SAN Vs NAS

Storage solutions for your network: SAN vs. NAS

No matter how much storage you have, it never seems like enough. In a typical network, users store data either on a locally attached (SCSI, FireWire, USB) hard drive/array or store data on a centralized server. However, these solutions have performance and management related issues that make them inefficient and cumbersome leading system administrators to look for new solutions to this old problem. Network Attached Storage (NAS) and Storage Area Networks (SAN) are two solutions that promise to address these needs.

The problems with the old way

In the past when a user needed more storage space, the systems administrator may have simply added a second drive, or for a high-end user added a storage array. In environments where there was already an established infrastructure of centralized servers, adding more space means adding more disks to the server. These solutions have problems though.

When we add more storage to a local desktop, backups become more complicated. All essential data must be backed up on a regular basis to protect against data loss. If we have numerous machines on the network, each with 10s or 100s of GB of storage, backing up the data over the network is no small task. If we look at the most popular backup package on the Mac platform, Retrospect, we realize that it can only backup one client machine at a time. This means that we may have to deploy a large number of servers in order to service a small number of clients. Each server can only use locally attached tape drives to store the backup data, increasing the overall cost and management complexity of the backup solution. Since the relationship between clients and servers is essentially hard-coded for a backup run, if one server finishes, the locally attached tape drives on that server sit idle while another server is struggling to complete its backup script.

Another issue associated with locally attached storage is the inability to allocate space dynamically. If storage needs fluctuate based on project demands, an administrator may want to move a storage array or disk between users. Doing this with locally attached storage is cumbersome because it means some downtime for both users. Using a centralized server pool minimizes the problem slightly, but again moving disks between servers requires downtime that may now affect hundreds of users.

Finally, with locally attached storage the network becomes a bottleneck. With backups running across the network and users sharing large files by sending them across the network, the network quickly becomes overloaded at various times during the day. Network protocols (TCP/IP) are more efficient than in the past, but still not as efficient as storage protocols such as SCSI. TCP/IP has packet size limits that ultimately affect how fast large files can be streamed across the network.

**** NAS ****

The idea behind NAS is to optimize the traditional client/server network model. In a NAS environment, administrators deploy dedicated servers that perform only one function - serving files. These servers generally run some form of a highly optimized embedded OS designed for file sharing. The disks are locally attached to the NAS box, usually with high-performance SCSI, and clients connect to the NAS server just like a regular file server.

NAS servers often support multiple protocols - AppleTalk, SMB, and NFS - to make sharing files across platforms easier. Some NAS servers also allow for directly attached tape drives for local backups, but more often a centralized backup server is used, requiring that data be sent across the LAN. Since backups only occur between the NAS servers and the backup server though, many administrators opt to install a secondary LAN dedicated to backups (this requires the NAS server to support multiple NICs and multi-homing, a common function).

There are a lot of companies that make NAS solutions that support the Mac. The leader on the low-end is Quantum with their SNAP line of servers. With Mac OS X's support of NFS, the number of NAS solutions available to Mac users now includes market leaders like Network Appliance, Auspex, and Sun to name a few.

**** SAN ****

The idea behind a SAN is radically different that NAS. To begin with, different protocols are used. In most SAN implementations, Fibre-channel (FC) adapters provide physical connectivity between servers, disk arrays, and tape libraries. These adapters support transfer rates up to 1 GB/s (along with trunking available for faster connections) and generally use Fiber cabling to extend distances up to 10km. Fibre-channel uses the SCSI command set to handle communications between the computer and the disks.

A SAN essentially becomes a secondary LAN, dedicated to interconnecting computers and storage devices. To implement a SAN, the administrator installs a FC card in each computer that will participate in the SAN (desktop or server). The computer then connects to a Fibre-Channel switch (hub solutions are also available). The administrator also connects the storage arrays and tape libraries to the FC switch (converters available for SCSI arrays). To improve redundancy, a second FC card can be installed in each device and a fully meshed FC switch fabric built. The final step is installing any necessary software components for managing the SAN and allocating storage pools.

We now have a second network that allows all computers to communicate directly with all the disks and tape drives as if they were locally attached. If a computer needs more storage, the administrator simply allocates a disk or volume from a storage array. This also improves backup flexibility because tape drives can be dynamically allocated to servers as needed, ensuring efficient use of resources.

The SAN represents a second network that supplements your existing LAN. The advantage of a SAN is that SCSI is optimized for transferring large chunks of data across a reliable connection. Having a second network also off-loads much of the traffic from the LAN, freeing up capacity for other uses. The most significant effective is that backups no longer travel over the regular LAN and thus have no impact on network performance.

Since the disk arrays are no longer locally attached, we can also implement more advanced data recovery solutions. Mirroring is a popular technique for protecting against disk failure. Now we can mirror between two disk arrays located in two different locations. If one array dies, the server immediately switches to the remote mirror. If a server dies, we can bring a new server online, attach it to the FC fabric, and allocate the disks to it from the array. Server back online without moving the storage array.

In a typical SAN, each server or desktop is allocated a set of disks from the central pool and no other computer can access those disks. If the administrator wants to shuffle space, they take the disks away from one computer and assign them to another. Adding more disks is as simple as adding more disks to the array or adding another array and allocating the disks to the server. Recent software advances though have made sharing of filesystems a reality. Now two or more computers can access the same files on the same set of disks, with the software acting as the traffic cop to guard against corruption. This allows for even more efficient use of space because users no longer maintain duplicate data. This also improves the ability to build clusters or other fault-tolerant systems to support 24x7 operations.

The next generation of SAN products promise to move storage traffic back to traditional network protocols like TCP/IP. Why do this when FC and SCSI are more efficient for moving large chunks of data? Well, FC switches are expensive compared to Ethernet switches and while FC has a performance advantage today, 10 Gigabit Ethernet will allow TCP/IP to surpass FC in overall transfer speed despite the higher overhead in transmitting data. The other advantage is that Ethernet and TCP/IP are well understood by most administrators and a little easier to troubleshoot than FC. Most administrators will still build a second LAN for storage needs, but switch to more standard protocols.

Unfortunately, the SAN market for the Mac is not quite as broad as on other platforms, but there are a number of good solutions available. These include arrays and cards from ATTO, AC & NC, Medea, Rorke Data, CharisMac, and 3ware. The biggest stumbling block for Mac users is the lack of support from FC adapter vendors. Emulex and JNI are the standards in the industry, yet they still don't offer direct Mac support.

Both NAS and SAN solutions offer a lot of potential for solving the problems associated with traditional storage solutions. Both solutions offer administrators new options in building high-performance, high-availability networks. Interoperability is often the most difficult issue to overcome when implementing a SAN, so evaluate complete solutions to ensure that you are successful in your endeavors.