Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()

On Tue, Mar 27, 2018 at 11:41 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Pavan Deolasee <pavan.deolasee@gmail.com> writes:
>> On Tue, Mar 27, 2018 at 7:28 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> If you have to search backwards, this breaks it.  Full stop.
>
>> We don't really need to fetch the previous record. We really need to find
>> the last checkpoint prior to a given LSN. That can be done by reading WAL
>> segments forward. It can be a little slow, but hopefully not a whole lot.
>
> This is ignoring the possibility of damaged data in between, ie
>
> A ... B ... CHKPT ... C ...  a few zeroed pages ... D ... CHKPT ... E ... F
>
> If A is the start of the WAL segment, and you ask what's the last
> checkpoint before F, you will get the wrong answer.  Or, if you're
> paranoid and verify that you can follow the chain all the way to F,
> you'll fail ... needlessly, and in a case where we previously succeeded.

It's hard for me to believe that this case matters very much.  If
you're trying to run pg_rewind on a system where the WAL segments
contain a few zeroed pages, you're probably going to be hosed anyway,
if not by this particular thing then by something else.

-- 
Robert Haas
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