Geoff Speicher wrote:
> On Thu, Apr 16, 2015 at 4:56 PM, Qingqing Zhou <zhouqq.postgres@gmail.com> wrote:
>> On Thu, Apr 16, 2015 at 5:09 AM, Geoff Speicher <gspeicher@umtechx.com> wrote:
>>> ZFS implements copy-on-write, so when PostgreSQL modifies a block on disk,
>>> the filesystem writes a new block rather than updating the existing block.
>>> 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.
I am quite certain that fillfactor < 100% will be a win even then (for the right load).
Upating one (heap) block should always be cheaper than updating one heap block
as well as (at least) one index block per index involved.
I doubt that you would be able to measure any performance improvement -
after all, you have to update at least one heap block, no matter what the
fillfactor setting is.
Yours,
Laurenz Albe