Re: Add statistics to pg_stat_wal view for wal related parameter tuning

At Tue, 20 Oct 2020 16:11:29 +0900, Masahiro Ikeda <ikedamsh(at)oss(dot)nttdata(dot)com> wrote in
> On 2020-10-20 12:46, Amit Kapila wrote:
> > I see that we also need to add extra code to capture these stats (some
> > of which is in performance-critical path especially in
> > XLogInsertRecord) which again makes me a bit uncomfortable. It might
> > be that it is all fine as it is very important to collect these stats
> > at cluster-level in spite that the same information can be gathered at
> > statement-level to help customers but I don't see a very strong case
> > for that in your proposal.

We should avoid that duplication as possible even if the both number
are important.

> Also about performance, I thought there are few impacts because it
> increments stats in memory. If I can implement to reuse pgWalUsage's
> value which already collects these stats, there is no impact in
> XLogInsertRecord.
> For example, how about pg_stat_wal() calculates the accumulated
> value of wal_records, wal_fpi, and wal_bytes to use pgWalUsage's
> value?

I don't think that works, but it would work that pgstat_send_wal()
takes the difference of that values between two successive calls.

WalUsage prevWalUsage;
...
pgstat_send_wal()
{
..
/* fill in some values using pgWalUsage */
WalStats.m_wal_bytes = pgWalUsage.wal_bytes - prevWalUsage.wal_bytes;
WalStats.m_wal_records = pgWalUsage.wal_records - prevWalUsage.wal_records;
WalStats.m_wal_wal_fpi = pgWalUsage.wal_fpi - prevWalUsage.wal_fpi;
...
pgstat_send(&WalStats, sizeof(WalStats));

/* remember the current numbers */
prevWalUsage = pgWalUsage;

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center