Conference Report: 2000 USENIX: Invited Talks: The Convergence of Networking and Storage: Will it Be SAN or NAS?

by Rod Van Meter

The goal of this talk was to provide models for thinking about SANs and NASes. Network-attached storage (NAS) is like NFS on the LAN; storage area networks (SAN) are like a bunch of Fibre Channel-attached disks.

The goal of people is to share their data. There are several patterns, such as 1-to-many users, 1-to-many locations, time slices, and fault tolerance; activities, such as read only, read-write, multiple simultaneous reads, and multiple simultaneous writes; and multiple ranges, of machines, CPUs, LAN versus WAN, and known versus unknown clients.

When sharing data over the network, how should you think about it? There are 19 principles that Levy and Silverschatz came up with that describe the file. These include the naming scheme, component unit, user mobility, availability, scalability, networking, performance, security, and so on. There is Garth Gibson's taxonomy of four cases: server-attached disks, like a Solaris machine; server integrated disks, like a Network Appliance machine; netSCSI, or SCSI disks shared across many hosts with one "trusted" host to do the writes, and network-attached secure devices (NASD). Over time, devices are evolving to become more and more network-attached, smarter, and programmable.

Rod went into several areas in more detail. Access models can be application specific (like databases or http), file-by-file (like most Unix file systems), logical blocks (like SCSI or IDE disks, or object-based (like NASD). Connections can be over any sort of transport, including Ethernet, HiPPI, Fibre Channel, ATM, SCSI, and more. Each connection model is at the physical and link layers and assumes there is a transport layer (such as TCP/IP), though other transport protocols are possible (like ST or XTP or UMTP). The issues of concurrency (are locks mandatory or advisory, is management centralized or distributed), security (authorization and authentication, data integrity, privacy, and nonrepudiation), and network ("it doesn't matter" versus "it's all that matters") all need to be considered.

Given all those issues, there are three major classes of solutions today. The first is a distributed file system (DFS), also known as NAS. This model is a lot of computers and lots of data; examples include NFS v2, AFS, Sprite, CIFS, and xFS. The bottleneck with these systems is the file manager or object store; drawbacks include the nonprogrammability of these devices and the fact that they are OS-specific and have redundant functionality (performing the same steps different times in different layers).

The second class of solution is storage area networks (SAN). These tend to have few computers and lots of data and tend to be performance critical. These are usually contained in a single server or machine room; the machines tend to have separate data and control networks. These devices' drawbacks are that they are neither programmable nor smart, they're too new to work well, they provide poor support for heterogeniety, and the scalability is questionable. However, there is a very low error rate and the application layer can perform data recovery. Examples of SANs include VAX clusters, NT clusters, CXFS from SGI, GFS, and SANergy.

The third solution class is NASD, developed at CMU. The devices themselves are more intelligent and perform their own file management. Clients have an NFS-like access model; disk drives enforce (but do not define) security policies. The problems with NASD is that it's too new to have reliable details, more invention is necessary, there are some OS dependencies, and some added functionality may be duplicated in different layers. Which solution is right for you? That depends on your organization's needs and priorities.

Slides from this talk will be made available through http://www.usenix.org/ shortly after the conference.

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