SAN, NAS, or SAS?
By Walt Hinton
A new type of network is emerging, called the storage-area network (SAN), that has radically different characteristics than traditional local- and wide-area networks. Traditional LANs/WANs provide connectivity between desktops and applications, while SANs provide connectivity between applications and storage.
The LAN/WAN is typically associated with asynchronous data transfer and is IP-based, which assumes that packet loss is handled at the transport layer. SANs, on the other hand, are typically based on synchronous channel protocols (ESCON, SCSI, or Fiber Channel), which do not depend on a higher-layer protocol like TCP. As a result, SANs:
- transfer data reliably
- provide fast data rates
- can be tuned effectively for the bulk data transfers that are characteristic of storage applications
There`s lots of talk about how great low-cost network-attached storage (NAS) will be. In fact, many view NAS as the Holy Grail of storage. I contend that very few people really know what NAS is all about and have considered the implications of deploying NAS solutions in large enterprise environments.
What is NAS? Essentially, NAS is a storage server with disk/tape drives that connects directly (via Ethernet, Gigabit Ethernet, FDDI, or ATM) to a LAN. The box is either used for backup and HSM services on the network or for file services such as NFS or CIFS.
And just how expensive is it? Each NAS server has its own embedded disk controller, dedicated operating system, processing engine, network interface, and disk/tape drives--sort of a scaled-down server on storage steroids.
Because LANs cannot accommodate high traffic, most NAS servers will be low capacity (roughly 100GB). To add capacity and relieve LAN congestion, more NAS boxes can be added to other segments, but at a cost. Additional controllers (the most expensive element in a disk subsystem) are necessary, there- by driving up the overall cost of incremental storage.
I also contend that NAS servers are inherently less reliable than storage servers attached to the SAN. Remember that a NAS server is at- tached to an asynchronous LAN, which is subject to broadcast storms and packet droppage. The NAS server is just another node on an already overcrowded packet network with email, print, Web, and other nonstorage traffic. In contrast, access to SAN-attached storage (SAS) is synchronous and arbitrated, not broadcast.
Furthermore, since we`re dealing with small-capacity NAS servers, which by nature will have to be inexpensive to become ubiquitous, the likelihood of enterprise-class reliability, availability, and serviceability is very low.
Also, are NAS solutions truly scalable? Since the LAN is a shared network, you will need to segment, or use LAN switches to segment, the network for maximum performance. You could attach NAS servers to each port of a Gigabit Ethernet switch; however, Ethernet/IP technology is not tuned for high-capacity storage applications. So, it`s unlikely that applications such as clustering and transaction processing will be run in this asynchronous and broadcast network environment. SAS, on the other hand, provides a clear alternative for inexpensive, highly reliable, and scalable storage.
In SAS environments, multiple servers can share common disk or tape susystems. The servers host such applications as backup and HSM as well as file services like NFS or CIFS, and they provide these services to clients in the LAN/ WAN environment. The storage behind these servers can be optimized for the lowest possible cost per megabyte. This is done with high-capacity disk subsys- tems: The cost of the controller is only incurred once and the incremental price per megabyte of data stored is tied to the price of each disk drive behind the controller.
SAS servers typically have storage capacities of 500GB to 1TB, compared to 100GB for NAS boxes. SAS storage is generally characterized by enterprise-class reliability, availability, and serviceability.
These systems are built with multiple power supplies, hot-swappable components, nondisruptive software loading, and phone-home servicing capabilities. In contrast, future low-cost NAS servers will likely lack hot swap and nondisruptive software updates and will require a box swap when failures occur.
SAS technology will also be scalable via switches. However, unlike NAS boxes, which use LAN switches, SAS servers will use Fibre Channel fabrics that provide synchronous data access in point-to-point or arbitrated network environments.
It may sound like I`m negative about NAS, but that`s not altogether true. There is a place for NAS boxes as alternatives to purchasing additional file servers or adding deskside disk pedestals. NAS will find a niche and coexist with SAS. The real question is whether a NAS box will be able to match the reliability, performance, scalability, and cost-effectiveness of a SAS solution for enterprise requirements.
Walt Hinton is vice president, product planning and development, at StorageTek`s network systems group in Minneapolis, MN.