In PG log file we can find some statistics about how vacuum works on a table.
Why vacuum needs to access WAL for cleaning up a table?
pages: 0 removed, 1640 remain, 0 skipped due to pins, 0 skipped frozen
tuples: 0 removed, 99960 remain, 0 are dead but not yet removable, oldest xmin: 825
index scan not needed: 0 pages from table (0.00% of total) had 0 dead item identifiers removed
avg read rate: 8.878 MB/s, avg write rate: 0.000 MB/s
buffer usage: 48 hits, 1 misses, 0 dirtied
**WAL usage: 1 records**, 0 full page images, 229 bytes
system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
