Unit 8A
Charles Burchess
Kaplan University
IT332 Principles of Information Systems
January 10, 2014

Unit 8A Backing up user data is vital to the company. To do this a backup server would be the way to go. There are several ways to do this one would be to install a NAS or use a Server to make sure the data is backed up. A NAS unit is a computer connected to a network that provides only file-based data storage services to other devices on the network. Although it may technically be possible to run other software on a NAS unit, it is not designed to be a general purpose server. For example, NAS units usually do not have a keyboard or display, and are controlled and configured over the network, often using a browser. A full-featured operating system is not needed on a NAS device, so often a stripped-down operating system is used. For example, FreeNAS, an open source NAS solution designed for commodity PC hardware, is implemented as a stripped-down version of FreeBSD. NAS systems contain one or more hard drives, often arranged into logical, redundant storage containers or RAID. NAS uses file-based protocols such as NFS, SMB/CIFS, AFP, or NCP. NAS units rarely limit clients to a single protocol. NAS is useful for more than just general centralized storage provided to client computers in environments with large amounts of data. NAS can enable simpler and lower cost systems such as load-balancing and fault-tolerant email and web server systems by providing storage services. The potential emerging market for NAS is the consumer market where there is a large amount of multi-media data. Such consumer market appliances are now commonly available. Unlike their rackmounted counterparts, they are generally packaged in smaller form factors. The price of NAS appliances has plummeted in recent years, offering flexible network-based storage to the home consumer market for little more than the cost of a regular USB or FireWire external hard disk. Direct-attached storage (DAS) refers to a digital storage system directly attached to a server or workstation, without a storage network in between. It is a retronym, mainly used to differentiate non-networked storage from the concepts of storage area network (SAN) and network-attached storage (NAS). Most functions found in modern storage do not depend on whether the storage is attached directly to servers (DAS), or via a network (SAN and NAS). A DAS device can be shared between multiple computers, as long as it provides multiple interfaces (ports) that allow concurrent and direct access. This way it can be used in computer clusters. In fact, most SAN-attachable storage devices or NAS devices can be easily used as DAS devices – all that is needed is to disconnect their ports from the data network and connect one or more ports directly to a computer (with a plain one-to-one cable). More advanced DAS devices, like SAN and NAS, can offer fault-tolerant design in many areas: controller redundancy, cooling redundancy, and storage fault tolerance patterns known as RAID. Some DAS systems provide embedded disk array controllers to offload RAID processing from the server's host bus adaptor (HBA). Basic DAS devices do not have such features. A DAS can, like SAN or NAS, enable storage capacity extension, while keeping high data bandwidth and access rate. High-availability clusters are groups of computers that support server applications that can be reliably utilized with a minimum of down-time. They operate by harnessing redundant computers in groups or clusters that provide continued service when system components fail. Without clustering, if a server running a particular application crashes, the application will be unavailable until the crashed server is fixed. HA clustering remedies this situation by detecting hardware/software faults, and immediately restarting the application on another system without requiring administrative intervention, a process known as failover. As part of this process, clustering software may configure the node before starting the application on it. For example, appropriate file systems may need to be imported and mounted, network hardware may have to be configured, and some supporting applications may need to be running as well. HA clusters usually use a heartbeat private network connection which is used to monitor the health and status of each node in the cluster. One subtle but serious condition all clustering software must be able to handle is split-brain, which occurs when all of the private links go down simultaneously, but the cluster nodes are still running. If that happens, each node in the cluster may mistakenly decide that every other node has gone down and attempt to start services that other nodes are still running. Having duplicate instances of services may cause data corruption on the shared storage.
RAID is a data storage virtualization technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy or performance improvement. Data is distributed across the drives in one of several ways, referred to as RAID levels, depending on the specific level of redundancy and performance required. The different schemes or architectures are named by the word RAID followed by a number (e.g. RAID 0, RAID 1). Each scheme provides a different balance between the key goals: reliability, availability, performance, and capacity. RAID levels greater than RAID 0 provide protection against unrecoverable (sector) read errors, as well as whole disk failure. RAID 0 consists of striping, without mirroring or parity. The capacity of a RAID 0 volume is the sum of the capacities of the disks in the set, the same as with a spanned volume. There is no added redundancy for handling disk failures, just as with a spanned volume. Thus, failure of one disk causes the loss of the entire RAID 0 volume, with reduced possibilities of data recovery when compared to a broken spanned volume. Striping distributes the contents of files roughly equally among all disks in the set, which makes concurrent read or write operations on the multiple disks almost inevitable. The concurrent operations make the throughput of most read and write operations equal to the throughput of one disk multiplied by the number of disks. Increased throughput is the big benefit of RAID 0 versus spanned volume. RAID 1 consists of mirroring, without parity or striping. Data is written identically to two (or more) drives, thereby producing a "mirrored set". Thus, any read request can be serviced by any drive in the set. If a request is broadcast to every drive in the set, it can be serviced by the drive that accesses the data first, improving performance. Sustained read throughput, if the controller or software is optimized for it, approaches the sum of throughputs of every drive in the set, just as for RAID 0. Actual read throughput of most RAID 1 implementations is slower than the fastest drive. Write throughput is always slower because every drive must be updated, and the slowest drive limits the write performance. The array continues to operate as long as at least one drive is functioning. RAID 5 consists of block-level striping with distributed parity. Unlike in RAID 4, parity information is distributed among the drives. It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks. RAID 5 is seriously affected by the general trends regarding array rebuild time and chance of failure during rebuild. In August 2012, Dell posted an advisory against the use of RAID 5 in any configuration and of RAID 50 with “Class 2 7200 RPM drives of 1 TB and higher capacity” for business-critical data.

References
Englander. Irv. (2009). The Architecture of Computer Hardware and System Software: An Information Technology Approach, (4th ed.). Boston, MA: Wesley.