Hi again,
Thanks very much for the replies last week. We’ve been continuing to
investigate this problem, and I thought I’d share an update on where we are.
To recap: the situation is that, looking at our backup from 2025-06-26
via pageinspect, we have btree index rows which point to either
non-existent heap TIDs, or to heap TIDs with data which does not
correspond to the index row. In fact it looks like we have entire index
pages which point only to non-existent heap TIDs.
(I previously said that these index rows were marked as ‘dead’ in the
backup. We now suspect this is an artifact of the restore process: we
believe they are live in the backup, but were marked as dead during the
restore.)
Empirically, and surprisingly to us, when one does a SELECT from an
index entry that points to a non-existent TID, the index entry is
quietly ignored.
We therefore suspect that this index corruption has been present for
some time (possibly years); more recently those non-existent heap TIDs
have been recycled, and that is when we have noticed the effects of the
problem.
As far as we can tell, the corruption only affects one index on one
table, and only a specific region of that index/table. Specifically, it
only appears to affect rows which would have been inserted between 2018
and January 2021. At least 1B rows appear to be affected (the table as a
whole has 29B rows).
One thing that surprised us is that ‘amcheck’ didn’t find any sign of
the corruption. We’re not completely sure if this is because we are
holding it wrong, or because it’s simply out of scope or unsupported for
amcheck. Any advice on this, or suggestions for other tooling we could
use to check the consistency of our other indexes, would be much
appreciated.
We’re still very interested in trying to understand the root cause of
the corruption, mostly to confirm that it’s not an ongoing problem.
Thanks Tom for the suggestion of
https://git.postgresql.org/gitweb/?p=postgresql.git&a=commitdiff&h=4934d3875.
We agree with your assessment that this is unlikely. For one thing, it
looks like that bug could only conceivably cause this corruption if it
affected an UPDATE query, and we’re reasonably sure we never do any
UPDATE queries on that table. (The table is mostly append-only. We do
sometimes run cleanup/compression jobs which amount to large amounts of
interleaved DELETEs and INSERTs, but no UPDATEs.)
Back in 2021, we were running Postgres 10.11. We’ve taken a pass through
the release notes since then to see if we can find any likely-looking
bugs. We found the one that causes BRIN index corruption (this is not a
BRIN index), and the one that causes CREATE INDEX CONCURRENTLY to end up
with too *few* entries (this one has the opposite problem), but no
particularly likely candidate. Any other suggestions would be welcome here.
At the moment, a historical hardware-level problem seems like it might
be the most likely culprit, though we are a bit mystified about how any
hardware failure could have caused such widespread damage to a single
index, whilst apparently leaving the rest of the database intact.
Any thoughts or suggestions are very much appreciated.
Thanks,
Erik
On 04/07/2025 15:59, Erik Johnston wrote:
On Fri, 4 Jul 2025, 15:38 Ron Johnson, <ronljohnso...@gmail.com> wrote:
On Fri, Jul 4, 2025 at 9:49 AM Erik Johnston <er...@element.io> wrote:
Hi, a quick update:
- We have discovered that the corruption was present from
before libicu update.
- We ran `pg_amcheck --index state_groups_state_type_idx
--heapallindexed matrix`, which returned nothing
- We believe that means that (and matches what we see
sampling) the index has gained extra entries, i.e. that for a
given state group it does return all the relevant rows in the
table /plus/ extra rows.
We are also seeing old state groups starting to point at rows
that have only just been inserted. For example, querying for
353864583 on the primary it returns that row plus four rows
that have been inserted today, but on the backup from last
week an index only scan for 353864583 only returns one row.
This makes it feel like the corruption is ongoing? Nothing
should have modified that state group in the interim (they are
generally immutable).
This naively feels like when inserting a new row we sometimes
add the row to the index twice: once pointing from the correct
state group to the new row, and once from an old state group
to the new row?
Are checksums enabled in the instance?
Alas not.
We've also now found that the index on the backup does in fact point
to those ctids after all, but they are marked as dead. So at some
point between then and when we inserted the new row at that ctid today
those entries were marked undead.
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