STORAGE AREA NETWORK 12SCS322 Unit 1 VTU

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STORAGE AREA NETWORK [12SCS322] Unit-1 VTU

1 INTRODUCTION 1.1 SERVERS-CENTRIC IT ARCHITECTURE AND ITS LIMITATIONS


In conventional IT architectures, storage devices are normally only connected to a single server. To increase fault tolerance, storage devices are sometimes connected to two servers, with only one server actually able to use the storage device at any one time. In both cases, the storage device exists only in relation to the server to which it is connected. Other servers cannot directly access the data; they always have to go through the server that is connected to the storage device. This conventional IT architecture is therefore called server-centric IT architecture. In this approach, servers and storage devices are generally connected together by SCSI cables.

As mentioned above, in conventional server-centric IT architecture storage devices exist only in relation to the one or two servers to which they are connected. The failure of both of these computers would make it impossible to access this data. Most companies find this unacceptable: at least some of the company data (for example, patient files, websites) must be available around the clock.


Although the storage density of hard disks and tapes is increasing all the time due to ongoing technical development, the need for installed storage is increasing even faster. Consequently, it is necessary to connect ever more storage devices to a computer. This throws up the problem that each computer can accommodate only a limited number of I/O cards (for example, SCSI cards). Furthermore, the length of SCSI cables is limited to a maximum of 25 m. This means that the storage capacity that can be connected to a computer using conventional technologies is limited. Conventional technologies are therefore no longer sufficient to satisfy the growing demand for storage capacity. In server-centric IT environments the storage device is statically assigned to the computer to which it is connected. In general, a computer cannot access storage devices that are connected to a different computer. This means that if a computer requires more storage space than is connected to it, it is no help whatsoever that another computer still has attached storage space, which is not currently used
Last, but not least, storage devices are often scattered throughout an entire building or branch. Sometimes this is because new computers are set up all over the campus without any great consideration and then upgraded repeatedly. Alternatively, computers may be consciously set up
where the user accesses the data in order to reduce LAN data traffic. The result is that the storage devices are distributed throughout many rooms, which are neither protected against unauthorised access nor sufficiently air-conditioned. This may sound over the top, but many system administrators could write a book about replacing defective hard disks that are scattered all over the country.
Last, but not least, storage devices are often scattered throughout an entire building or branch. Sometimes this is because new computers are set up all over the campus without any great consideration and then upgraded repeatedly. Alternatively, computers may be consciously set up
where the user accesses the data in order to reduce LAN data traffic. The result is that the storage devices are distributed throughout many rooms, which are neither protected against unauthorised access nor sufficiently air-conditioned. This may sound over the top, but many system administrators could write a book about replacing defective hard disks that are scattered all over the country.

1.2 STORAGE-CENTRIC IT ARCHITECTURE AND ITS ADVANTAGES

Storage networks can solve the problems of server-centric IT architecture that we have just discussed. Furthermore, storage networks open up new possibilities for data management. The idea behind storage networks is that the SCSI cable is replaced by a network that is installed in
addition to the existing LAN and is primarily used for data exchange between computers and storage devices
In contrast to server-centric IT architecture, in storage networks storage devices exist completely independently of any computer. Several servers can access the same storage device directly over the storage network without another server having to be involved. Storage devices are thus placed at the centre of the IT architecture; servers, on the other hand, become an appendage of the storage devices that ‗just process data. IT architectures with storage networks are therefore known as storage-centric IT architectures.
When a storage network is introduced, the storage devices are usually also consolidated. This involves replacing the many small hard disks attached to the computers with a large disk subsystem. Disk subsystems currently (in the year 2009) have a maximum storage capacity of up to a petabyte. The storage network permits all computers to access the disk subsystem and share it. Free storage capacity can thus be flexibly assigned to the computer that needs it at the time. In the same manner, many small tape libraries can be replaced by one big one.
More and more companies are converting their IT systems to a storage-centric IT architecture. It has now become a permanent component of large data centres and the IT systems of large companies. In our experience, more and more medium-sized companies and public institutions are now considering storage networks. Even today, most storage capacity is no longer fitted into the case of a server (internal storage device), but has its own case (external storage device).

1.3 CASE STUDY: REPLACING A SERVER WITH STORAGE NETWORKS

In the following we will illustrate some advantages of storage-centric IT architecture using a case study: in a production environment an application server is no longer powerful enough. The ageing computer must be replaced by a higher-performance device. Whereas such a measure can be very complicated in a conventional, server-centric IT architecture, it can be carried out very elegantly in a storage network.
1. Before the exchange, the old computer is connected to a storage device via the storage network, which it uses partially.
2. First, the necessary application software is installed on the new computer. The new computer is then set up at the location at which it will ultimately stand. With storage networks it is possible to set up the computer and storage device several kilometers apart.
3. Next, the production data for generating test data within the disk subsystem is copied. Modern storage systems can (practically) copy even terabyte-sized data files within seconds. This function is called instant copy and is explained in more detail in Chapter 2. To copy data it is often necessary to shut down the applications, so that the copied data is in a consistent state. Consistency is necessary to permit the application to resume operation with the data. Some applications are also capable of keeping a consistent state on the disk during operation (online backup mode of database systems, snapshots of file systems).
4. Then the copied data is assigned to the new computer and the new computer is tested intensively (Figure 1.5). If the storage system is placed under such an extreme load by the tests that its performance is no longer sufficient for the actual application, the data must first be transferred to a second storage system by means of remote mirroring. Remote mirroring is also explained in more detail in Chapter 2.
5. After successful testing, both computers are shut down and the production data assigned to the new server. The assignment of the production data to the new server also takes just a few seconds (Figure 1.6 shows steps 5 and 6).
6. Finally, the new server is restarted with the production data.




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