All of the vendors provide replication from a primary machine to a backup machine. The last real hands on experience and testing I did took 45 minutes to synch up a MIMIX based machine after cutting over to the backup machine.
Sadly, unlike Linux or Unix you do not have real-time replication or load balanced system sharing that actually run parallel systems instead of primary & backup. This has been a huge shortcoming of OS/400 and IBM i forever. There may have been some improvement, but I've not seen them.
There are some good data replication products for IBM, I like Mimix which I used. The key to my comments are the fact that this has been an area of weakness since the AS/400 was first shipped. While Mimix and other products can provide data replication between machines there is no true HA solution that allows you to take a machine out of your network and continue processing as you can with other platforms and OS's.
For many IBM i customers data and object replication can be sufficient if you thoroughly review your program change management, security administration, network administration, etc. and make sure that:
1. You do not lose in flight data or that you know what you lose when you cut over.
2. Any programs that have been updated or changed on your primary machine are replicated to your back up machine.
3. Any other resources besides data or programs are replicated and synchronized.
4. User profiles and other security settings are synchronized, and
5. You have a network configuration that allows you to switch users to your backup machine.
This is not trivial and this really sets the vendors apart from each other. But keep in mind. Unlike modern could based scenarios you CANNOT balance your work load across multiple IBM i based systems and simply take one offline and keep on processing. You can do this on other platforms including Unix, Linux, and Windows.