Motors & Drives Information

n other words, you have sent a request for the web page through the web browser. This request is answered by the web server, by sending the required page to the web browser, which in turn displays it.

The web server works thus; there are many other types of servers that are used commonly.

Types of Servers

Probably, the simplest of servers are the file servers. You can send your request for a file or a file record, over the network to the file server. (File servers require a large bandwidth.) Another equally simple server is the disk server.

A comparatively more advanced version is a database server. In a database server, the user passes on a message to the server (in SQL - 'structure query language') and the results are returned over the network. The data resides on the server, and enables the code that processes the request to find the requested data. The client is not passed on all the files to scan for the required information.

The other server is the transaction server, which contains a SQL data base engine. The procedural statements on the servers either execute the entire SQL transactions or they fail it as a group. As the applications on the transaction servers are mission critical they necessitate an enhanced level of security and integrity of the database. The exchange usually consists of a single request and reply.

A server that is much commonly used unlike the transaction server is the e-mail server. This receives mail from clients and other email servers. The mail server consists of a storage area, a list of users, a series of communication modules and a set of rules defined by the user. The storage area, which is a database of information, consists of the received mail and those in transit. The user defined rules determine the reaction of the mail server to specific messages, for example, only certain e-mails can be sent to a company while the rest get blocked. The communication modules (their number is determined by the mail server) handle the transfer of messages to and fro the email clients.

Servers usually function without manual intervention and they are expected to work almost constantly, therefore, RAID is used.

RAID (Redundant Array of Independent Drives)

This is also known as 'redundant array of inexpensive disks.' This term defines a computer data storage scheme that divides and also replicates data among varied hard disks. In a RAID, the different physical hard disks are combined into a single logical unit by using a special hardware or software.

The three key RAID concepts are:

· Mirroring - that of copying data on to different disks in the array

· Striping - the splitting of data in more than one disk, and finally

· Error correction - redundant data is stored for error detection if the need arises

RAID systems are designed to work reliably; therefore, they are ideally suited for servers. However, at times, RAID array also fail and data recovery from a (RAID) server is a complicated process.

Data Recovery from a (RAID) Server

In event of data loss, an initial diagnosis is performed which determines whether each device is accessible. If the media is accessible, a raw image is created of all the data onto a new device so that the nature of information loss can be ascertained. At times, some devices may be inaccessible; these are closely tested to assess the extent of physical damage. Data recovery from a crashed RAID usually involves the replacement of the damaged components like the read/ write head, the drive motors, etc. Specialised tools are used to recover the lost data.

Servers with multiple drives are de-striped on to a device to enable performance of file system repairs and to extract the data files.

Data loss usually induces panic. Losing data from a RAID server necessitates an important precaution. The disk array order should not be changed. Even if they are removed from the server, they should be placed back in the same order and the data recovery experts must be contacted.