The “Problem” with NAS
Introduction
Computer storage has evolved from Directly Attached (DAS) to Storage Area Networks (SAN). Along the way, Sun in 1984 invented NFS, and Network Area Storage (NAS) was born. Since then other NAS protocols have been added, most notably the Windows-based Server Message Block (SMB), aka CIFS. But throughout the history of storage, NAS has been regarded as poorly performing and unreliable compared to SAN and DAS. Certainly Auspex’s creation and NetApp’s advancement of NAS “appliances” helped move NAS from being a science project to a mainstream production solution, but in my opinion NAS is still under-appreciated and under-deployed. Perhaps in light of the new generation of NAS appliances, that should change.
At a more philosophical level, it’s worth asking “what is SAN” and “what is NAS.” Fundamentally, they are storage arrays that make disk space available via varying protocols over varying interconnect media. For the most part, both technologies are available with Fibre Channel (FC), SATA, and SAS disks. Both have disks of varying speeds, capacities, and performance. Traditionally, SANs have been FC connected and NAS appliances connected via Ethernet, but many current products provide both interconnects—block transactions occur via FC or iSCSI and file transactions over Ethernet. A proof point of this merger of NAS and SAN is the FCOE protocol which places Fibre Channel frames over Ethernet networks. Perhaps the most straightforward definition is that “SAN” is block-based storage and “NAS” is file storage, and that a given datacenter should choose which to use for any given application or function. After those decisions are made, it is easier to determine the best products to implement the resulting storage architecture. Now let’s consider the problem with NAS as well as the solutions it can provide.
The “Problem”
Over the years I’ve seen many, many computing infrastructures. Back in the “old days” (say, the 1980s), we had servers and SANs for production, and NAS was pushed to the side. It was typically used for home directories and the storage of utility programs, if at all. In those cases, NAS storage was mounted to all servers as well as all workstations.
That helped NAS gain a reputation for unreliability—probably because any failure caused everyone to notice it, and failures were difficult to recover from (with hard mounts never timing out, for example, taking down all computing until the NAS server could be fixed). Also, many situations called for “cross mounts,” where servers would mount each other’s directories via NFS. If one server then failed, all servers would eventually end up hanging until the failed one recovered. NFS also had quirks like “stale file handles” that left a bad taste in the mouth.
So failures of NFS servers were quite painful to the computing infrastructure. Why did NAS servers fail as often as they did? Well, they were non-clustered, while their SAN brethren typically had more redundant components and automatic recovery from problems. Originally, a “NAS server” was just a general-purpose Sun server running NFS. SAN originally and usually still is a purpose-built storage array. Also, they were and still are network- connected. Back in the day, there was typically one network connection to each workstation (and frequently between servers as well). That one link was used for NAS and non-NAS network traffic. Even if there was a separate network carved out for storage communication between the servers and NAS, it was rarely redundant. Multiple use and single points of failure meant NAS was more prone to failure than SAN. Thus the lingering impression that SAN is more reliable than NAS.
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