Re: About to add WAL write/fsync statistics to pg_stat_wal view

On 2021-02-08 14:26, Fujii Masao wrote:
> On 2021/02/08 13:01, Fujii Masao wrote:
>>
>>
>> On 2021/02/05 8:45, Masahiro Ikeda wrote:
>>> I pgindented the patches.
>>
>> Thanks for updating the patches!
>>
>> +       <function>XLogWrite</function>, which nomally called by an
>> +       <function>issue_xlog_fsync</function>, which nomally called by
>> an
>>
>> Typo: "nomally" should be "normally"?
>>
>> +       <function>XLogFlush</function> request(see <xref
>> linkend="wal-configuration"/>)
>> +       <function>XLogFlush</function> request(see <xref
>> linkend="wal-configuration"/>),
>>
>> Isn't it better to add a space character just after "request"?
>>
>> +                    INSTR_TIME_SET_CURRENT(duration);
>> +                    INSTR_TIME_SUBTRACT(duration, start);
>> +                    WalStats.m_wal_write_time =
>> INSTR_TIME_GET_MICROSEC(duration);
>>
>> If several cycles happen in the do-while loop, m_wal_write_time should
>> be
>> updated with the sum of "duration" in those cycles instead of
>> "duration"
>> in the last cycle? If yes, "+=" should be used instead of "=" when
>> updating
>> m_wal_write_time?
>>
>> +            INSTR_TIME_SET_CURRENT(duration);
>> +            INSTR_TIME_SUBTRACT(duration, start);
>> +            WalStats.m_wal_sync_time =
>> INSTR_TIME_GET_MICROSEC(duration);
>>
>> Also "=" should be "+=" in the above?
>
> + /* Send WAL statistics */
> + pgstat_send_wal();
>
> This may cause overhead in WAL-writing by walwriter because it's called
> every cycles even when walwriter needs to write more WAL next cycle
> (don't need to sleep on WaitLatch)? If this is right, pgstat_send_wal()
> should be called only when WaitLatch() returns with WL_TIMEOUT?

Thanks, I didn't notice that.
I'll fix it.

> - <function>XLogFlush</function> request(see <xref
> linkend="wal-configuration"/>)
> + <function>XLogFlush</function> request(see <xref
> linkend="wal-configuration"/>),
> + or WAL data written out to disk by WAL receiver.
>
> So regarding walreceiver, only wal_write, wal_write_time, wal_sync, and
> wal_sync_time are updated even while the other values are not. Isn't
> this
> confusing to users? If so, what about reporting those walreceiver stats
> in
> pg_stat_wal_receiver?

OK, I'll add new infrastructure code to interect with wal receiver
and stats collector and show the stats in pg_stat_wal_receiver.

> if (endofwal)
> + {
> + /* Send WAL statistics to the stats collector */
> + pgstat_send_wal();
> break;
>
> You added pgstat_send_wal() so that it's called in some cases where
> walreceiver exits. But ISTM that there are other walreceiver-exit
> cases.
> For example, in the case where SIGTERM is received. Instead,
> pgstat_send_wal() should be called in WalRcvDie() for those all cases?

Thanks, I forgot the case.
I'll fix it.

Regards,
--
Masahiro Ikeda
NTT DATA CORPORATION