Alta Disponibilidade - NAS

Network-attached storage (NAS) é um dispositivo, conectado a rede, capaz de prover tanto as funcionalidades de storage quanto as de file-system, provendo acesso a dados para clientes de rede heterogêneos.

NAS hardware is similar to the traditional file server equipped with direct attached storage. However it differs considerably on the software side. The operating system and other software on the NAS unit provides only the functionality of data storage, data access and the management of these functionalities. Use of NAS devices for other purposes (like scientific computations or running database engine) is strongly discouraged. Many vendors also purposely make it hard to develop or install any third-party software on their NAS device by using closed source operating systems and protocol implementations. In other words, NAS devices are server appliances.
NAS units also usually have a web interface as opposed to monitor/keyboard/mouse.
Often minimal-functionality or stripped-down operating systems are used on NAS devices. For example FreeNAS, which is open source NAS software meant to be deployed on standard computer hardware, is in fact a "leaned-out" version of FreeBSD.
NAS systems usually contain one or more hard disks, often arranged into logical, redundant storage containers or RAIDs (redundant arrays of independent disks), as do traditional file servers. NAS removes the responsibility of file serving from other servers on the network.
NAS uses file-based protocols such as NFS (popular on UNIX systems) or SMB (Server Message Block) (used with MS Windows systems). NAS units rarely limit clients to only one protocol. NAS provides both storage and filesystem. This is often contrasted with SAN (Storage Area Network), which provides only block-based storage and leaves filesystem concerns on the "client" side.

SAN protocols are SCSI, Fibre Channel, iSCSI, ATA over Ethernet, or HyperSCSI.

The boundaries between NAS and SAN systems are also starting to overlap, with some products making the obvious next evolution and offering both file level protocols (NAS) and block level protocols (SAN) from the same system. However a SAN device is usually served through NAS as one large flat file, not as a filesystem per se. An excellent example of this is Openfiler the opensource product running on Linux.

Availability of data might potentially be increased with NAS if it provides built-in RAID and clustering. Performance can be increased by NAS because the file serving is done by the NAS and not done by a server responsible for also doing other processing. The performance of NAS devices, though, depends heavily on the speed of and traffic on the network and on the amount of cache memory (RAM) on the NAS computers or devices. It should be noted that NAS is effectively a server in itself, with all major components of a typical PC – a CPU, motherboard, RAM, etc. – and its reliability is a function of how well it is designed internally. A NAS without redundant data access paths, redundant controllers, redundant power supplies, is probably less reliable than DAS connected to a server which does have redundancy for its major components.

Due to the multiprotocol, and the reduced CPU and OS layer, the NAS has its limitations compared to the DAS/FC systems. If the NAS is occupied with too many users, too many I/O operations, or CPU processing power that is too demanding, the NAS reaches its limitations. A server system is easily upgraded by adding one or more servers into a cluster, so CPU power can be upgraded, while the NAS is limited to its own hardware, which is in most cases not upgradeable. NAS will also fail to expose well-known services that are typical of a file server, or enable them in a way that is not efficient. Examples are: ability to compute disk usage of separate directories, ability to index files rapidly (locate), ability to mirrorize efficiently with rsync. You may still rsync, but through a NFS client; that method fails to enumerate huge file hierarchies at the nominal speed of local drives and induces important network traffic. The key difference between DAS and NAS is that DAS is simply an extension to an existing server and is not networked while NAS sits on a network as its own entity; it is easier to share files with NAS. NAS typically has less CPU and I/O power compared to DAS.

Security when connected to Internet

NAS drives are generally safe to place on a network if connected to the Internet, however there are some security concerns that need to be addressed by administrators. Some NAS drives (like the LANdrive for example) are based on a Linux operating system and are open source. There are a numerous firmware upgrades on the Internet produced by various sources and NAS drive users are often tempted to upgrade their firmware to overcome problems and issues they have experienced. Inadvertently extra features may be added which can compromise the security of the NAS drive and network. Some NAS drives also have FTP servers built in.

NAS heads
A NAS head refers to a NAS which does not have any on-board storage, but instead connects to a SAN. In effect, it acts as a translator between the file-level NAS protocols (NFS,CIFS,etc.) and the block-level SAN protocols (Fibre Channel, iSCSI). Thus it can combine the advantages of both technologies. The term "NAS head" is sometimes also used to refer to the portion of a self-contained NAS system other than its storage. An example would be the ONStor Bobcat.
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