Re: Amcheck verification of GiST and GIN

On 7/10/24 18:01, Tomas Vondra wrote:
> ...
>
> That's all for now. I'll add this to the stress-testing tests of my
> index build patches, and if that triggers more issues I'll report those.
>

As mentioned a couple days ago, I started using this patch to validate
the patches adding parallel builds to GIN and GiST indexes - I scripts
to stress-test the builds, and I added the new amcheck functions as
another validation step.

For GIN indexes it didn't find anything new (in either this or my
patches), aside from the assert crash I already reported.

But for GiST it turned out to be very valuable - it did actually find an
issue in my patches, or rather confirm my hypothesis that the way the
patch generates fake LSN may not be quite right.

In particular, I've observed these two issues:

ERROR: heap tuple (13315,38) from table "planet_osm_roads" lacks
matching index tuple within index "roads_7_1_idx"

ERROR: index "roads_7_7_idx" has inconsistent records on page 23723
offset 113

And those consistency issues are real - I've managed to reproduce issues
with incorrect query results (by comparing the results to an index built
without parallelism).

So that's nice - it shows the value of this patch, and I like it.

One thing I've been wondering about is that currently amcheck (in
general, not just these new GIN/GiST functions) errors out on the first
issue, because it does ereport(ERROR). Which is good enough to decide if
there is some corruption, but a bit inconvenient if you need to assess
how much corruption is there. For example when investigating the issue
in my patch it would have been great to know if there's just one broken
page, or if there are dozens/hundreds/thousands of them.

I'd imagine we could have a flag which says whether to fail on the first
issue, or keep looking at future pages. Essentially, whether to do
ereport(ERROR) or ereport(WARNING). But maybe that's a dead-end, and
once we find the first issue it's futile to inspect the rest of the
index, because it can be garbage. Not sure. In any case, it's not up to
this patch to invent that.

I don't have additional comments, the patch seems to be clean and likely
ready to go. There's a couple committers already involved in this
thread, I wonder if one of them already planned to take care of this?
Peter and Andres, either of you interested in this?

--
Tomas Vondra
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