On 04/23/2015 06:38 PM, Bruce Momjian wrote:
> On Thu, Apr 23, 2015 at 10:42:59AM +0300, Heikki Linnakangas wrote:
>> On 04/22/2015 09:24 PM, Robert Haas wrote:
>>>> I would feel safer if we added a completely new "epoch" counter to the page
>>>>> header, instead of reusing LSNs. But as we all know, changing the page
>>>>> format is a problem for in-place upgrade, and takes some space too.
>>> Yeah. We have a serious need to reduce the size of our on-disk
>>> format. On a TPC-C-like workload Jan Wieck recently tested, our data
>>> set was 34% larger than another database at the beginning of the test,
>>> and 80% larger by the end of the test. And we did twice the disk
>>> writes. See "The Elephants in the Room.pdf" at
>>> https://sites.google.com/site/robertmhaas/presentations
>>
>> Meh. Adding an 8-byte header to every 8k block would add 0.1% to the
>> disk size. No doubt it would be nice to reduce our disk footprint,
>> but the page header is not the elephant in the room.
>
> Agreed. Are you saying we can't find a way to fit an 8-byte value into
> the existing page in a backward-compatible way?
I'm sure we can find a way. We've discussed ways to handle page format
updates in pg_upgrade before, and I don't want to get into that
discussion here, but it's not trivial.
- Heikki