On Fri, Apr 20, 2018 at 11:40 AM, Kyotaro HORIGUCHI
<horiguchi.kyotaro@lab.ntt.co.jp> wrote:
> By the way, I think I found a bug of FPW.
>
> The following steps yields INSERT record that doesn't have a FPI
> after a checkpoint.
>
> (Start server with full_page_writes = off)
> CREATE TABLE t (a int);
> CHECKPOINT;
> INSERT INTO t VALUES (1);
> ALTER SYSTEM SET full_page_writes TO on;
> SELECT pg_reload_conf();
> CHECKPOINT;
> INSERT INTO t VALUES (1);
>
> The last insert is expected to write a record with FPI but it
> doesn't actually. No FPI will be written for the page after that.
>
> It seems that the reason is that XLogInsertRecord is forgetting
> to check doPageWrites' update.
>
> In the failure case, fpw_lsn has been set by XLogRecordAssemble
> but doPageWrite is false at the time and it considers that no FPI
> is required then it sets fpw_lsn to InvalidXLogRecPtr.
>
> After that, XLogInsertRecord receives the record but it thinks
> that doPageWrites is true since it looks the shared value.
>
>> if (fpw_lsn != InvalidXLogRecPtr && fpw_lsn <= RedoRecPtr && doPageWrites)
>
> So this line thinks that "no FPI is omitted in this record" but
> actually the record is just forgotten to attach them.
>
> The attached patch lets XLogInsertRecord check if doPageWrites
> has been turned on after the last call and cause recomputation in
> the case.
>
I think you have correctly spotted the problem and your fix looks good
to me. As this is a separate problem and fix is different from what
we are discussing here, I think this can be committed it separately.
--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com