Many folks have compiled the full list of options, both commercial and free, shared and non-shared disk. There are Wikipedia entries covering the high level clustered filesystem concept is here and an exhaustive list of filesystems.
It sounds like you are operating with non-shared storage by using disks local to each system. You will probably want a dedicated network connection between the two hosts so that synchronization traffic doesn't interfere with your normal web application.
Now my real problem is that you have two independent filesystems now and you presumably want to merge them into one, reliable, better performing filesystem, without causing any downtime to your web site. The tough problem is that you're about to cut your storage space in half and you'll have to go through some variation of these steps:
- Select the filesystem of your choice
- Build a test rig so that you can practice; replicate networking as much as possible
- Test everything you see here on the practice system first so as to minimize downtime and mistakes
- Migrate all the data to one NFS server
- The one NFS server now serves both addresses
- The second machine becomes the first node of a new clustered filesystem
- All the data from NFS is copied to the new filesystem
- Client migrations to the new cluster begin
- Continuous migration of data from NFS to the clustered filesystem until all clients are on board
- Drop the NFS filesystem
- Rebuild the now-unused server and join it to the cluster
- Ensure that replication and/or load balancing works as planned
- Test the system by disconnecting the original node of the cluster and ensure that the clients stay up and running
If your only clients are a couple of web servers, then it might not be real bad. You can also continue to access the clustered filesystem over a variety of protocols like FTP, Samba, SSH/SCP, NFS.
One important tip - clustered filesystem make a poor storage location for database files of a live database. Use a clustering technology specific to your database engine if this applies to you.
The migration to a new filesystem is never short or easy. Good luck choosing your underlying filesystem and getting it in.