Is it safe to initially rsync without pg_start_backup, provided you snapshot and rsync afterwards? Trying to think of ways to speed up slave creation without master downtime.

The normal way of doing is:


pg_start_backup
rsync database files
pg_stop_backup
rsync wal files and start slave


Since 2 can take a long time over the network, is it safe to:


rsync
pg_start_backup
rsync again
pg_stop_backup
rsync wal files and start slave


?
Yes, completely safe. The subsequent rsync performed under snapshot conditions will ensure that the data is properly aligned using the delta method.

One might want to ensure that there isn't an accumulation of irrelevant data files by using the --delete option (think carefully about what you are excluding) but this is a mostly separate issue.