Re: Patch: Write Amplification Reduction Method (WARM)

On Fri, Feb 24, 2017 at 4:06 PM, Pavan Deolasee
<pavan(dot)deolasee(at)gmail(dot)com> wrote:
>> Wow, OK. In my view, that makes the chain conversion code pretty much
>> essential, because if you had WARM without chain conversion then the
>> visibility map gets more or less irrevocably less effective over time,
>> which sounds terrible.
>
> Yes. I decided to complete chain conversion patch when I realised that IOS
> will otherwise become completely useful if large percentage of rows are
> updated just once. So I agree. It's not an optional patch and should get in
> with the main WARM patch.

Right, and it's not just index-only scans. VACUUM gets permanently
more expensive, too, which is probably a much worse problem.

>> But it sounds to me like even with the chain
>> conversion, it might take multiple vacuum passes before all visibility
>> map bits are set, which isn't such a great property (thus e.g.
>> fdf9e21196a6f58c6021c967dc5776a16190f295).
>
> The chain conversion algorithm first converts the chains during vacuum and
> then checks if the page can be set all-visible. So I'm not sure why it would
> take multiple vacuums before a page is set all-visible. The commit you quote
> was written to ensure that we make another attempt to set the page
> all-visible after al dead tuples are removed from the page. Similarly, we
> will convert all WARM chains to HOT chains and then check for all-visibility
> of the page.

OK, that sounds good. And there are no bugs, right? :-)

--
Robert Haas
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