Storage 102: Storage Network Overview-SAN, NAS and Beyond


The Good Old Days
In a direct attached storage (DAS) model, storage is connected directly to the server (computer). In the good old days, when you needed more storage capacity, it was "relatively painless" to add, and the fact that storage was cheap didn't hurt either. DAS worked just fine since the amount of storage used in applications didn't amount to much, and the IT administrator was still able to manage the additional storage resources. I say relatively painless because in the 1980's and early 1990's, companies had barely scratched the surface to leverage computers to store important customer data, perform complex data mining or run mission-critical applications. All that has changed during the past decade as companies rely heavily on IT infrastructure to run their business and to sustain competitive advantages in their marketplace.

As more servers were added to run business-critical applications, the problems with DAS became more exaggerated. In a client-server architecture, storage is local to each server. Each storage device is connected to only one server and only that server can open files, read and write data, create backup tapes, and perform administrative functions on data stored on the device. If other servers need access to a server's local data, it would not be an easy task. As data growth grew rapidly in the mid-1990's, the maintenance became a nightmare for the IT administrator as each server with local storage became an island, requiring the IT administrator to manage each system individually.

Storage Networking Builds Momentum
With the explosive demand for more storage capacity and the strategic importance data took on in business critical applications, more and more companies are deploying storage networking over traditional DAS. There are two storage networking technologies: network attached storage (NAS) and storage area network (SAN)

NAS is widely deployed by both small organizations and large enterprises. We can safely say that NAS technology is embraced and will continue to thrive in the future. SAN is still an evolving technology and is typically deployed in the data center. Contrary to popular misconception, NAS and SAN are not competing. They solve different problems and can coexist in any organization's storage strategy. Both NAS and SAN share these common goals:

1. Enable sharing of files or storage from a pool of physical storage devices

2. Streamline and centralize IT administration of storage resources

3. Increase storage utilization

4. Lower storage cost of ownership

NAS Before and Now
The growth of NAS caught many people by surprise. Before NAS, there were NFS and CIFS file servers. Large data centers usually had many dedicated NFS (Unix) and CIFS (Windows) file servers, each serving thousands of users. Both have been around for years and are common in many organizations. However, there are a number of issues associated with managing NFS and CIFS.

The first problem with NFS and CIFS servers is that each is a dedicated server providing Unix file services or Windows file services, respectively. Each file server represents another island to manage, so the limitations of data sharing and inefficiencies of storage management are issues to contend with.

Secondly, few environments are strictly Unix or strictly Windows.-rather a mixture of both. In this mixed environment, companies find themselves in a quandary: Do they incur the extra cost of loading PC NFS software on their Windows desktops, or do they use Samba on their Unix boxes to provide CIFS services to their workstations? The result is CIFS data being served by a Windows machine, and NFS data by a Unix machine. Given the drastic differences between those two operating systems, each is usually managed by a different person, or an entirely different group of people.

For the record, NAS refers to dedicated storage appliances that provide file access to clients (Unix workstations and Windows PCs) using NFS and CIFS protocols transported on Ethernet and TCP/IP.

NAS appliances have evolved to the point that they offer all the advantages of NFS and CIFS file servers without any of the previously mentioned disadvantages. Companies are consolidating all their dedicated NFS and CIFS file servers under a single NAS engine. This results in fewer systems for IT administrators to manage. With one fell swoop, you eliminate all the disparate backups, patch upgrades and other administrative overhead. Today's NAS appliance allows Unix and Windows clients to store, access and share files under a single pool of storage. This makes it very simple and painless to add storage capacity when the time comes to upgrade. With enterprise class NAS appliances, multiple users can access identical files concurrently thereby eliminating multiple copies of files and enhancing storage utilization. In businesses where cross-functional groups must share data to complete a project, this is a significant advantage over older file systems. For example, an automotive manufacturer requires graphics designers, materials engineers, electrical engineers, procurement, and safety engineers to share designs on a car door for a new automobile.

Another area where NAS appliances have improved over traditional NFS/CIFS servers is in the area of high availability and scalability. Today's NAS appliances are RAID configurable, offer failover protection, provide snapshot capability, and can replicate data to another location in case of disaster recovery.

Using SANs To Maximize Your Storage
SANs certainly qualify as leading-edge technology, so it's no surprise that many people don't fully understand them yet. A SAN is a network of interconnected computers and storage devices. Just as a LAN allowed the introduction of a shared file system with NAS, SAN introduces the concept of a shared storage device. A shared storage device is a raw storage device (such as a disk, optical or tape drive) that is connected to a SAN and appears as a locally attached device to any computer connected to the SAN. To put it in simpler terms, connecting a tape drive to a SAN allows any computer attached to that SAN to perform a backup to that tape drive just as if that tape drive were physically attached to that computer via a SCSI cable.

SANs allow you to maximize your investment in both online and offline storage. Three things SANs make possible are online storage maximization, offline storage maximization and highly available systems

Online storage maximization allows you to create a pool of disk systems that can be allocated to the servers that need them. Because a SAN allows for multiple servers to access multiple physical disks via multiple physical paths, it also allows for highly available systems. Offline storage maximization means being able to configure your backup and recovery system in ways that you were never able to do before.

Online Storage Maximization
Many large data centers have thousands of discrete disks between all of their servers. There are a lot of disadvantages to this that can be overcome by SANs. When one disk fails, the server often has to be taken down in order to repair it. While buying RAID arrays with hot-swappable disks can solved this--and at least one hot spare for each array--allocating a hot spare for each array can be expensive. Also, you can't automatically use the spare from one server to fix a bad drive in another server.

Another manageability issue is space allocation. It's common to have one server starving for disk space, while another has disks to spare. Putting your disks behind a SAN can solve those problems. By putting all your disks on a large storage array behind the SAN, this array can have redundant power supplies, paths and disks. It can contain a small common pool of hot spare disks. This arrangement requires far fewer disks than having a hot spare for each system.

Perhaps the greatest benefit of consolidating your storage behind a SAN is dynamic space allocation. Disk drives can be allocated only to the servers that need more storage. If you find that you've allocated too much storage for a given server, simply de-allocate that storage from that server, and return it to the pool of storage that is available for servers that need it.

Offline Storage Maximization
LAN-free backups allow each server connected to the SAN to back up its entire data set directly to its own tape drive. It can do this without transferring the data via the LAN (local area network) and without the administrator to allocate one or more tape drives to its exclusive use.

LAN-free backups allow you to maximize the return on your investment in tape libraries. Instead of being forced to buy many smaller libraries and connect a few drives to each server, you can buy one or two really large tape libraries and dynamically allocate the tape drives to the servers that need them. By connecting your tape drives to a SAN, you also increase the availability of tape drives to all servers.

Highly Available Systems
Another area that has benefited quite a bit from the advent of SANs has been highly available systems. Since SANs allow for multiple paths from multiple servers to multiple physical devices, it's much easier to build a highly available system using SANs.