HDD: Hard Disk Drive By Tyler Beckett Janaki Ramachandran
Why Are We Here? • To teach the class background information about HDDs, including some background about them, how HDDs have changed over time in regards to various aspects such as capacity and cost, the hardware and materials used to build HDDs, and how HDDs work.
A Short Background of HDDs • Hard Disk refers to one plate, while drive refers to a stack of them • A hard disk stores data that is able to be changed, in a permanent form; that is, the data is still there even if the power goes out. • Invented in 1950’s • Used to be large discs of approx. 20 cm with just a few MB of space
Magnetic Tapes vs. Hard Disks Disks store a lot more info, and they’re much faster; they’re also more precise, because information is stored in much smaller “magnetic domains” than in a tape. Both use same magnetic method of recording and storage.
The internals of a hard-drive primarily consist of: -A row of platters that spin very fast-A "comb" of heads in between the platters The industry standard rotational speed today is:7,200 RPMs or approximately 150 MPH. The old standard was 5,400 RPMs and the fastest consumer hard drives today run up to 15,000 RPMs. Platters are made from:-Non-magnetic material(usually glass or aluminum)-Coated with a thin layer of magnetic materialGaps of equal size are the data, stored as 1's or 0's via magnetism.
Heads are tiny Read-and-Write magnetic heads, held just above the surface of the fast-spinning disks by an actuator arm (the “comb”). They detect and modify the magnetism of the area directly below the head. The actuator arm moves side to side (slightly circular because of the motion) to reach the entire surface of the platter. The whole head setup runs on a cushion of air created by the air being pushed by the friction between the fast-spinning platters. This keeps the heads incredibly close to the platters.
Hard drives are mostly airtight, to prevent dust, hair, fingerprints and the like to enter into the very delicate environment inside a hard disk. The slightest interference of a particle of dust can offset a head and cause it to “crash” into the hard drive, causing even more debris to go flying into the device, resulting in a “hard drive crash.” A small, highly filtered valve on the outside of the shell is used to regulate the air pressure with that of the surrounding environment.
USB and Firewire are the standard connections for external hard drives today. Internal Hard drives pretty much all use ATA today, but the trend has been towards SATA and SATA2. USB2 = 60 MB/s Firewire 400 = 50 MB/s ATA = 100 MB/s or 133 MB/s SATA = 1.5 GB/s SATA2 = 3 GB/s
RAID stands for Redundant Array of Independent (sometimes Inexpensive) Disks. RAID 0 - instance mirror-backup (often used by servers and mainframes) RAID 1 - Striped HDDs (often used by gamers) RAID 0+1 - Mirrored Striped HDDs(often used by both mainframes and gamers) RAID 5 - 3-drive parry bit combination (designed to safe check your striped drives) JBOD - Just a Bunch Of Drives
Seek/Access Time • seek time – (access time)…amount of time between when the CPU asks for a file and when the first byte of the file is sent to the CPU. This happens in ms. (10-20 ms). • All other computer processes run on the scale of ns…so the CPU has to send the message to get something, do other computations while waiting, and then receive the file. • Data rate – the number of bits the computer can deliver per second • Despite the increase in space, throughput has not increased as much, and access times have not decreased much at all. • The most common way to decrease r/w time is to increase the number of r/w heads (there are two parts on the head, one for a read and one for a write).
Seek/Access Time cont’d. • However, these r/w heads are the most expensive part of a HD, so increasing the number of heads would only increase the cost of production, so this isn’t exactly feasible. • Two ways to increase throughput and reduce access time are: • NVRAM (non-volatile RAM) • holographic memory
Holographic Memory • Holographic memory – instead of only using a 2D surface to store, like we do with CDs, the entire volume of the storage medium is used. This enables more data to be stored in a smaller space, so transfer times are faster. • Holographic memory can store up to 1 Tb in a storage medium the size of a sugar cube crystal. [1000 CDs]
Capacity Capacity – the number of bytes a disk can hold The capacity of Hard Drives has increased dramatically over the years.
Capacity cont’d. • The first hard disk was the one in the IBM 350 RAMAC (Random Access Method of Accounting and Control) computer. It was invented in 1956, and it stored 5 million characters (about 5 megabytes) on fifty 24-inch diameter platters.
Capacity cont’d. • In the 80’s, a PC with more memory had 20 MB • In the 90s, they had 1 Gb and above • …And in 2006, the lowest memory capacity in production was 20 GB, and the highest was 768 GB (0.75 TB, with 4 platters) • Nowadays, many PCs are between 40 – 80 GBs, on average. • Hitachi announced that in 2007, they would release a 5 platter 1 TB Hard Drive • Exponential increases in capacity have made it possible to market such things as the Apple iPod. Without the space, it would not be feasible. • Has also changed how we program; since space is cheaper, we can afford to use more space in order to speed up processes. • In the past, it would not have been as practical.
REFERENCES • http://www.pcguide.com/ref/hdd/ • http://en.wikipedia.org/wiki/Hdd • http://en.wikipedia.org/wiki/Redundant_Array_of_Independent_Disks • http://computer.howstuffworks.com/hard-disk.htm • http://www.pcmech.com/show/harddrive/296/ • http://www.pcmech.com/show/harddrive/65/ • http://www.techweb.com/encyclopedia/defineterm.jhtml?term=harddisk
References • http://computer.howstuffworks.com/hard-disk.htm • http://en.wikipedia.org/wiki/Hard_disk • http://www.public.iastate.edu/~mcfong/Works/How_Stuff_Works/Hard_Drive/history.html • http://membres.lycos.fr/patrickgelinas/minis_e.html • http://www.acsdata.com/readwrite.htm • http://www.newscientist.com/blog/technology/2006_03_01_archive.html • http://www.apple-history.com/?page=gallery&model=ipod_mini • http://www.soldam.net/2006/item/hdd/whd/images/hdd.jpg