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Network-attached storage boxes or filers need backing up like every other file server. If they use standard Windows or Unix/Linux file formats then existing backup applications, the mainstream ones, can be used by the NAS administrator. But if they use proprietary file formats then the backup app has a problem. It can't make sense of the filers' files.

In that case the backup application supplier has to write a new interface for its backup software. Why should they bother? They won't unless the likely sales are going to be large enough to make it worthwhile.

This was the problem facing NetApp with its proprietary WAFL - Write Anywhere File Layout. Customers wanted to use NetApp's filers without having to alter existing backup processes.

NDMP was invented by NetApp and PDC Software - which became part of Legato, now an EMC division - to solve this problem. It is an open protocol to be used by filers. There is an NDMP organisation charged with spreading the word.

This is what its web site says about the origin and benefit of NDMP:

"The objective of the protocol is to help address the problem of backing up networks of heterogeneous file servers, including dedicated file servers ("filers"), with any of several backup applications. Prior to the existence of the protocol, backup vendors would port to and track many different platforms and OS releases, with filers presenting a special challenge because of the desire to have them be backup-ready (eliminating the need to specially install backup client software). Dedicated file server vendors tried to make sure that all the newest, most important backup applications were available for their current and new releases."

"This network-based backup protocol enables the creation of a "universal agent" for the network- attached file servers to be used by any of the centralized backup administration applications. The filer vendors must only be concerned with maintaining compatibility with one, well-defined protocol. The backup vendors can place their primary focus on the sophisticated central backup administration software."

It's all pretty straightforward. Backup vendors are extending their applications to have NDMP support. For example, EMC's Legato has just enhanced NetWorker's support of NDMP to include disk as a target device for backups of filerrs. They could already be backed up to tape. Veritas' NetBackup also has this feature.

The NDMP Product Directory contains a list of all shipping NDMP-compliant products.

NDMP is specifically a filer format. A backup application takes this and then writes it to whatever tape format is specified by the backup app user. Or the NDMP data stream is written to tape as an NDMP data stream. The NDMP client has a NDMP tape and SCSI interface set that allow it to position tapes and interface to a library.

A white paper describing NDMP can be downloaded from here. There are several other white papers available.