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NETWORK BASICS

Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  


Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…

HARD DISK

HARD DISK
A hard disk is part of a unit, often called a "disk drive," "hard drive," or "hard disk drive," that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today's computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage. 

A hard disk is really a set of stacked "disks," each of which, like phonograph records, has data recorded electromagnetically in concentric circles or "tracks" on the disk. A "head" (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, which is an operation called a "seek." (Data already in a disk cache, however, will be located more quickly.)A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today's hard disks. 

Solid-State Drives (SDD)

Solid-state drives are a relatively new alternative to more traditional hard disk drives. Solid-state drives do not have moving parts, and data is stored electrically instead of magnetically. Most solid-state drives use flash memory, which is also used in memory cards for digital cameras and USB flash drives. Since there are no moving parts, solid-state drives are much less vulnerable to damage from physical shock. The major downside of solid-state drives is that they are a lot more expensive than hard disk drives, although prices are gradually coming down. 

Different type of HDD

As per technology, Speed & storage there are basically five types of Hard Disk available at market. We will discuss about those various hard disks.  
1. IDE: Integrated Drive Electronics. IDE drives are also known as PATA drives (Parallel advance technology attachment)  
2.  SATA: Serial advance technology attachment  
3. SCSI: Small Computer System Interface. SCSI is pronounced as scuzzy. 
4. SAS: Serial Attached SCSI 
5. External removable Hard Disk Drive. 

1. IDE / PATA (Integrated Drive Electronics Drive / Parallel Advance Technology Attachment Drive) 
•    IDE/PATA Drives have usually 40 pins. 
•    IDE/PATA Drives offer 133 MB/sec transfer rate. 
•    It sends 8 bit data at a time.  
•    PATA Cables are used to connect PATA HDD. Two drives can be connected in a single pata cable. One as master and other as slave. The configuration of master and slave is done by different combination of jumpers in the HDD 

2. SATA (Serial Advance Technology Attachment Drive)

 •  SATA Drives have usually 7 pins, 4 pins in pair of two for sending and receiving data and rest 3 pins are grounded.  
•   SATA Drives offers generally 300MB/sec transfer rate. 
•   It sends data bit by bit. 
•    SATA Cables are used to connect SATA HDD. Only one drive can be connected in a single sata cable.

3. SCSI (Small Computer System Interface Drive)

•    SCSI Drives have usually 50 to 68 pins. 
•    SCSI Drive offers generally 640MB/sec transfer rate. 
•    This drives are hot swappable (means it can be attached or detached from system in running                condition) .  
•    SCSI cables are used to connect SCSI HDD. Maximum of 16 drives can be connected in a single SCSI cable. Each HDD have a 8 bytes hexadecimal code known as WWN (worldwide name) for its identification in the cable.

 4. SAS(Serial Attached SCSI Drive)

 •    SAS Drives generally offers 805 MB/sec transfer rate.  
 •    This drives are hot swappable.  
 •    SAS Cables are used to connect SAS Drives. Maximum of 128 drives can be connected in a single sas cable. 

5. External removable HDD

    This is the hard disk drives external to system typically connect via USB cable. It is removable in         nature and features large storage options and portable design.

 It can be used for :  
•    Backup 
•    Data storage 
•    External boot disk for system 
•    Data cloning/recovery 

HARD DISK internal parts

Hard drives have two kinds of components: internal and external. External components are located on a printed circuit board called logic board while internal components are located in a sealed chamber called HDA or Hard Drive Assembly. 

The fancy piece of green woven glass and copper with SATA and power connectors called Printed Circuit Board or PCB. PCB holds on place and wires electronic components of HDD. The black painted aluminum case with all stuff inside called Head and Disk Assembly or HDA. The case itself called Base. Now let's remove PCB and see electronic components on the other side. 
The heart of PCB is the biggest chip in the middle called Micro Controller Unit or MCU. On modern HDDs MCU usually consists of Central Processor Unit or CPU which makes all calculations and Read/Write channel - special unit which converts analog signals from heads into digital information during read process and encodes digital information into analog signals when drive needs to write. MCU also has IO ports to control everything on PCB and transmit data through SATA interface. The Memory chip is DDR SDRAM memory type chip. Size of the memory defines size of the cache of HDD. This PCB has Samsung 32MB DDR memory chip which theoretically means HDD has 32MB cache (and you can find such information in data sheet on this HDD) but it's not quite true. Because memory logically divided on buffer or cache memory and firmware memory. CPU eats some memory to store some firmware modules and as far as we know only Hitachi/IBM drives show real cache size in data sheets for the other drives you can just guess how big is the real cache size. Next chip is Voice Coil Motor controller or VCM controller. This fellow is the most power consumption chip on PCB. It controls spindle motor rotation and heads movements. The core of VCM controller can stand working temperature of 100C/212F. Flash chip stores part of the drive's firmware. When you apply power on a drive, MCU chip reads content of the flash chip into the memory and starts the code. Without such code drive wouldn't even spin up. Sometimes there is no flash chip on PCB that means content of the flash located inside MCU. Shock sensor can detect excessive shock applied on a drive and send signal to VCM controller. VCM controller immediately parks heads and sometimes spins down the drive. It theoretically should protect the driver from further damage but practically it doesn't, so don't drop you drive - it wouldn't survive. On some drives shock sensors used for detection even light vibrations and signals from such sensors help VCM controller tune up heads movements. Such drives should have at least two shock sensors. Another protection device called Transient Voltage Suppression diode or TVS diode. It protects PCB from power surges from external power supply. When TVS diode detects power surge it fries itself and creates short circuit between power connector and ground. There are two TVS diodes on this PCB for 5V and 12V protection. Let's take a quick look on HAD. 

You can see motor and heads contacts which were hiding under the PCB. There is also small almost unnoticeable hole on HDA. This hole called Breath hole. You may be heard old rumor which says that HDD has vacuum inside, well that is not true. HDD uses Breath hole to equalize pressure inside and outside HDA. From the inside Breath hole closed by Breath filter to make air clean and dry. 

 Precious information stored on platters, you can see top platter on the picture. Platters made of polished aluminum or glass and covered with several layers of different compounds including ferromagnetic layer which actually stores all the data. As you can see part of the platter covered with the Dumper. Dumpers sometimes called as Separators located between platters, they reduce air fluctuations and acoustic noise. Usually dumpers made of aluminum or plastic. Aluminum dumpers better for cooling air inside HDA.  

Heads mounted on Head Stack Assembly or HSA. This drive has parking area closer to the spindle and if power is not applied on a drive, HSA normally parked like on the picture. 

HDD is a precision mechanism and in order to work it requires very clean air inside. During work HDD may create some very small particles of metal and oil inside. To clean air immediately a drive uses Re-circulation filter. This hi-tech filter permanently collects and absorbs even finest particles. The filter located on the way of air motion created by platters rotation. 

HDDs use very strong Neodymium magnets. Such a magnet is so strong it could lift up to 1300 times its own weight, so don't put your fingers between magnet and steel or another magnet - it can develop great impact. You can see on this picture there is a HSA stopper on the magnet. HSA stoppers limit HSA movements, so heads wouldn't bang on the platters clamp and on the other side they wouldn't just fly off the platters. HSA stoppers may have different construction but there are always two of them and they always present on modern HDDs. On this drive the second HSA stopper located on HDA under the top magnet. 

There is the other HSA stopper. And you also can the second magnet. The Voice coil is a part of HSA, Voice coil and the magnets form Voice Coil Motor or VCM. VCM and HSA form the Actuator a device which moves the heads. Tricky black plastic thingy called Actuator latch is a protection device - it will release HSA when drive un-parking (loading) heads normally and it should block HSA movements in the moment of impact if drive was dropped. Basically it protects (should, at least) heads from unwanted movements when HSA is in parking area.

HSA has precision bearing to make movements nice and smooth. The biggest part of HSA milled from piece of aluminum called the Arm. Heads Gimbal Assembly or HGA attached to the Arm. HGAs and Arms usually produced on different factories. Flexible orange widget called Flexible Printed Circuit or FPC joins HSA and plate with heads contacts. 

The gasket makes connection airtight. The only way for air to go inside HDA is through the breathing hole. On this drive contacts covered with thin layer of gold, for better conductivity.  

The black small things at the end of HGAs called Sliders. In many sources you can find that sliders claimed as actual heads but a slider itself is not a head it's a wing which helps read and write elements fly under the platter's surface. Heads flying height on modern HDDs is about 5-10 nano-meters. For example: an average human's hair is about 25000 nano-meters in diameter. If any particle goes under the slider it could immediately overheat (because of friction) the heads and kill them that's why clean air inside HDA is so important. The actual read and write elements located at the end of the slider and they are so small that can only be seen under a good microscope.

 As you can see slider's surface is not flat, it has aerodynamically grooves. These grooves help a slider fly on the certain height. Air under the slider forms Air Bearing Surface or ABS. ABS makes slider fly almost parallel to the platter's surface.

There is very important part of HSA which we haven't discussed yet. It called the preamplifier or preamp. The preamp is a chip, which controls heads and amplifies signals from/to them.

The reason why the preamp located inside HDA is simple - signals from heads are very weak and on modern HDDs have more than 1GHz frequency, if take the preamp out of HDA such weak signals wouldn't survive, they will disappear on the way to PCB. The preamp has much more tracks going to the heads (right side) than to the HDA (left side), it's because HDD can work only with one "head" (pair of read and write elements) at a time. HDD sends control signals to the preamp and the preamp selects the head which HDD needs at the current moment. This HDD has six contacts per "head", why so many? One contact is for ground, other two for read and write elements. Other two for micro actuators - special piezoelectric or magnetic devices which can move or rotate slider, it helps tune up heads position under a track. And finally the last contact is for a heater. The heater can help adjust heads flying height. The heater can heat the gimbal special joint which connects slider to HGA, the gimbal made from two stripes of different alloys with different thermal expansion. Once gimbal got heated it bents itself toward platter's surface and this action reduces flying height. After cooling down the gimbal straights itself. 

  File Storage hardware & Disk organization 

In this segment we will discuss about HDD basics & low-level disk organization. 

  Hard disk drive basics

 A hard disk is a sealed unit containing a number of platters in a stack. Hard disks may be mounted in a horizontal or a vertical position. Let’s discuss about various terms related to hard disk storage organization. 

Tracks

On a hard disk, data is stored in thin, concentric bands. A drive head, while in one position can read or write a circular ring, or band called a track. There can be more than a thousand tracks on a 3.5-inch hard disk. Sections within each track are called sectors. A sector is the smallest physical storage unit on a disk, and is almost always 512 bytes (0.5 kB) in size. The tracks, stacked on top of each other form a cylinder. This scheme is slowly being eliminated with modern hard drives. 

Sector

A sector, being the smallest physical storage unit on the disk, is almost always 512 bytes in size because 512 is a power of 2 (2 to the power of 9). The number 2 is used because there are two states in the most basic of computer languages — on and off. Each disk sector is labelled using the factory track-positioning data. Sector identification data is written to the area immediately before the contents of the sector and identifies the starting address of the sector.  

Cluster: 

A cluster can consist of one or more consecutive sectors. The number of sectors is always an exponent of 2. A cluster could consist of 1 sector (2^0), or, more frequently, 8 sectors (2^3). The only odd number a of sectors a cluster could consist of is 1. It could not be 5 sectors or an even number that is not an exponent of 2. It would not be 10 sectors, but could be 8 or 16 sectors.  

Cylinder

The tracks with the same diameter on the various surfaces are called cylinder because of the shape they would form if connected in space. The set of tracks of a disk drive which can be accessed without changing the position of the access arm are called cylinder.  

RAID (Redundant array of inexpensive disks)

RAID (redundant array of independent disks; originally redundant array of inexpensive disks) provides a way of storing the same data in different places (thus, redundantly) on multiple hard disks (though not all RAID levels provide redundancy). By placing data on multiple disks, input/output (I/O) operations can overlap in a balanced way, improving performance. Since multiple disks increase the mean time between failures (MTBF), storing data redundantly also increases fault tolerance. RAID arrays appear to the operating system (OS) as a single logical hard disk. RAID employs the technique of disk mirroring or disk striping, which involves partitioning each drive's storage space into units ranging from a sector (512bytes) up to several megabytes. The stripes of all the disks are interleaved and addressed in order. In a single-user system where large records, such as medical or other scientific images, are stored, the stripes are typically set up to be small (perhaps 512 bytes) so that a single record spans all disks and can be accessed quickly by reading all disks at the same time. In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical or maximum size record. This allows overlapped disk I/O across drives. There are many different ways to organize data in a RAID array. These ways are called "RAID levels". Different RAID levels have different speed and fault tolerance properties. RAID level 0 is not fault tolerant. Levels 1, 5, 6, and 1+0 are fault tolerant to a different degree - should one of the hard drives in the array fail, the data is still reconstructed on the fly and no access interruption occurs. RAID levels 2, 3, and 4 are theoretically concept but not used in practical. Now we will discuss about various RAID 1, 5, 6, and 1+0 categories in this segment. 

Striping & Block

Striping & block both are very useful term which are related to the RAID technology. Striping is a technique to store data on the disk array. The contiguous stream of data is divided into blocks, and blocks are written to multiple disks in a specific pattern. Striping is used with RAID levels 0, 5, 6, and 10. Block size is selected when the array is created. Typically, blocks are from 32KB to 128KB in size. 

RAID level 0(Stripe Set)

In a RAID0, the data is divided into blocks, and blocks are written to disks in turn. RAID0 provides the most speed improvement, especially for write speed, because read and write requests are evenly distributed across all the disks in the array. Note that RAID1, Mirror, can provide the same improvement with reads but not writes. So if the request comes for, say, blocks 1, 2, and 3, each block is read from its own disk. Thus, the data is read three times faster than from a single disk. However, RAID0 provides no fault tolerance at all. Should any of the disks in the array fail, the entire array fails and all the data is lost.  

RAID level 1(Mirror)

Mirroring (RAID1) stores two identical copies of data on two hard drives. Should one of the drives fail, all the data can be read from the other drive. Mirroring does not use blocks and stripes. Read speed can be improved in certain implementations, because read requests are sent to two drives in turn. Similar to RAID0, this should increase speed by the factor of two. However, not all implementations take advantage of this technique. Write speed on RAID1 is the same as the write speed of a single disk, because all the copies of the data must be updated. RAID1 uses the capacity of one of its drives to maintain fault tolerance. This amounts to 50% capacity loss for the array. E.g. if you combine two 500GB drives in RAID1, you'd only get 500GB of usable disk space. If RAID1 controller fails you do not need to recover neither array configuration nor data from it. To get data you should just connect any of the drives to the known-good computer. 

RAID level 5(Stripe with parity)

RAID5 writes data blocks evenly to all the disks, in a pattern similar to RAID0. However, one additional "parity" block is written in each row. This additional parity, derived from all the data blocks in the row, provides redundancy. If one of the drives fails and thus one block in the row is unreadable, the contents of this block can be reconstructed using parity data together with all the remaining data blocks. Write speed of a RAID5 is limited by the parity updates. For each written block, its corresponding parity block has to be read, updated, and then written back. Thus, there is no significant write speed improvement on RAID5, if any at all. The capacity of one member drive is used to maintain fault tolerance. E.g. if you have 10 drives 1TB each, the resulting RAID5 capacity would be 9TB. If RAID5 controller fails, you can still recover data from the array with RAID 5 recovery software. Unlike RAID0, RAID5 is redundant and it can survive one member disk failure. 

RAID level 6(Stripe with dual parity)

RAID6 is a large, highly reliable, relatively expensive storage. RAID6 uses a block pattern similar to RAID5, but utilizes two different parity functions to derive two different parity blocks per row. If one of the drives fails, its contents are reconstructed using one set of parity data. If another drive fails before the array is recovered, the contents of the two missing drives are reconstructed by combining the remaining data and two sets of parity. The capacity of two member drives is used to maintain fault tolerance. For an array of 10 drives 1TB each, the resulting RAID6 capacity would be 8TB. The recovery of the RAID6 from a controller failure is fairly complicated. 

RAID level 10(Mirror over stripes)

RAID10 is a large, fast, reliable, but expensive storage. RAID10 uses two identical RAID0 arrays to hold two identical copies of the content. Writes are two times slower than reads, because both copies have to be updated. As far as writes are concerned, RAID10 of N disks is the same as RAID0 of N/2 disks. Half the array capacity is used to maintain fault tolerance. In RAID10, the overhead increases with the number of disks, contrary to RAID levels 5 and 6, where the overhead is the same for any number of disks. This makes RAID10 the most expensive RAID type when scaled to large capacity. If there is a controller failure in a RAID10, any subset of the drives forming a complete RAID0 can be recovered in the same way the RAID0 is recovered.
  
Hard disk installation steps 

The hard drive is the primary storage location for all of your computer’s data. If you run out of that storage, installing a new drive may be the most practical solution. If your computer is dead, you may need to replace your old failed hard drive. In either case, you can do it yourself instead of taking your computer into a repair shop. So let’s go through the installation steps.  

Step 1: 
Power down the computer 

Shut off the power supply in the back. Even if you can access the inside where it stands, you need to unplug the computer. Otherwise, unplug the computer and place it somewhere that allows you to get inside.  

Step 2: 
Remove the case panels 

You may need a Phillips head screwdriver, but most new computers have thumbscrews. You will need to remove both sides so that you can screw the hard drive in on both sides. 

Step 3: 
Ensure that you do not have any static electricity.  

Touch a metal object (other than your computer), such as a doorknob, to discharge any existing static electricity that you may have.  

Step 4:
Make sure you are grounded. 

If your computer is still plugged in (but the power supply is switched off), you can ground yourself by touching any metal part of the case. Otherwise, make sure that you are grounded before starting to work on the inside of the computer. This will prevent electrostatic shock from damaging the components of your computer. 

Step 5: 
Remove the old drive (if applicable)

If you are removing an old hard drive, make sure all of the cables are disconnected from both the motherboard and the power supply. Unscrew all the screws on both sides of the hard drive, and then slide it out of the housing. You may need to remove more cables or cards in order to access the hard drives in a tight case. 

Step 6: 
Insert your new drive. 

Remove it from the anti-static packaging and slide it into an open space in the hard drive housing. The drive should slide directly in, and the holes on the side of the drive should line up to the screw guides on the housing. If you can, use a slot that has some space around it. This will improve airflow and lead to a cooler system, but is not a critical concern. 

Step 7: 
Secure the hard drive 

Once the hard drive has been inserted, use the screws that came with it to secure the hard drive in the housing. Ideally you should use two screws on each side of the hard drive. If the hard drive is loose, it can rattle and cause more noise and lead to physical damage. Tighten the screws to a firm tightness, but don’t over-tighten as that may cause damage as well. 

Step 8: 
Connect a SATA hard drive to the motherboard.  

Newer hard drives will use SATA cables, which are thin and resemble USB cables. Use a SATA cable to connect the hard drive to the motherboard. SATA cables can be connected in either direction.  

➣ If you are connecting your primary hard drive, the SATA cable should be plugged into the first SATA channel. This may be labeled SATA0 or SATA1. Refer to your motherboard documentation for detailed information for your motherboard. 
➣ Secondary drives should be connected to the next available SATA channel. 

Step 9: 
Connect a PATA (IDE) hard drive to the motherboard.

IDE drives are older model hard drives that can be identified by the long rows of pins on the back. IDE Drives are connected via an IDE cable, which is wide and flat. The cable is usually gray in color.  
➣ The blue end of the cable plugs into the motherboard. The black connector plugs into your primary (Master) drive, while the black connector plugs into the secondary (Slave) drive if applicable. 
➣ Set the jumper for your primary drive to Master. The jumper diagram should be printed onto the hard drive. If you are installing a secondary drive but it is the only drive connected to the cable, it should be set to Master as well. 

Step 10: 
Connect the power supply to the hard drive. 

Most new power supplies have SATA power connectors, though older power supplies typically only have Molex (4 pin) connectors. If this is the case, and you are installing a SATA drive, you will need a Molex-to-SATA adapter. IDE drives will use the Molex connector.  Make sure that all of your connections are secure. Ensure that none of the cables can come undone by wiggling them a little bit. 

Step 11: 
Close up your computer case 

Replace the case sides and reconnect your cables if you had to move the case to work on the inside. Turn the power supply back on and then turn your computer on.  


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