I have two different FreeNAS boxes that are being used for storage. It is possible to link them in a manner that the linked connection can be presented as a single storage destination for clients? For example, I have one FreeNAS installation that has 6 TB of storage and another one that has 4 TB of storage. I would like to be able to present or have a single mapped drive for clients that would be able to span across both of them; however, the client would only see it as one drive. Is this possible? If not, is there a different way I should be approaching this problem? Is there a different OS distribution that would work better for this?
In theory, yes. FreeNAS 8 supports NFSv4, and NFSv4 provides features called "replication", "Migration" and "referrals". From RFC 5661 / "Network File System (NFS) Version 4 Minor Version 1 Protocol", section 11.4:
11.4.1. File System Replication
The fs_locations and fs_locations_info attributes provide alternative locations, to be used to access data in place of or in addition to the current file system instance. On first access to a file system, the client should obtain the value of the set of alternate locations by interrogating the fs_locations or fs_locations_info attribute, with the latter being preferred.
Referrals provide a way of placing a file system in a location within the namespace essentially without respect to its physical location on a given server. This allows a single server or a set of servers to present a multi-server namespace that encompasses file systems located on multiple servers. Some likely uses of this include establishment of site-wide or organization-wide namespaces, or even knitting such together into a truly global namespace.
This might do what you want in terms of namespace. Your users would, in theory, simply browse to a single network share, and the physical location of the NFS servers would be largely invisible to them.
Keep in mind that features mentioned in the FreeNAS GUI or commandline tools may not encompass all of NFSv4. You might need to hack it in, which can get you into trouble.
I haven't used these NFSv4 features at all, but I am looking at them now.
Note that "NFSv4 replication" is different then what most FreeNAS users mean by "Replication" (They mean ZFS snapshot/filesystem replication).
Note that some of the FreeNAS folks just finished up at the MeetBSD conference in Sunnyvale, CA and are brimming with new energy. It might be worth asking your question on the FreeNAS forums. I don't usually refer people to other forum sites, but in your case you might get more knowledgeable answers there.
I'm not sure how you would do this with FreeNAS so I would use Ubuntu, Fedora or CentOS to do one of the following:
Gluster provides mirroring and striping across network connected devices.
Export one of the RAID arrays as an iSCSI "share". On the other box use mdadm to RAID 0 the local disks with the remote iSCSI disks.
Beware: As you're effectively creating a RAID 0 array you'll have no resilience if one backend node dies. Also, to share with Windows clients, one node will have to provide the Samba service for both nodes.