GlusterFS, XtreemFS look like they're cloud storage systems. Are you wanting to store your data on cloud servers or are you wanting to ensure 2 directories on 2 different servers are in synch? Also, is it ok for them to be synched on a schedule or do they need to be in synch at all times? You can use rsync to just make sure both folders are in synch, and use Cron to schedule the synchronization.