Re: Spreading full-page writes

On Tue, May 27, 2014 at 3:57 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> On 25 May 2014 17:52, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> wrote:
>
>> Here's an idea I tried to explain to Andres and Simon at the pub last night,
>> on how to reduce the spikes in the amount of WAL written at beginning of a
>> checkpoint that full-page writes cause. I'm just writing this down for the
>> sake of the archives; I'm not planning to work on this myself.
> ...
>
> Thanks for that idea, and dinner. It looks useful.
>
> I'll call this idea "Background FPWs"
>
>> Now, I'm sure there are issues with this scheme I haven't thought about, but
>> I wanted to get this written down. Note this does not reduce the overall WAL
>> volume - on the contrary - but it ought to reduce the spike.
>
> The requirements we were discussing were around
>
> A) reducing WAL volume
> B) reducing foreground overhead of writing FPWs - which spikes badly
> after checkpoint and the overhead is paid by the user processes
> themselves
> C) need for FPWs during base backup
>
> So that gives us a few approaches
>
> * Compressing FPWs gives A
> * Background FPWs gives us B
> which look like we can combine both ideas
>
> * Double-buffering would give us A and B, but not C
> and would be incompatible with other two ideas

Double-buffering would allow us to disable FPW safely but which would make
a recovery slow. So if we adopt double-buffering, I think that we would also
need to overhaul the recovery.

Regards,

--
Fujii Masao