What is Virtual Memory Management

VIRTUAL MEMORY MANAGEMENT(VMM)

Real, or physical, memory exists on RAM chips inside the computer. Virtual memory, as its name suggests, doesn't physically exist on a memory chip. It is an optimization technique and is implemented by the operating system in order to give an application program the impression that it has more memory than actually exists. Virtual memory is implemented by various operating systems such as Windows, Mac OS X, and Linux.

So how does virtual memory work? Let’s say that an operating system needs 120 MB of memory in order to hold all the running programs, but there’s currently only 50 MB of available physical memory stored on the RAM chips. The operating system will then set up 120 MB of virtual memory, and will use a program called the virtual memory manager (VMM) to manage that 120 MB. The VMM will create a file on the hard disk that is 70 MB (120 – 50) in size to account for the extra memory that’s needed. The O.S. will now proceed to address memory as if there were actually 120 MB of real memory stored on the RAM, even though there’s really only 50 MB. So, to the O.S., it now appears as if the full 120 MB actually exists. It is the responsibility of the VMM to deal with the fact that there is only 50 MB of real memory.

The paging file and the RAM

Now, how does the VMM function? As mentioned before, the VMM creates a file on the hard disk that holds the extra memory that is needed by the O.S., which in our case is 70 MB in size. This file is called a paging file (also known as a swap file), and plays an important role in virtual memory. The paging file combined with the RAM accounts for all of the memory. Whenever the O.S. needs a ‘block’ of memory that’s not in the real (RAM) memory, the VMM takes a block from the real memory that hasn’t been used recently, writes it to the paging file, and then reads the block of memory that the O.S. needs from the paging file. The VMM then takes the block of memory from the paging file, and moves it into the real memory – in place of the old block. This process is called swapping (also known as paging), and the blocks of memory that are swapped are called pages. The group of pages that currently exist in RAM, and that are dedicated to a specific process, is known as the working set for that process.

As mentioned earlier, virtual memory allows us to make an application program think that it has more memory than actually exists. There are two reasons why one would want this: the first is to allow the use of programs that are too big to physically fit in memory. The other reason is to allow for multitasking – multiple programs running at once. Before virtual memory existed, a word processor, e-mail program, and browser couldn’t be run at the same time unless there was enough memory to hold all three programs at once. This would mean that one would have to close one program in order to run the other, but now with virtual memory, multitasking is possible even when there is not enough memory to hold all executing programs at once.

Storage Technology(SAN & NAS a Complete Reference)

Storage Area Network and SAN Protocols

Storage Area Network (SAN) is a high-speed network or subnetwork whose primary purpose is to transfer data between computer and storage systems. A storage device is a machine that contains nothing but a disk or disks for storing data. A SAN consists of a communication infrastructure, which provides physical connections; and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust.

Typically, a storage area network is part of the overall network of computing resources for an enterprise. A storage area network is usually clustered in close proximity to other computing resources but may also extend to remote locations for backup and archival storage. SANs support disk mirroring, backup and restore, archival and retrieval of archived data, data migration from one storage device to another, and the sharing of data among different servers in a network. SANs can incorporate subnetworks with network-attached storage (NAS) systems.

There are a few SAN technologies available in today's implementations, such as IBM's optical fiber ESCON which is enhanced by FICON architecture, or the newer Fibre Channel technology. High speed Ethernet is also used in the storage Area Network for connection. SCSI and iSCSI are popular technologies used in the Storage Area Network.

A typical SAN architecture is diplayed as follows:

NAS: Network Attached Storage

Network Attached Storage (NAS) is a data storage mechanism that uses special devices connected directly to the network media. These devices are assigned an IP address and can then be accessed by clients via a server that acts as a gateway to the data, or in some cases allows the device to be accessed directly by the clients without an intermediary.NAS servers use small, specialized, and proprietary operating systems instead of general-purpose operating systems (OSs) such as the UNIX and Windows NT operating systems. Compared to the general-purpose OSs, NAS server OSs are smaller, faster, and optimized for the specialized task of file serving.

NAS servers support a variety of network file protocols such as Sun's Network File System (NFS) and Microsoft's Server Message Block / Common Internet File System (SMB/CIFS). NAS severs use open standard protocols, which allow them to interface with multiple types of client computers and operating systems.

Computer Networking Cables - Types and Usage FAQ

Some unanswered questions and there answers..enjoy.... 

Q. What is an unshielded network cable?

A. Unshielded Twisted Pair cables are widely used with the Ethernet networks. The twisted wire pairs in the cable, as suggested by the name, are not shielded. Eight individual copper wires (22 or 24 gauge) are twisted to make four pairs that restrict the signal corruption due to electromagnetic interference. UTP cables must adhere to the specifications regulating the number of twists that are permitted in defined length. A relatively smaller diameter of this cable is beneficial during installation. The speed and throughput of this cable is 10 to 1000 Mbps and is the most cost effective networking cable. This cable is extremely popular because it can be effectively used with most of the major networking infrastructure.

Q. What is a shielded network cable?

A. In the Shielded Twisted Pair cables, a metallic foil is used for wrapping each pair of copper wire which are then blanketed altogether in a metallic braid. STP not only reduces the outside disturbance like the EMI and RFI radiations but also the electrical noises in the cables like crosstalk and pair-to-pair to coupling. It is generally installed with the STP data connector but can also use RJ connector base. Unlike UTP, number of twisted pairs in this cable are only two and are comparatively more difficult to install. When installing, you must ensure that the metallic shield on both the ends is properly grounded.

Q. When do I need a shielded cable and which type do I need?

A. The main use of shielded cable is to strengthen the signal transmission via the cable and protect it from disturbances like crosstalk, Electromagnetic Interference (EMI) and Radio Frequency Interference. This interference usually occurs when there are any kinks or crimps from the neighbouring structures. These cause the transmission of data to slowdown. Common sources causing the disturbances are large motors and generators, radio towers and high-voltage lighting fixtures.

Q. What is a crossover cable?

A. A crossover cable, also known as cross wired cable, is a network cable that is used for the connection of two network devices without the need of any hub or switch. These cables are terminated distinctively on one of its ends with pairs switched green and orange. This cable also enables you to directly access the hardware on the same network.

Q. What is a solid cable?

A. Within the jacket of a solid cable, also known as one strand cable, there's only one solid piece of copper wire. Solid cables give mechanical ruggedness and because it's a single conductor, this cable can be used for structured wiring inside a building. In an insulation displacement connector, it's easier to place solid cables.

Q. What is a stranded cable?

A. A stranded cable consists of numerous thin copper wires bundled within a jacket. Each one of these thin strands within the cable can be bent and flexed conveniently. Owing to their flexibility, these cables are mainly to make patch cables that can easily be plugged, unplugged and installed. Even if the stranded cables are twisted frequently, these neither corrupt nor affect the signal strength.

Q. Which type do I need, solid or stranded?

A. Though both the cables, solid and stranded have the same signal strengths, stranded cables are considered better for short lengths due to their flexibility. On the other hand, solid cables are much better for long cable runs.

Q. What is the maximum length?

A. The maximum recommended length for networking cables is 330ft (100m) which is based upon the solid cable as it's the preferred choice for maximum length. Always make it a point to use repeaters at the length of patch cables. The distance between two of the patch cords should be 30ft (8m). another thing you have to pay heed to is the bending of the cables. Since these are rated with the least bending capacities, the signals are slowed down in case of a severe bending.

Q. What is RJ45?

A. Just like the lines on the road control the traffic, connectors, at the end of cables, line up the wires in physical locations inside themselves. In network installation, RJ45 connectors are used specifically for connecting the Ethernet devices.