@benbjohnson Sure. 
We keep some old logs after snapshot (to help slow machine) and they will store on disk.

When the machine restarts, it need to restore the `startIndex` and  `startTerm` and `commitIndex` from snapshot and another `commitIndex` from conf. We will use the larger `commitIndex`.

And we only want to keep the log after `startIndex`. So we need to compare it against `startIndex` we got from snapshot.