On Sun, Jan 16, 2022 at 8:12 PM Noah Misch <noah@leadboat.com> wrote:
> For specifics of the kernel bug, see the attached test program. In brief, the
> bug arises if one process is write()ing or pwrite()ing a file at about the
> same time that another process is read()ing or pread()ing the same. POSIX
> says the reader should see the data as it existed before the write or the
> newly-written data. On this kernel, the reader can see zeros instead. That
> leads to the $SUBJECT failure. PostgreSQL processes write out a given WAL
> block 20-30 times in ~10ms, and COMMIT PREPARED reads that block. The writers
> aren't changing the bytes of interest to COMMIT PREPARED, but the zeros from
> the kernel bug yield the failure. We could opt to work around that by writing
> only the not-already-written portion of a WAL block, but I doubt that's
> worthwhile unless it happens to be a performance win anyway.
>
> Separately, while I don't know of relevance to PostgreSQL, I was interested to
> see that CentOS 7 pwrite()/pread() fail to have the POSIX-required atomicity.
FWIW there was some related discussion over here:
https://www.postgresql.org/message-id/flat/17064-bb0d7904ef72add3%40postgresql.org