On 9/17/21, 1:32 PM, "Alvaro Herrera" <alvherre@alvh.no-ip.org> wrote:
> On 2021-Sep-17, Bossart, Nathan wrote:
>
>> I gave the patch a read-through. I'm wondering if the
>> XLOG_OVERWRITE_CONTRECORD records are actually necessary. IIUC we
>> will set XLP_FIRST_IS_ABORTED_PARTIAL on the next page, and
>> xlp_pageaddr on that page will already be validated in
>> XLogReaderValidatePageHeader(). Does adding this new record also help
>> ensure the page header is filled in and flushed to disk?
>
> That was the first implementation, a few versions of the patch ago. An
> added benefit of a separate WAL record is that you can carry additional
> data for validation, such as -- as suggested by Andres -- the CRC of the
> partial data contained in the message that we're skipping. I didn't
> implement that, but it should be trivial to add it.
I see. IMO feels a bit counterintuitive to validate a partial record
that you are ignoring anyway, but I suppose it's still valuable to
know when the WAL is badly broken. It's not expensive, and it doesn't
add a ton of complexity, either.
Nathan