DAS, NAS and SAN

Three similar sounding acronyms to describe the three popular ways to connect data storage to servers.

DAS is traditional Direct Attached Storage, which is connected directly and usually exclusively to a general purpose file server and is accessed through the IP address of the hosting server.

Network-attached storage (NAS) is a combination of disk storage possibly in a RAID configuration, and operating system and software for configuration and file mapping. NAS also provides centralised file system functions. In other words, NAS delivers files to servers. NAS is attached to the local area network with its own IP address. So NAS is a device, not a network infrastructure, and shared storage is either internal to the NAS device or attached to it, it is not attached to the file servers.

SANs work on a private, usually fibre channel network and connect storage devices to servers with switches. SANs transfer data in fixed size blocks as the file systems reside on the servers.

It is possible to have a hybrid SAN/NAS system where an appliance delivers both blocks and files.

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NAS components

NAS Head or Gateway

The hardware that performs the NAS control functions is called a NAS head or NAS gateway. The clients always connect to the NAS head, as it is the NAS head is addressable on the network. A NAS head is usually a discrete hardware device that is independent of the storage devices and contains an imbedded operating system that does not need a keyboard, mouse or monitor. A storage administrator accesses the appliance and manages the disk resources from a remote console. Disks and in some cases tape drives are attached to the NAS head for capacity. NAS Heads are also sometimes called NAS appliances, based on the ideas that NAS is a commodity item like a toaster or washing machine.

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NAS file systems and protocols

The most popular NAS protocols are

  • NFS (Sun Network File System) which was developed for UNIX but also supports other systems
  • CIFS (Common Internet File System) which was developed Windows operating systems
  • HTTP (Hypertext Transfer Protocol) which supports web browsers and is commonly used for administrator interfaces

NAS devices support true file sharing between NFS and CIFS, which covers the majority of computers today. The NAS vendors can either implement these protocols with their own software, or use a freeware program like SAMBA.

NAS suppliers

Enterprise systems cost about $3,000 for 24TB. Some vendors are NetApp, Oracle, Seagate, QNAP, Synology

Home systems cost about $200 for a 4TB system. You connect to your wireless router so the data storage can be accessed by devices such as laptops, tablets and phones that you have defined in your local phone network. Some vendors are Synology, LaCie, Western Digital

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