Something changed for the better in create index between MySQL 8.0.18 and 8.0.20

I include MongoDB, Postgres, MySQL/InnoDB and MySQL/RocksDB (MyRocks) in the things I test via Linkbench and the insert benchmark. Hopefully I add another test later this year. I have been using MySQL 8.0.18 this year and recently started to use 8.0.20.

The new insert benchmark workflow is create tables with PK indexes, load tables, create 3 secondary indexes on each table, continue the load and then run several rounds of reads+inserts. For this test there were 8 clients with a table per client and 10M rows/table after the initial insert.

I think something changed for the better from MySQL 8.0.18 to 8.0.20 and I didn't spot the change in the release notes. I see:

• Create index in 8.0.20 is 10% to 20% faster (nice, but not why I wrote this)
• Create index in 8.0.18 uses ~7X more read IO with innodb_flush_method = O_DIRECT (14G vs 2G) and ~2X more read IO with it set to O_DIRECT_NO_FSYNC (4G vs 2G). The table was cached at the start of create index and the indexed table should be less than 2G. Tests used innodb_sort_buffer_size = 64M.
• Something changed with the sort used by create index. Maybe it does fewer passes. I am not sure there is a way to monitor that.
• I don't understand the impact from O_DIRECT vs O_DIRECT_NO_FSYNC in 8.0.18, while it had no impact in 8.0.20.
• When the load was restarted after create index there was initially a lot of read IO with 8.0.20. Either the indexed table or the newly created indexes were not in cache and my bet is on the new indexes. This doesn't happen with 8.0.18. 
• For 8.0.20, create index is faster with innodb_use_native_aio set to ON, but I am ignoring that topic for now.

Tests are run for 3 configurations (c10b40, c10b40a, c10b40b). The InnoDB options for those configs are here. Perhaps because of the way I compiled MySQL from source, innodb_use_native_aio is off by default for 8.0.18 but on for 8.0.20 -- so 8.0.18 and 8.0.20 differ at runtime only for that option with the c10b40 config. For the c10b40a and c10b40b configs, the options are the same at run time because innodb_use_native_aio is set. The differences between the configs are:

• c10b40 - use O_DIRECT
• c10b40a - start with c10b40, add innodb_use_native_aio=FALSE
• c10b40b - start with c10b40b, change to O_DIRECT_NO_FSYNC

Performance metrics are here. The slightly out of date legend for the metrics is here. Similar to my favorite InnoDB performance expert, the results are compressed so I can compare many configurations on one screen. The metrics don't include it but the time to create the index for 8.0.18 is (275, 281, 233) seconds and for 8.0.20 is (157, 228, 209) seconds for the (c10b40, c10b40a, c10b40b) configs. The ips metric in the tables for create index is indexed rows per second computed as (number of rows in table / time to create all indexes).