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Backup Procedures. Many organizations follow traditional file backup procedures that can be implemented across the LAN. Some of these procedures include performing file backups at night — full backups if possible, incremental backups otherwise. Archival backups of all disk drives are typically done at least monthly; multiple daily saves of critical databases may be warranted in some cases. The more data users already have stored on their hard disks, the longer it takes to save. For this reason, LAN managers encourage users to offload unneeded files and consolidate file fragments with utility software to conserve disk space, as well as to improve overall system performance during backups. Some LAN managers have installed automatic archiving facilities that move files from users’ hard disks to a backup database if they have not been opened in the past 90 days.

Retrieving files from archival storage is typically not an easy matter; users forget file names, the date the file was backed up, or in which directory the file was originally stored. In the future, users can expect to see intelligent file backup servers that permit files to be identified by textual content. Graphics files, too, are retrieved without having the name, backup date, or location of the file. In this case, the intelligent file backup system compares the files with bit patterns from a sample graphic with the bit patterns of archived files to locate the right file for retrieval.

As the amount of stored information increases, there is the need for LAN backup systems that address such strategic concerns as tape administration, disaster recovery, and the automatic movement of files up and down a hierarchy of network storage devices. Such capabilities are currently available and are referred to as system storage management or hierarchical storage management.

Levels of Fault Tolerance

Protecting data at the server has become a critical concern for most network managers; after all, a failure at the server can result in lost or destroyed data. Considering that some servers are capable of holding vast quantities of data in the gigabyte range, loss or damage can have disastrous consequences for an information-intensive organization.

Depending on the level of fault tolerance desired and the price the organization is willing to pay, the server may be configured in several ways: unmirrored, mirrored, or duplexed.

Unmirrored Servers. An unmirrored server configuration entails the use of one disk drive and one disk channel, which includes the controller, a power supply, and interface cabling, as shown in Exhibit 2. This is the basic configuration of most servers. The advantage is chiefly one of cost: the user pays only for one disk and disk channel. The disadvantage of this configuration is that a failure in either the drive or anywhere on the disk channel could cause temporary or permanent loss of the stored data.

Mirrored Servers. The mirrored server configuration entails the use of two hard disks of similar size. There is also a single disk channel over which the two disks can be mirrored together, as shown in Exhibit 3. In this configuration, all data written to one disk is then automatically copied onto the other disk. If one of the disks fails, the other takes over, thus protecting the data and ensuring all users have access to the data. The server’s operation system issues an alarm notifying the network manager that one of the mirrored disks is in need of replacement.

The disadvantage of this configuration is that both disks use the same channel and controller. If a failure occurs on the channel or controller, both disks become inoperative. Because the same disk channel and controller are shared, the writes to the disks must be performed sequentially — that is, after the write is made to one disk, a write is made to the other disk. This can degrade overall server performance under heavy loads.

Disk Duplexing. In disk duplexing, multiple disk drives are installed with separate disk channels for each set of drives, as shown in Exhibit 4, malfunction occurs anywhere along a disk channel, normal operation continues on the remaining channel and drives. Because each disk uses a separate disk channel, write operations are performed simultaneously, offering a performance advantage over servers using disk mirroring.

Disk duplexing also offers a performance advantage in read operations. Read requests are given to both drives. The drive that is closest to the information responds and answers the request. The second request given to the other drive is canceled. In addition, the duplexed disks share multiple read requests for concurrent access.


Exhibit 2.  Unmirrored Disk Drive Configuration.

The disadvantage of disk duplexing is the extra cost for multiple disk drives, also required for disk mirroring, as well as for the additional disk channels and controller hardware. However, the added cost for these components must be weighed against the replacement cost of lost information plus costs that accrue from the interruption of critical operations and lost business opportunities. Faced with these consequences, an organization might discover that the investment of a few hundred or even a few thousand dollars to safeguard valuable data is negligible.

REDUNDANT ARRAYS OF INEXPENSIVE DISKS

One method of data protection is growing in popularity: redundant arrays of inexpensive disks (RAID). Instead of risking all of its data on one high-capacity disk, the organization distributes the data across multiple smaller disks, offering protection from a crash that could wipe out all data on a single, shared disk. Exhibit 5 illustrates redundant arrays of inexpensive disks. Other benefits of RAID include:

  Increased storage capacity per logical disk volume.
  High data transfer or input/output rates that improve information throughput.
  Lower cost per megabyte of storage.
  Improved use of data center floor space.


Exhibit 3.  Configuration for Disk Mirroring.


Exhibit 4.  Disk Duplexing Configuration.


Exhibit 5.  Redundant Arrays of Inexpensive Disks.


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