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Network-attached storage (NAS) is file-level computer data storage connected to a computer network providing data access to a heterogeneous group of clients.NAS differs from the traditional file serving and Direct Attached Storage in that the operating system and other software on the NAS unit provide only the functionality of data storage, data access and the management of these functionalities.

Benefits to NAS (Network-attached storage)

Availability of data can potentially be increased with NAS because data access is not dependent on a server: the server can be down and users will still have access to data on the NAS. 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 (the equivalent of RAM) on the NAS computers or devices. Scalability of NAS is not limited by the number of internal or external ports of a server's data bus, as a NAS device can be connected to any available network jack. NAS can be more reliable than DAS because it separates the storage from the server. If the server fails, there is unlikely to be file system corruption, although partially-created files may linger. However, if the power source or OS of the NAS fails, corruption is still possible.

What is RAID?

RAID (Redundant Array of Independent Disks) is a set of technology standards for teaming disk drives to improve fault tolerance and performance.

RAID Levels

Level Name
0 Striping
1 Mirroring
2 Parallel Access with Specialized Disks
3 Synchronous Access with Dedicated Parity Disk
4 Independent Access with Dedicated Parity Disk
5 Independent Access with Distributed Parity
6 Independent Access with Double Parity

Choosing a RAID Level

Each RAID level represents a set of trade-offs between performance, redundancy, and cost.

RAID 0 -- Optimized for Performance

RAID 0 uses striping to write data across multiple drives simultaneously. This means that when you write a 5GB file across 5 drives, 1GB of data is written to each drive. Parallel reading of data from multiple drives can have a significant positive impact on performance.

The trade-off with RAID 0 is that if one of those drives fail, all of your data is lost and you must retore from backup.

RAID 0 is an excellent choice for cache servers, where the actual data being stored is of little value, but performance is very important.

RAID 1 -- Optimized for Redundancy

RAID 1 uses mirroring to write data to multiple drives. This means that when you write a file, the file is actually written to multiple disks. If one of the disks fails, you simply replace it and rebuild the mirror.

The tradeoff with RAID 1 is cost. With RAID 1, you must purchase double the amount of storage space that your data requires.

RAID 5 -- A Good Compromise

RAID 5 stripes data across multiple disks. RAID 5, however, adds a parity check bit to the data. This slightly reduces available disk capacity, but it also means that the RAID array continues to function if a single disk fails. In the event of a disk failure, you simply replace the failed disk and keep going.

The tradeoffs with RAID 5 are a small performance penalty in write operations and a slight decrease in usabable storage space.

RAID 6 -- Better Redundancy

RAID 6 stripes data across multiple disks. RAID 5, however, adds double parity check bits to the data. This slightly reduces available disk capacity, but it also means that the RAID array continues to function if a two disk fails. In the event of two disks failure, you simply replace the failed disks and keep going.

RAID 1+0 -- Optimize for Performance and Redundancy

RAID 1+0 combines the performance of RAID 0 with the redundancy of RAID 1.

To build a RAID 1+0 array, you first build a set of RAID 1 mirrored disks and you then combine these disk sets in a RAID 0 striped array.

A RAID 1+0 array can survive the loss of one disk from each mirrored pair. RAID 1+0 cannot survive the loss of two disks in the same mirrored pair.

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