Re: Usage of epoch in txid_current

On Wed, Dec 6, 2017 at 11:26 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>
>> Either way, it is not clear to me how we will keep it
>> updated after recovery. Right now, the mechanism is quite simple, at
>> the beginning of a recovery we take the value of nextXid from
>> checkpoint record and then if any xlog record indicates xid that
>> follows nextXid, we advance it. Here, the point to note is that we
>> take the xid from the WAL record (which means that it assumes xids are
>> non-contiguous or some xids are consumed without being logged) and
>> increment it. Unless we plan to change something in that logic (like
>> storing 64-bit xids in WAL records), it is not clear to me how to make
>> it work. OTOH, recovering value of epoch which increments only at
>> wraparound seems fairly straightforward as described in my previous
>> email.
>
> I think it should be fairly simple if simply add the 64bit xid to the
> existing clog extension WAL records.
>

IIUC, you mean to say that we should log the 64bit xid value in
CLOG_ZEROPAGE record while extending clog and that too we can do only
at wraparound. Now, maybe doing it every time also doesn't hurt, but
I think doing it at wraparound should be sufficient.

Just to be clear, I am not planning to pursue writing a patch for this
at the moment. So, if anybody else is interested or if Andres wants
to write it, I can help in the review.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com