NAS: more than just an appliance


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TCP/IP accelerators - NAS takes a performance hit when it's attached to the Internet and uses the TCP/IP. Basically, these network protocols ensure the delivery of data by segmenting and reassembling the transmitted packets, as well as checking the packets for errors. The software to accomplish these tasks is implemented on the requesting server and in the NAS controller. These transmission and error checking tasks eat up many processor cycles and degrade performance. Recently, vendors have started to move some of the TCP and IP processing software out of the server and onto a host bus adapter - called a NIC, for Ethernet networking, which lets the processor do other useful work and provided a significant performance boost. Of course, the old NICs and drivers must be replaced, but the performance gains are well worth the effort and expense.

DAFS - DAFS is a new technology (see "

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How DAFS will change networked storage," April 2002) that speeds up access to storage over a network by using the virtual interface (VI) architecture rather than TCP/IP. It uses the remote direct memory access (RDMA) implementation of VI to transfer data directly between a NAS device and the application memory space or a layer of software to handle the VI if the application isn't VI-aware. By setting up a direct VI, the overhead of the operating system and the TCP and IP protocols is eliminated. To utilize DAFS, the NAS appliance must support DAFS and have a VI-capable NIC or HBA card installed along with the appropriate driver.

NAS with metadata serving - When NAS is combined with a metadata server, it provides the simplicity of file-level access with the performance of a SAN. Here's how it works: A metadata server is accessed with normal remote file system semantics such as NFS or CIFS, but that access is used to identify the location of the data on the SAN-attached storage and that information is returned to redirector software on the requesting server. The redirector software does the high-speed block I/O over the SAN. This is also termed a SAN-NAS convergence and from the user standpoint, it becomes transparent as to whether the data is being accessed with block I/O or file I/O. The administration would be different because the redirector software must be installed and the metadata server administered as a special purpose server, usually in a clustered environment or with an internal failover capability. Examples of this product include IBM's TotalStorage 300G with SANergy enabled, EMC's Celerra HighRoad or ADIC's CentraVision.

LDAP (lightweight directory access protocol) - To improve security, some vendors are implementing the LDAP, which provides authentication for access to files on a NAS appliance. In addition, some vendors are tying the security controls for CIFS access into the Active Directory. This technology is in its early stages for NAS usage.

Representative high-end NAS appliances

These new NAS technologies open up a wider range of applications that high-end NAS devices can now tackle. As a result, NAS has steadily worked its way into enterprise environments.
Capacity supported
12TB 64TB 28TB 22TB 9TB 17TB Depends on Sun server
Cache memory
(max per controller)
3GB 4GB 3GB 2GB 3GB 2GB Depends on Sun server
Connectivity types Gigabit, 10/100 Ethernet, ATM Gigabit, 10/100 Ethernet Gigabit, 10/100 Ethernet, ATM, FDDI Gigabit, 10/100 Ethernet Gigabit, 10/100 Ethernet, ATM Gigabit, 10/100 Ethernet Depends on Sun server
Failover (cluster models
have 2x capacity, cache, etc.)
Yes - NS3000XA model Yes - basic Yes - basic Yes - Model G26 Yes - F880C model Yes - 3600C model Clustered via Sun servers
File sharing Yes Yes Yes Yes Yes Yes Yes
Locking type implemented NFS style CIFS style NFS or CIFS - user's choice CIFS style NFS and CIFS styles NFS style NFS style
PIT copy Yes Yes Yes Yes Yes Yes Yes
Remote copy Yes Host-based Yes No Yes Yes Yes
NEW TECHNOLOGY              
DAFS No No No No No No No
TCP/IP accelerator Not standard Not standard Not standard Not standard Not standard Not standard Not standard
Metadata server option No No Yes (with HighRoad) Yes (with HighRoad) No No No
NAS Yes Yes Yes Yes No No No
Gateway option Yes Yes Yes Yes Yes No No
LDAP standard No No Yes No No Yes No
Other N/A Virtual software for file system expansion Up to 14 NAS controllers N/A Aggreg. software with Data Fabric Manager Allows block access to attached disks N/A

This was first published in July 2002

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