get forced. > >> > >> So here's an example of how this could happen. ?Someone sets > >> max_wal_size = 480MB. ?Then, they hear about the > >> checkpoint_completion_target parameter, and say, ooh, goody, let me > >> boost that. ?So they raise it from 0.5 to 0.9. ?Now, all of a sudden, > >> they're getting more frequent checkpoints. ?Performance may get worse > > > > Uh, checkpoint_completion_target only controls flushing of buffers > > between checkpoints, not the frequency of checkpoints. > > According to the formula in our fine documentation, if you increase > checkpoint_completion_target, the maximum number of WAL files also > increases. This makes sense: the files from the last checkpoint can't > be removed until further along into the next cycle. Therefore, if you > wanted to increase the checkpoint_completion_target while keeping the > maximum amount of WAL on disk the same, you'd need to trigger > checkpoints more frequently.
Do we recycle WAL files between checkpoints or just at checkpoint time? I thought it was only at checkpoint time. Also, there was talk that a larger WAL directory would slow recovery, but I thought it was only the time since the last checkpoint that controlled that. [ Again, sorry for my late reading of this and other threads. ] -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
