Re: Patch: Write Amplification Reduction Method (WARM)

On 21/03/17 18:14, 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.
>

This is why I like the idea of pluggable storage, if we ever get that it
would buy us ability to implement completely different heap format
without breaking pg_upgrade.

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