Fault Tolerance and Back Ups

Developing and Implementing a Backup and Recovery System

• Be the guardian of the information stored on the local area network system

• Develop system specifications that will provide a reliable and fault-tolerant system

• Plan for disasters by implementing a backup and recovery system using NOS utilities

RAID - Short for Redundant Array of Independent (or Inexpensive) Disks, a category of disk drives that employ two or more drives in combination for fault tolerance and performance.

RAID can be implemented either in dedicated hardware or custom software running on standard hardware. Additionally, there are hybrid RAIDs that are partly software- and partly hardware-based solutions.

With a software implementation, the operating system manages the disks of the array through the normal drive controller. With present CPU speeds, software RAID can be faster than hardware RAID, though at the cost of using CPU power which might be best used for other tasks

A hardware implementation of RAID requires at a minimum a special-purpose RAID controller. On a desktop system, this may be a PCI expansion card, or might be a capability built in to the motherboard. In larger RAIDs, the controller and disks are usually housed in an external multi-bay enclosure.

Both hardware and software versions may support the use of a hot spare, a preinstalled drive which is used to immediately (and almost always automatically) replace a failed drive. This reduces the mean time to repair period during which a second drive failure in the same RAID redundancy group can result in loss of data.

RAID disk drives are used frequently on servers but aren't generally necessary for personal computers.

Level 0 -- Striped Disk Array without Fault Tolerance: Provides data striping (spreading out blocks of each file across multiple disk drives) but no redundancy. This improves performance but does not deliver fault tolerance. If one drive fails then all data in the array is lost.

Level 1 -- Mirroring and Duplexing: Provides disk mirroring. Level 1 provides twice the read transaction rate of single disks and the same write transaction rate as single disks.

mirroring - RAID 1: Duplicates a partition on another physical disk with one data channel, 2 drives, 1 used for data, 1 for parity, advantages are fault tolerance; disadvantages are it's expensive and requires twice the disk space

duplexing - RAID 1: Duplicates a partition on another physical disk that is connected to another Hard Drive Controller using two data channels simultaneously, two data cables and two DASD, 1 used for data, 1 for parity, advantages and disadvantages are the same as mirroring but duplexing provides much faster read speeds than mirroring.

Level 5 -- Block Interleaved Distributed Parity: Provides data striping at the byte level and also stripe error correction information. This results in excellent performance and good fault tolerance. Level 5 is one of the most popular implementations of RAID.

striping (with and without parity) - data striping is when blocks or bits of data are written to each drive in the array in succession. It's used in most RAID levels and is great for improving read/write speeds because the I/O request are being distributed between all I/O data channels. Parity checking relies on an extra bit called a parity bit, which is used to compare the bit string to an odd or even count. If the odd or even count is not matched based on the setting of the parity bit, then the data string is sent again. Extra drive space is used for the parity bits. Not using parity will improve overall data transmission because of the omission of the parity bit calculation, but should be used when speed is of greater importance than fault tolerance. RAID 5 provides the best fault tolerance because it uses several drives with block interleaving, a distributed check sum for parity and has fast reads.

Data backup -

A good data backup solution depends on the amount of data generated and how often it changes. Daily backups may suffice for small companies, whereas larger companies that produce a high volume of data may need to back up data several times per day, or on a continuous basis. Tape or disk, or a combination of the two, can be used for backup.

tape backup - offline storage and is easily removable, slow read/write compared to hard disk, high capacity on magnetic tape, excellent choice for fault tolerance because it's cheap and the media can be sent elsewhere for protection.

Backup Strategies

Full - copies all files and marks them as being backed up.

Incremental - copies only files created or changed since last full backup and marks them as being backed up.

Differential - copies only files created or changed since last full backup and doesn’t mark them as being backed up.

volumes - are active segments of a physical server hard drive which may be fully contained in a single hard drive, spanned over several disks or multiple volumes can occupy one hard drive.

Another option is a backup service that allows you to back up your data to a remote location across the Internet.

An alternative for off-site backup storage is to physically move a copy of each day's backup to another location.

For more information on Networking Basics, the OSI Model and Server Administration:
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