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Using NAS for virtual machines

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Common wisdom says you need block storage for virtual servers, but with most hypervisors supporting the NFS protocol, NAS may work just as well.

By Eric Siebert

Shared storage is a requisite for virtualized servers if you want to use any of the advanced features server virtualization offers, such as high availability or the ability to move a running virtual machine (VM) from one host to another. This typically meant you had to invest in an expensive Fibre Channel SAN (FC SAN). But all server virtualization products also support using network-attached storage (NAS) devices, which can provide a worthy, cost-effective alternative to FC SANs for shared storage.

Another alternative is iSCSI storage which, like NAS, uses TCP/IP over a standard Ethernet network, but iSCSI is block storage like Fibre Channel and tends to be costlier than NAS. NAS systems generally support both NFS and CIFS file-sharing protocols, but server virtualization products prefer -- or are limited to -- NFS.

Inside NFS

NFS was developed in the late 1980s and has been revised several times over the years; NFS Version 4 (NFSv4) is the most recent version. The NFS architecture consists mainly of three components:

• Remote procedure calls (RPCs)
• External data representation (XDR)
• NFS procedures

The NFS protocol uses an RPC system that allows a server (NFS client) to make a call that's executed on another

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server (NFS server). XDR is the data encoding standard for NFS and serves as the universal language used between clients and servers. NFS procedures are software instructions used to perform storage-related tasks.

An NFS server may be a dedicated NAS appliance such as those sold by Net-App and all major storage vendors, or it can be a common server running an operating system. NFS is commonly used in Unix and Linux systems, but is also available with other operating systems such as Windows. NFS is a stateless protocol, which means the server doesn't store any client information and each RPC event contains all the necessary information to complete the call. In this manner, no open connections between the client and server exist, and crash recovery is as simple as having the client resend requests until the server responds.

This was first published in February 2011

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