Today's personal computers (PCs) usually have hard disk storage built directly into the computer's casing, where they store their directories and files. This is referred to as local storage, which is usually only available to users who work directly on the computer. If a hard disk storage with the files stored on it is to be available network-wide, i.e. accessible from all computers in the network, the storage system must first be directly connected to the network and must also be able to handle transfer protocols that make the files on this storage space available in the connected computer network.
This functionality is provided by file-based network protocols such as SMB and NFS or block-based network protocols such as iSCSI and FCoE. One then speaks of a "storage connected to a network", i.e. Network Attached Storage or NAS for short. NAS systems are therefore connected directly to the network and work autonomously, i.e. without requiring a dedicated PC or server.
The file systems of the NAS, i.e. all files and directories created there, appear on the target system like a mounted share or a local file system. NAS systems in the narrower sense are server services that make ready-to-use file systems available to the clients connected via a network service, depending on the operating system. This distinguishes the service from Direct Attached Storage (DAS) and Storage Area Network (SAN). NAS systems in the broader sense, as they are offered in practice, provide central storage on disk arrays with both file-based and block-based access via the general network. The general network is usually the Ethernet-based LAN, increasingly also WLAN.
A NAS generally provides file server functions. This is understood to mean user access to files via a local network. Especially when used in a professional environment, the systems must be able to take into account access rights (ACL) for users registered in the network (data protection).
A common occurrence is personal data that is only accessible to one user or group data that is accessible to groups of several users. A NAS thus provides far more functions than just allocating storage to a computer via the network. Therefore, in contrast to direct attached storage, a NAS is always either a stand-alone computer (host) or a virtual computer (virtual storage appliance, or VSA) with its own operating system.
Many systems have RAID functions to prevent data loss due to defects and/or to increase transfer speeds. File-based services such as NFS or SMB/CIFS are the core function. Some more comprehensive NAS implementations such as FreeNAS, OpenMediaVault or Openfiler additionally handle block-based data access, as is common with DAS or SAN, and offer an iSCSI implementation for this.
This variety of functions is often summarised in the term filer. For use in small home and so-called SoHo networks, Network Direct Attached Storage has also been developed with additional functions that go beyond the term filer, for example print servers. However, some of these functions are limited to small networks with few computers, because depending on the system, special device drivers are required on each connected computer.
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