Re: block-level incremental backup

Hi,

> > Wow.  I have to admit that I feel completely opposite of that- I'd
> > *love* to have an independent tool (which ideally uses the same code
> > through the common library, or similar) that can be run to apply WAL.
> >
> > In other words, I don't agree that it's the server's problem at all to
> > solve that, or, at least, I don't believe that it needs to be.
> 
> I mean, I guess I'd love to have that if I could get it by waving a
> magic wand, but I wouldn't love it if I had to write the code or
> maintain it.  The routines for applying WAL currently all assume that
> you have a whole bunch of server infrastructure present; that code
> wouldn't run in a frontend environment, I think.  I wouldn't want to
> have a second copy of every WAL apply routine that might have its own
> set of bugs.

I'll fight tooth and nail not to have a second implementation of replay,
even if it's just portions.  The code we have is complicated and fragile
enough, having a [partial] second version would be way worse.  There's
already plenty improvements we need to make to speed up replay, and a
lot of them require multiple execution threads (be it processes or OS
threads), something not easily feasible in a standalone tool. And
without the already existing concurrent work during replay (primarily
checkpointer doing a lot of the necessary IO), it'd also be pretty
unattractive to use any separate tool.

Unless you just define the server binary as that "independent tool".
Which I think is entirely reasonable. With the 'consistent' and LSN
recovery targets one already can get most of what's needed from such a
tool, anyway.  I'd argue the biggest issue there is that there's no
equivalent to starting postgres with a private socket directory on
windows, and perhaps an option or two making it easier to start postgres
in a "private" mode for things like this.

Greetings,

Andres Freund