I recently had a talk with Apple Computer's Tom Goguen, director of server and storage software, about Xsan, the software-based SAN solution that Apple's putting out in the fall for US$999 per server. I went into the discussion knowing what a SAN is, but after about five minutes I felt like I was in short pants and learning my ABCs.
I came out of that meeting with two key bits of knowledge. First, Xsan is really a SAN file system, which makes SANs useful and accessible beyond their core capabilities. And second, Xsan is precisely the right way to turn inexpensive disk arrays (like Xserve RAID) into shared, consolidated network storage.
Here's the elevator pitch: Xsan does the SAN thing, consolidating and virtualizing storage. But Xsan presents that storage in its most readily usable form, as disks that are partitioned into OS X HFS+ (HFS second-generation) volumes. Each of Xsan's virtual volumes permits simultaneous read/write use by multiple servers.
Apple chose the StorNext FS technology from Advanced Digital Information Corporation as the model for Xsan. ADIC's solution sits between SAN hardware and the servers that access it. Xsan knocks the SAN hardware out of the loop, replacing it with software that runs on Xserve and Power Mac machines. Xsan is fully compatible with ADIC's StorNext line. Optional Xsan support for Linux, Unix, and Windows servers can be purchased from ADIC.
Although the Mac sees only a formatted disk, Xsan is silently handling most of the work that administrators usually have to do; no more worrying about replication and synchronization of redundant and load-sharing servers. There's no fiddling with RAID partitions and network shares when you bring a new server online. Spanning of physical storage and cascading fail-over just come along for the ride. For high availability, Xsan stores state information on disk, and takes advantages of Xserve RAID's battery-backed cache.
Xsan puts SAN file systems within the reach of midsize businesses and high-demand workgroups or server clusters. The initial buy-in is manageable at around $35,000 for the Xserves, Xserve RAID, Xsan software, FC (Fibre Channel) network adapters, and the fiber switch needed to get a 3TB Xsan running. After that, you can join Xserve, Xserve G5, Power Mac G4, and Power Mac G5 systems to the Xsan for a flat $999 per machine plus the cost of the FC adapter. You can add Xserve RAID storage to your heart's content as long as you have open ports on your fiber switches. There is no cost associated with publishing traditional OS X, Unix, or Windows shared folders from Xsan servers.
What gives me pause is Apple's professional services network. Xsan is, by far, the most complex technology that Apple has produced. Only a percentage of Apple dealers and consultants will have the skill to configure, expand, and support Xsan. Although Xsan should be a solution that opens SANs to those who can't afford them, it's going to be a do-it-yourself exercise until dealers get up to speed. There's a yellow flag here. But I got Xsan in 90 minutes over the phone with almost no understanding of SAN file systems.
If you can give up "plug, play, and don't touch" in favor of five or six figures' worth of reduced buy-in and upgrade costs, rolling up your sleeves might seem less inconvenient.