Alternative to DFS-R on Amazon EC2 Windows Servers

For hosting our web sites in a Windows load-balanced environment we've traditionally used DFSR to sync folders between servers with our hosting company.

We're looking to migrate a lot of our sites to Amazon EC2 and I'm playing around with load balanced setups at the moment. That's all going fine, but now I'm onto trying to get folders/drives to sync between web servers.

Ideally I'd like to use DFSR, but it relies on having a domain controller in order to sync folders. After some research into setting up a DC on EC2 it appears that this is quite tricky (and somewhat outside my area of expertise as a developer) but I've seen it said once or twice that it's not necessarily that reliable?

So far alternatives I've looked into:

• Network share from www1 to www2 – if this is on a single web server then it provides a single point of failure (plus we've had performance issues with this).
• Network share on a separate "micro" Windows instance – again, provides a single point of failure as if this instance goes down then the whole site
• Cached network drive – I investigated using a cached network drive to get around the "single point of failure" issue, however, it's tricky to get a persistent networked drive that IIS will be able to see, and even then, I couldn't see a way to cache it (plus I'm not sure how quickly changes would be replicated between machines).
• Robocopy – from what I've seen this this could be useful if I ran it every minute, but it looks like it won't copy locked files? Also, there would be a lag in sync-times

Sorry if that's not explained very well or I've missed something obvious. Has anyone come up with a good, reliable method of syncing folders between web servers on Amazon EC2 which is as good as DFSR (or close)?