Re: WAL usage calculation patch

On Thu, Mar 19, 2020 at 09:03:02PM +0900, Fujii Masao wrote:
>
> On 2020/03/19 2:19, Julien Rouhaud wrote:
> >
> > I'm attaching a v5 with fp records only for temp tables, so there's no risk of
> > instability. As I previously said I'm fine with your two patches, so unless
> > you have objections on the fpi test for temp tables or the documentation
> > changes, I believe those should be ready for committer.
>
> You added the columns into pg_stat_database, but seem to forget to
> update the document for pg_stat_database.

Ah right, I totally missed that when I tried to clean up the original POC.

> Is it really reasonable to add the columns for vacuum's WAL usage into
> pg_stat_database? I'm not sure how much the information about
> the amount of WAL generated by vacuum per database is useful.

The amount per database isn't really useful, but I didn't had a better idea on
how to expose (auto)vacuum WAL usage until this:

> Isn't it better to make VACUUM VERBOSE and autovacuum log include
> that information, instead, to see how much each vacuum activity
> generates the WAL? Sorry if this discussion has already been done
> upthread.

That's a way better idea! I'm attaching the full patchset with the 3rd patch
to use this approach instead. There's a bit a duplicate code for computing the
WalUsage, as I didn't find a better way to avoid that without exposing
WalUsageAccumDiff().

Autovacuum log sample:

2020-03-19 15:49:05.708 CET [5843] LOG: automatic vacuum of table "rjuju.public.t1": index scans: 0
pages: 0 removed, 2213 remain, 0 skipped due to pins, 0 skipped frozen
tuples: 250000 removed, 250000 remain, 0 are dead but not yet removable, oldest xmin: 502
buffer usage: 4448 hits, 4 misses, 4 dirtied
avg read rate: 0.160 MB/s, avg write rate: 0.160 MB/s
system usage: CPU: user: 0.13 s, system: 0.00 s, elapsed: 0.19 s
WAL usage: 6643 records, 4 full page records, 1402679 bytes

VACUUM log sample:

# vacuum VERBOSE t1;
INFO: vacuuming "public.t1"
INFO: "t1": removed 50000 row versions in 443 pages
INFO: "t1": found 50000 removable, 0 nonremovable row versions in 443 out of 443 pages
DETAIL: 0 dead row versions cannot be removed yet, oldest xmin: 512
There were 50000 unused item identifiers.
Skipped 0 pages due to buffer pins, 0 frozen pages.
0 pages are entirely empty.
1332 WAL records, 4 WAL full page records, 306901 WAL bytes
CPU: user: 0.01 s, system: 0.00 s, elapsed: 0.01 s.
INFO: "t1": truncated 443 to 0 pages
DETAIL: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
INFO: vacuuming "pg_toast.pg_toast_16385"
INFO: index "pg_toast_16385_index" now contains 0 row versions in 1 pages
DETAIL: 0 index row versions were removed.
0 index pages have been deleted, 0 are currently reusable.
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
INFO: "pg_toast_16385": found 0 removable, 0 nonremovable row versions in 0 out of 0 pages
DETAIL: 0 dead row versions cannot be removed yet, oldest xmin: 513
There were 0 unused item identifiers.
Skipped 0 pages due to buffer pins, 0 frozen pages.
0 pages are entirely empty.
0 WAL records, 0 WAL full page records, 0 WAL bytes
CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s.
VACUUM

Note that the 3rd patch is an addition on top of Kirill's original patch, as
this is information that would have been greatly helpful to investigate in some
performance issues I had to investigate recently. I'd be happy to have it land
into v13, but if that's controversial or too late I'm happy to postpone it to
v14 if the infrastructure added in Kirill's patches can make it to v13.