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

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?

- <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?

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?

Regards,

--
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION