Unsupported redo log format (0). The redo log was created before MySQL 5.7.9 I have not started MySQL on my Mac Mojave in while. When today I ran mysql.server start I got the following error:

2019-10-03T21:42:11.410763Z 1 [System] [MY-011012] [Server] Starting upgrade of data directory.
2019-10-03T21:42:11.464825Z 1 [ERROR] [MY-013090] [InnoDB] Unsupported redo log format (0). The redo log was created before MySQL 5.7.9
2019-10-03T21:42:11.464850Z 1 [ERROR] [MY-012930] [InnoDB] Plugin initialization aborted with error Generic error.
2019-10-03T21:42:11.465724Z 1 [ERROR] [MY-011013] [Server] Failed to initialize DD Storage Engine.
2019-10-03T21:42:11.466028Z 0 [ERROR] [MY-010020] [Server] Data Dictionary initialization failed.
2019-10-03T21:42:11.466203Z 0 [ERROR] [MY-010119] [Server] Aborting


My online research suggested that I need to rename ib_logfile1 and ib_logfile1 in my /usr/local/var/mysql directory.

I did that and then I got the following error:

2019-10-03T21:27:39.506301Z 1 [System] [MY-011012] [Server] Starting upgrade of data directory.
2019-10-03T21:27:40.301335Z 1 [ERROR] [MY-013168] [InnoDB] Cannot upgrade server earlier than 5.7 to 8.0
2019-10-03T21:27:45.304727Z 1 [ERROR] [MY-011013] [Server] Failed to initialize DD Storage Engine.
2019-10-03T21:27:45.305119Z 0 [ERROR] [MY-010020] [Server] Data Dictionary initialization failed.
2019-10-03T21:27:45.305571Z 0 [ERROR] [MY-010119] [Server] Aborting


What is going on here? (BTW, I thought I already had mysql8 installed but I am not sure anymore)
You must upgrade to 5.7 as a first step.  Then you can upgrade to 8.0.

If Mac is trying to make a bigger leap, then they are doing it wrong.

You can probably see what version is installed (without hitting that problem) by doing

mysqld --version