Re: finding changed blocks using WAL scanning

Hi,

On 2019-04-23 18:07:40 +0200, Tomas Vondra wrote:
> Well, the thing is that for prefetching to be possible you actually have
> to be a bit behind. Otherwise you can't really look forward which blocks
> will be needed, right?
> 
> IMHO the main use case for prefetching is when there's a spike of activity
> on the primary, making the standby to fall behind, and then hours takes
> hours to catch up. I don't think the cases with just a couple of MBs of
> lag are the issue prefetching is meant to improve (if it does, great).

I'd be surprised if a good implementation didn't. Even just some smarter
IO scheduling in the startup process could help a good bit. E.g. no need
to sequentially read the first and then the second block for an update
record, if you can issue both at the same time - just about every
storage system these days can do a number of IO requests in parallel,
and it nearly halves latency effects. And reading a few records (as in a
few hundred bytes commonly) ahead, allows to do much more than that.

Greetings,

Andres Freund