Repercussions of Cancelled Autovacuum

Hello,

We have a large(ish) table that had a series of delete statements executed against it (culling most of the rows in the table). This triggered a time consuming autovacuum, which we allowed to run for 2+ days (and were planning to allow to run to completion). However, it ended up getting cancelled unexpectedly, and is now running once again. I have some questions:

1. My understanding is the autovacuum will block most (all?) DDL statements executed on it, and pg will cancel it if a user requests a conflicting lock. I don't believe either of these things occurred. Are there any other circumstances that will cancel an autovacuum?
2. Is autovacuum incremental? Or, does it effectively roll back and start fresh, having to redo all of the work of the last couple of days?

Thank you!

Andrew