On Tue, Oct 14, 2014 at 01:21:53PM -0400, Bruce Momjian wrote: > On Tue, Oct 14, 2014 at 09:20:22AM -0700, Jeff Janes wrote: > > On Mon, Oct 13, 2014 at 12:11 PM, Bruce Momjian <br...@momjian.us> wrote: > > > > > > I looked into this, and came up with more questions. Why is > > checkpoint_completion_target involved in the total number of WAL > > segments? If checkpoint_completion_target is 0.5 (the default), the > > calculation is: > > > > (2 + 0.5) * checkpoint_segments + 1 > > > > while if it is 0.9, it is: > > > > (2 + 0.9) * checkpoint_segments + 1 > > > > Is this trying to estimate how many WAL files are going to be created > > during the checkpoint? If so, wouldn't it be (1 + > > checkpoint_completion_target), not "2 +". My logic is you have the old > > WAL files being checkpointed (that's the "1"), plus you have new WAL > > files being created during the checkpoint, which would be > > checkpoint_completion_target * checkpoint_segments, plus one for the > > current WAL file. > > > > > > WAL is not eligible to be recycled until there have been 2 successful > > checkpoints. > > > > So at the end of a checkpoint, you have 1 cycle of WAL which has just become > > eligible for recycling, > > 1 cycle of WAL which is now expendable but which is kept anyway, and > > checkpoint_completion_target worth of WAL which has occurred while the > > checkpoint was occurring and is still needed for crash recovery. > > OK, so based on this analysis, what is the right calculation? This? > > (1 + checkpoint_completion_target) * checkpoint_segments + 1 + > max(wal_keep_segments, checkpoint_segments)
Now that we have min_wal_size and max_wal_size in 9.5, I don't see any value to figuring out the proper formula for backpatching. -- Bruce Momjian <br...@momjian.us> http://momjian.us EnterpriseDB http://enterprisedb.com + Everyone has their own god. + -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
