> On Aug 29, 2020, at 3:27 AM, Andrey M. Borodin <x4...@yandex-team.ru> wrote:
>
>
>
>> 29 авг. 2020 г., в 00:56, Robert Haas <robertmh...@gmail.com> написал(а):
>>
>> On Fri, Aug 28, 2020 at 2:10 PM Andrey M. Borodin <x4...@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
