On Thu, Apr 16, 2015 at 5:09 AM, Geoff Speicher <gspeicher@umtechx.com> wrote: > Therefore one might posit that PostgreSQL should be configured to use 100% > fillfactor and avoid clustering on ZFS. Can anyone comment on this? >
Even with COW, I can see fillfactor < 100% still have its virtues. For example, HOT update can avoid adding an extra index item on the index page if it finds the new item can be inserted in the same heap page.
That's true, the new physical location on disk is transparent to the DBMS so it has no more or less housekeeping with or without COW, but the housekeeping still has to be done somewhere, so it helps to understand which is more efficient. I'll see if I can produce some empirical data unless anyone thinks it's a waste of time.
When you do CLUSTER command, engine will overwrite table into new files any way, so COW does not affect here.
I was thinking about CLUSTER more in terms of its positive side-effects to the effective fillfactor but your point is taken.
