Found in the last couple of days.
1. If you have more than one active database, and you're using text format logs, make sure you have %d (the database name) in your log line prefix. Not knowing which database the log line comes from can be confusing and makes life more difficult.
2. If you report on a possible database error, report from the Postgres logs, not from your client's logs. We need to see things from the database's point of view. In the case in point, the Postgres logs conatined no relevant errors at all, i.e. this wasn't a Postgres error.
3. Found when trying to resize a data volume on Amazon: certain Linux disk partitioning utilities don't optimally align partitions by default, notably sfdisk and cfdisk. Stay away from those, or make sure you explicitly choose proper alignment for partitions. Programs that do the right thing include fdisk and parted. In general you are probably best to stick to using parted.