Re: Patch: Write Amplification Reduction Method (WARM)

On 21/03/17 18:19, Bruce Momjian wrote:
> On Tue, Mar 21, 2017 at 01:14:00PM -0400, Robert Haas wrote:
>> On Tue, Mar 21, 2017 at 1:08 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>>> On Tue, Mar 21, 2017 at 10:04 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>>>> I think that's a good question. I previously expressed similar
>>>> concerns. On the one hand, it's hard to ignore the fact that, in the
>>>> cases where this wins, it already buys us a lot of performance
>>>> improvement. On the other hand, as you say (and as I said), it eats
>>>> up a lot of bits, and that limits what we can do in the future. On
>>>> the one hand, there is a saying that a bird in the hand is worth two
>>>> in the bush. On the other hand, there is also a saying that one
>>>> should not paint oneself into the corner.
>>>
>>> Are we really saying that there can be no incompatible change to the
>>> on-disk representation for the rest of eternity? I can see why that's
>>> something to avoid indefinitely, but I wouldn't like to rule it out.
>>
>> Well, I don't want to rule it out either, but if we do a release to
>> which you can't pg_upgrade, it's going to be really painful for a lot
>> of users. Many users can't realistically upgrade using pg_dump, ever.
>> So they'll be stuck on the release before the one that breaks
>> compatibility for a very long time.
>
> Right. If we weren't setting tuple and tid bits we could imrpove it
> easily in PG 11, but if we use them for a single-change WARM chain for
> PG 10, we might need bits that are not available to improve it later.
>

I thought there is still couple of bits available.

--
Petr Jelinek http://www.2ndQuadrant.com/
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