Tom Lane wrote:
Maybe I misread the patch, but I thought that if someone requested an
immediate checkpoint, the checkpoint-in-progress would effectively flip
to immediate mode.  So that could be handled by offering an immediate vs
extended checkpoint option in pg_start_backup.  I'm not sure it's a
problem though, since as previously noted you probably want
pg_start_backup to be noninvasive.  Also, one could do a manual
CHECKPOINT command then immediately pg_start_backup if one wanted
as-fast-as-possible (CHECKPOINT requests immediate checkpoint, right?)


Yeah, that's possible.

and recovery would need to process on average 1.5 as much WAL as before. Though with LDC, you should get away with shorter checkpoint intervals than before, because the checkpoints aren't as invasive.

No, you still want a pretty long checkpoint interval, because of the
increase in WAL traffic due to more page images being dumped when the
interval is short.

If we do that, we should remove bgwriter_all_* settings. They wouldn't do much because we would have checkpoint running all the time, writing out dirty pages.

Yeah, I'm not sure that we've thought through the interactions with the
existing bgwriter behavior.

I searched the archives a bit for the discussions when the current bgwriter settings were born, and found this thread:

http://archives.postgresql.org/pgsql-hackers/2004-12/msg00784.php

The idea of Load Distributed Checkpoints certainly isn't new :).

Ok, if we approach this from the idea that there will be *no* GUC variables at all to control this, and we remove the bgwriter_all_* settings as well, does anyone see a reason why that would be bad? Here's the ones mentioned this far:

1. we need to keep 2x as much WAL segments around as before.

2. pg_start_backup will need to wait for a long time.

3. Recovery will take longer, because the distance last committed redo ptr will lag behind more.

1. and 3. can be alleviated by using a smaller checkpoint_timeout/segments though as you pointed out that leads to higher WAL traffic. 2. is not a big deal, and we can add an 'immediate' parameter to pg_start_backup if necessary.

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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