Horton

Re: new heapcheck contrib module

Mark Dilger <mark.dilger@enterprisedb.com>

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From: Mark Dilger <mark.dilger@enterprisedb.com>
To: "Andrey M. Borodin" <x4mmm@yandex-team.ru>
Cc: Robert Haas <robertmhaas@gmail.com>, Amul Sul <sulamul@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Peter Geoghegan <pg@bowt.ie>, Andres Freund <andres@anarazel.de>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2020-08-29T17:48:44Z
Lists: pgsql-hackers 

> On Aug 29, 2020, at 3:27 AM, Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
> 
> 
> 
>> 29 авг. 2020 г., в 00:56, Robert Haas <robertmhaas@gmail.com> написал(а):
>> 
>> On Fri, Aug 28, 2020 at 2:10 PM Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
>>> I don't think so. ISTM It's the same problem of xmax<relfrozenxid actually, just hidden behind detoasing.
>>> Our regular heap_check was checking xmin\xmax invariants for tables, but failed to recognise the problem in toast (while toast was accessible until CLOG truncation).
>> 
>> The code can (and should, and I think does) refrain from looking up
>> XIDs that are out of the range thought to be valid -- but how do you
>> propose that it avoid looking up XIDs that ought to have clog data
>> associated with them despite being >= relfrozenxid and < nextxid?
>> TransactionIdDidCommit() does not have a suppress-errors flag, adding
>> one would be quite invasive, yet we cannot safely perform a
>> significant number of checks without knowing whether the inserting
>> transaction committed.
> 
> What you write seems completely correct to me. I agree that CLOG thresholds lookup seems unnecessary.
> 
> But I have a real corruption at hand (on testing site). If I have proposed here heapcheck. And I have pg_surgery from the thread nearby. Yet I cannot fix the problem, because cannot list affected tuples. These tools do not solve the problem neglected for long enough. It would be supercool if they could.
> 
> This corruption like a caries had 3 stages:
> 1. incorrect VM flag that page do not need vacuum
> 2. xmin and xmax < relfrozenxid
> 3. CLOG truncated
> 
> Stage 2 is curable with proposed toolset, stage 3 is not. But they are not that different.

I had an earlier version of the verify_heapam patch that included a non-throwing interface to clog.  Ultimately, I ripped that out.  My reasoning was that a simpler patch submission was more likely to be acceptable to the community.

If you want to submit a separate patch that creates a non-throwing version of the clog interface, and get the community to accept and commit it, I would seriously consider using that from verify_heapam.  If it gets committed in time, I might even do so for this release cycle.  But I don't want to make this patch dependent on that hypothetical patch getting written and accepted.

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Add pg_amcheck, a CLI for contrib/amcheck.

  2. Refactor and generalize the ParallelSlot machinery.

  3. Generalize parallel slot result handling.

  4. Move some code from src/bin/scripts to src/fe_utils to permit reuse.

  5. Factor pattern-construction logic out of processSQLNamePattern.

  6. Doc: clean up verify_heapam() documentation.

  7. Fix more portability issues in new amcheck code.

  8. Fix portability issues in new amcheck test.

  9. Try to avoid a compiler warning about using fxid uninitialized.

  10. Extend amcheck to check heap pages.

  11. Adjust walsender usage of xlogreader, simplify APIs

  12. Improve checking of child pages in contrib/amcheck.

  13. Sanitize line pointers within contrib/amcheck.

  14. Fix possible sorting error when aborting use of abbreviated keys.