Hard Disk Performance

How Hard Drives Work

A+ Guide to Managing and Maintaining Your PC, Fifth Edition

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Hard disk also called fixed disk, inside the head disk assembly they are 1 Disk Platter are covered with thin layer of magnetically retentive called media ,which magnetic information & data is store. 2 Read write head are electromagnetic devices that read ,records & erase data on the media, number of platters depend on numbers of read write head, when drive is at rest the head are forced into direct contact with the platters by the spring tension, when the drive is spinning ,air pressure develops below the head & lift them off the surface of the platter. 3 Head Actuator Mechanism Can be categorized into 2 type

Stepper –Motor Actuators is the older drive Voice –coil Actuators nowadays people use this because they are faster ,efficient & quieter than oldest drive, they know where the head by reading special magnetic pattern on dedicated area on the disk surface(closed –loop system),it used a feedback signal from the drive to determine the head position & to adjust them accurately if necessarily this system is (closed-loop) is similar to typical audio speaker ,the electromagnetic coil is attached to the end of the head rack & placed near a stationary magnet.

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Spindle Motors spins the platters that are mounted on the spindle ,the revole speed at 3600 to 7200rpm or higher.

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Drive Electronics control the spindle & head actuator system & present data to the controller ,it also translates commands from operating system to voltage fluctuations to control the actuator ,read/write head & spindle motor. 6 Hard disk capacity is the number of bytes it can store ranges from megabytes to gigabytes(1 gbytes eaquel to 1024 mbyte),it use hard metal platter to store data ,as number of platters is equal to higher the capacity, the surface of the platter is divided into tracks & sector ,each sector of the same size holds 512 bytes of data ,sector is the smallest manageable piece of data storage. 7 Partition is the process of dividing the hard disk into smallest disk, so hard disk may have different combination of heads ,tracks & sectors, the first sector of the hard disk is on head 0,track0,sector 1. 8 As read write head moves along the track ,it must reach each sector & pass it to the controller ,the controller will deliver this data to the host computer thought the buses ,if failed “interleaving” is employed to order the sector so they will not sequential ,enable a slow controller to keep up without missing the next sector,”interleaving is not used in new drive electronics can process the data fast enough to be able to read the data sequentially from sector to sector. 9 Disk caching is the t for speed up access time on the hard disk, there are 2 types of disk caching 1) Hardware disk cache (memory 64k-1m on the disk) 2) Software disk caching,is done by SMARTDRV.SYS using MSDOS to set DRAM on the motherboard ,the larger size of the memory ,the more files it can store & access by the CPU. 10 Performance factor is about (internal factor) & (external factor) about the performance hard disk, in computer system hard disk is the slowest component because in (internal factor) are 1) Mechanical Design Factor as we know platter store data & required to spin, actuators arms carry the read/write heads to move the correct cylinder ,all need extra times . The most factors affect the performance of the drive are I. Size & number of platters surface the heavier & larger the platter will slow down the rotation speed of the spindle because it add more weight for the platter to spin. II. Actuator Characteristics the speed of the actuator assembly will affect the seek time of the hard disk. III. Spindle Motor Speed & Power affect the platter spin ,faster the motors turns ,the faster the platters spin, the spindle speed affect the driver’s rotational latency,position performance & internal transfer rate, the speed of the hard disk is measured as (Revolutions per minutes)RPM, modern speed is 4,200rpm to 15,000 rpm & the common speed is 5,400 to 10,000RMP, for the SCSI standard they are using 15,000RPM & IDE/ATA are 5,400 & 7,200 & 10,000RPM.

2) Data Recording & Encoding Factors I. Recording Density ,Track Density & Areal Density ,recording density also called bit density meaning number of bits can be placed on the length of track on a hard disk, track density is number of tracks can be placed across a radial length of 1 inch ,& it’s unit is track per inch, Areal density is amount of data that can be store in amount of platter space, & it’s measured by multiplying bit density & track density(BPSI),the higher areal density equal to higher transfer rate, the improvement in transfer rate is increase in bit density not track density ,the meaning of transfer rate is amount of data being transferred from hard disk to memory in 1 second. II. Read /write technology hard disk specifications based upon it’s reading ,it’s spend more time on reading than writing & it’s much faster than writing, the average seek time for writing than reading is about 1 millisecond ,the virtually affect on the PC is 10% which pc user don’t realised it. III. Encoding Method Data recorded on the hard disk surface will affect the speed of read write, Encoding will also affect the amount of data space to be store on the hard disk ,a good code increase recording density therefore allow more data to be store. IV. Track & Sector Layout A. Interleaving (go to number 8) B. Zone Bit Recording the outer track is more than inner track & outer track is divided more track & sector so it can record more data & have a higher transfer rate, & that the reason media transfer rate depends on what part of hard disk being accessed C. Cylinder & Head Skew help to remove the latency time when switching from 1 track to another. D. Sector Format each sector can store 512bytes of data, sector perform store data ,identify data & save data ,if the data is reduced the sector can be smaller then can create more sector on platter to save more data. 3) Integrated Controller Factor I. Controller is much faster than mechanical but circuitry need to maintain same rotational speed when transferring a large file & Alogrithm is refers to software that manages hard disk when more than 1 command given at the same time. II. Cache Size affect burst transfer of data transfer, if the file size is smaller than the cache memory the retrieval will be fast, cache can be use when writing on the hard disk,this will free the cpu to do other task when the hard disk is taking time to store the information on the platter.

11 Hard Disk Specification .I Spindle Speed

.II Area Density is amount of data that can be store in amount of platter space, & it’s measured by multiplying bit density(bit density meaning number of bits can be placed on the length of track on a hard disk,) & track density(BPSI), (number of tracks can be placed across a radial length of 1 inch ,& it’s unit is track per inch) .III Seek Time is most popular in the system , it’s amount of time for the read/write head to move between track over the surface of the platters, it’s measure in milliseconds (“msec” or” ms”)modern seek time is 8 to 10ms ,1 ms is a long time in electronic terms ,a small reductions in seek time can result in improvements in overall system performance, in specification seek time refers to reading data,seek time for writing data is 1 ms longer .IV Settle Time amount of time required ,after actuator has moved the head assembly during a seek ,for the head to stabilize sufficiently for the data to being to be read,it take less than 0.1 msec ,it is dwarfed in importance by seek time & rotational latency .manufacture don’t brother the settle time they lump it with seek time. .V Command Overhead time command given & the respond ,normally it takes0.5ms .VI Latency after the actuator assembly has completed its seek to the correct track, the driver must wait for the correct sector to come around to where the read/write heads are located the time is called latency, latency is related to spindle speed of the drive, on average ,latency will be half time it takes for a full rotation of the disk, latency is for multiple ,frequent reads of random sectors on the disk, for reading large continuous blocks of data ,& first sector of a file. .VII Access Time hard disk access time is range from 8 to 15ms, Access Time =Command Overhead Time +Seek Time Settle Time latency

.VIII Data Transfer Rate is the most important in the system .it’s the rate that drive & controller can send data to the system ,it’s primary depends on the HAD(Hard Drive Assembly) & secondarily on the controller .IX Storage capacity numbers of platters * 2 * cylinder*sector*bytes=bytes