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

On Tue, Mar 27, 2018 at 10:06 PM, Michael Paquier <michael@paquier.xyz> wrote:
> On Tue, Mar 27, 2018 at 02:02:10PM -0400, Tom Lane wrote:
>> Robert Haas <robertmhaas@gmail.com> writes:
>>> On Tue, Mar 27, 2018 at 11:41 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>>> This is ignoring the possibility of damaged data in between, ie
>>>> A ... B ... CHKPT ... C ...  a few zeroed pages ... D ... CHKPT ... E ... F
>>
>>> 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.
>>
>> Well, the point of checkpoints is that WAL data before the last one
>> should no longer matter anymore, isn't it?
>
> I have to agree with Tom here.  If you force pg_rewind to replay more
> WAL records from a checkpoint older than the checkpoint prior to where
> WAL has forked at promotion then you have a risk of losing data.

Oh!  I see now.  Good point.

-- 
Robert Haas
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