Re: Online checksums verification in the backend

On Thu, Oct 15, 2020 at 3:59 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
>
> On Thu, Oct 15, 2020 at 1:37 PM Julien Rouhaud <rjuju123@gmail.com> wrote:
> >
> > On Thu, Oct 1, 2020 at 1:07 PM Michael Paquier <michael@paquier.xyz> wrote:
> > >
> > > On Fri, Sep 25, 2020 at 06:11:47PM +0800, Julien Rouhaud wrote:
> > > > Thanks a lot for the tests!  I'm not surprised that forcing the lock
> > > > will slow down the pg_check_relation() execution, but I'm a bit
> > > > surprised that holding the buffer mapping lock longer in a workload
> > > > that has a lot of evictions actually makes things faster.  Do you have
> > > > any idea why that's the case?
> > >
> > > That's still a bit unclear to me, but I have not spent much time
> > > thinking about this particular point either.
> > >
> > > > I'm assuming that you prefer to remove both the optimization and the
> > > > throttling part?  I'll do that with the next version unless there's
> > > > objections.
> > >
> > > Yeah, any tests I have done tends to show that.  It would be good to
> > > also check some perf profiles here, at least for the process running
> > > the relation check in a loop.
> > >
> > > > I agree that putting the code nearby ReadBuffer_common() would be a
> > > > good idea.  However, that means that I can't move all the code to
> > > > contrib/  I'm wondering what you'd like to see going there.  I can see
> > > > some values in also having the SQL functions available in core rather
> > > > than contrib, e.g. if you need to quickly check a relation on a
> > > > standby, so without requiring to create the extension on the primary
> > > > node first.
> > >
> > > Good point.  This could make the user experience worse.
> > >
> > > >  Then, I'm a bit worried about adding this code in ReadBuffer_common.
> > > > What this code does is quite different, and I'm afraid that it'll make
> > > > ReadBuffer_common more complex than needed, which  is maybe not a good
> > > > idea for something as critical as this function.
> > > >
> > > > What do you think?
> > >
> > > Yeah, I have been looking at ReadBuffer_common() and it is true that
> > > it is complicated enough so we may not really need an extra mode or
> > > more options, for a final logic that is actually different than what a
> > > buffer read does: we just want to know if a page has a valid checksum
> > > or not.  An idea that I got here would be to add a new, separate
> > > function to do the page check directly in bufmgr.c, but that's what
> > > you mean.  Now only the prefetch routine and ReadBuffer_common use
> > > partition locks, but getting that done in the same file looks like a
> > > good compromise to me.  It would be also possible to keep the BLCKSZ
> > > buffer used to check the page directly in this routine, so as any
> > > caller willing to do a check don't need to worry about any
> > > allocation.
> >
> > I made all the suggested modifications in attached v14:
> >
> > - moved the C code in bufmgr.c nearby ReadBuffer
> > - removed the GUC and throttling options
> > - removed the dubious optimization
> >
> > All documentation and comments are updated to reflect those changes.
> > I also split the commit in two, one for the backend infrastructure and
> > one for the SQL wrappers.
>
> And I did miss a reference in the sgml documentation, fixed in v15.

I forgot to add the modified file in the previous attachment, sorry
for the noise.