Disk Drives

1. Disk Drives

2. Hard drive • In our original view of a computer as being comprised of ALU, Control, Memory, Input and Output, the hard drive is a device connected either as input or output. • But the hard drive is also sometimes viewed as a logical extension of memory. • The hard drive is the primary storage device. • Compared to RAM, storage is non-volatile • Compared to ROM, storage is more easily written.

3. Speed and Capacity • The two main characteristics of a hard drive are • Its capacity: how much data can it hold • Its speed: how quickly can it be read from or written to • Data intensive applications such as databases, graphics and so on will require pages to swapped in and out of memory. The speed of the hard drive will be an important factor for determining how efficiently such programs run.

4. Analog of Moore • Recall that Moore’s Law concerns the exponential growth of the number of transistors on an integrated chip. • There has been similar exponential growth in the capacity of hard drives.

5. Exponential Improvement in Hard Drive Capacity

6. Paper Tape and Cards • Prior to hard drives, programs and data could be stored on cards or paper tape. • In both cases a hole could correspond to a 1 and the absence of a hole to a 0.

7. Magnetic Tape • Magnetic tape was an improvement upon punch cards and paper tape, both in terms of speed and capacity. • In magnetic tape, a long thin piece of plastic is covered with a ferromagnetic material, such as Ferric oxide (Fe2O3).

8. Magnetic Tapes versus Hard drives • The writing or reading of a individual bit is similar whether we are talking about a hard drive or a magnetic tape. • The difference is one of addressing: • Magnetic tapes have sequential access • Hard drives have random access

9. Magnets • Little magnets align themselves in a particular way when in the presence of a magnetic field • E.g. a compass points North because it is a magnet aligning itself with the Earth’s magnetic field

10. Ferromagnetic • Many atoms are like tiny little magnets, but the little magnets point in random directions and tend to cancel out any magnetic effect on a large scale. • An external magnetic field can make these little magnets line up and produce a large-scale (macroscopic) effect. • If the little magnets remain aligned even when the external magnetic field is removed, then the material is said to be ferromagnetic. • The lining up of the magnets is called magnetization. • A ferromagnetic material “holds” or “remembers” its magnetization state.

11. Writing/Recording • The signal (data changing over time) is fed to a magnet which magnetizes the material on the region of the tape that corresponds to that time. • The tape may record analog or digital information.

12. Reading/Playing • The magnetized region produces a magnetic field of its own. • Any device that can sense this magnetic field can read the information encoded on the tape. • It is important to note that the reading device (head) does not have to be in physical contact with the tape in order to sense the tape region’s magnetic field – just in its vicinity.

13. Floating height/Flying height • As opposed to floppies, VCR and cassette tapes, hard disk heads do not come in contact with the medium they are reading from or writing to. • The distance the head is from the material is one of the important design parameters and is known as the floating height or flying height.

14. Reading, Sensitivity and Density • As the heads are made sensitive to smaller magnetic fields, the region corresponding to a unit of information can be made smaller. • Also as the heads are made to approach the material more closely (where the field is stronger) without touching it, the region corresponding to a unit of information can be made smaller. • If the regions grow smaller, the number of such regions per area (the density) increases. • There are other ways to measure density, so this version is sometimes called the areal density.

15. Exponential Improvement in Hard Drive Density The units for areal density are bits per square inch (BPSI) or in this case MBPSI (mega)

16. Shrinking Bit Size

17. IBM RAMAC • To get a sense of this improvement, consider an early disk drive. • One of the first commercially available hard disks was IBM's RAMAC (Random Access Method of Accounting and Control) introduced in 1956. • Capacity: about 5 MB • Used 50 24" disks!!!!!!!! • Areal density: 2,000 bits per square inch • Data throughput: 8,800 bits/s.

18. Form Factor • The areal density improvement has allowed the capacity to increase while the size has decreased. • Drive’s form factors (basically their width and height) have continued to grow smaller and smaller. • The 5.25-inch width was a standard. It came in three standard heights • Full-height: 3.25 inch • Half-height: 1.625 inch • Third height: 1 inch

19. 3.5 inch Form Factor • But now the 3.5-inch width has replaced it as the standard in PCs. • This width comes in two standard heights • Third height: 1-inch which is standard (slim-line) • Half-height: 1.625-inch which is used for higher capacity drives • Besides the overall benefits of miniaturization, smaller widths allow the platters to spin faster and thus help improve the speed.

20. Platters and Spindle Speed • Instead of the long plastic strip of magnetic tape, hard disks have a collection of circular shaped aluminum or glass platters (which serve as the substrate) that are covered with magnetic material (the media layer). • Data is accessed by having the head float over the platter as its spins. • Access speed is thus related to rotational or spindle speed which is measured in RPM (revolutions per minute). • Spindle speeds in the thousands to tens of thousands are typical these days.

21. Hard versus floppy disk • The disks in a hard drive are fairly rigid and hence the name “hard” disk. • The disk in a floppy disk is made of a more flexible material.

23. Anatomy of a Hard Drive • The data is stored on platters: hard, flat circular-shaped piece of aluminum coated with magnetic material on one or both sides. • The spindle serves as a rotational axis for the platters. • The rotational motion is driven by the spindle motor. • The information is accessed by a read/write head. • Typically there are two heads per platter, one on the top, one on the bottom.

24. Anatomy of a Hard Drive (Cont.) • The head is attached to a part called the slider, which is in turn attached to the actuator arm, which is used to position the head in the desired region. • The position of the actuator arm is controlled by the actuator. • There is actually a parallel array of heads which are moved in unison by the actuator. • The actuator in controlled by the logic board which communicates the rest of the PC.

26. Don’t try this at home • Reading requires the head to come very, very close to the platter without touching it. • Hard drives should not be opened because a typical speck of dust is larger than the head-to-platter reading distance. • Opening a drive will almost assuredly ruin it. • Head crash: when the head touches the platter

27. Dust particle versus Flying Height

28. In both hard and floppy disks, the data is written in concentric circular paths known as tracks A typical floppy disk has 80 (double-density) or 160 (high-density) tracks. Tracks track

29. Tracks (cont.) • The density of tracks is measured in units of tracks per inch (TPI). • Each track is further divided into sectors. • The location of information is remembered by noting its track and sector numbers.

30. Sectors • Radial lines break the tracks up into sectors, each of which holds 512 bytes of information. Sector Usually 17 sectors per track for a floppy

31. Not all sectors are created equal • A sector on the outer portion of the platter has a greater area than a sector on the inner portion. • More and more of the storage area is wasted as one moves out in the radial direction. • More modern drive technologies divide the outer tracks into more sectors to make use of this storage area.

32. Zoned Bit Recording • Zoned bit recording (ZBR), a.k.a. multiple zone recording or zone recording. • Tracks are broken into groups called zones based on their radial position. Tracks with greater radii are broken into more sectors so that storage area is not wasted. • Requires more sophisticated controller.

33. Larger Radius, Faster Access • In ZBR, the outer tracks have more sectors and thus hold more data, but the data is accessed by the spinning of the disk. So more data goes by per revolution when reading the outer tracks. • The outer tracks tend to be used first. So disk access performance may go down as one starts to use the inner/slower tracks.

34. ZBR

35. Cylinders • Each platter has tracks with the various radii. • All the tracks on different platters but with the same radius make up a cylinder. • For example, if a hard drive has • Four platters • And each platter has 600 tracks • Then • There will be 600 cylinders • Each cylinder will have 8 tracks (assuming that each platter has tracks on both sides).

36. Why cylinders are important • If the data being written does not fit in a single track, it can be spread across the cylinder. • Activating different heads (at the same radius/cylinder) is an electronic process and thus is faster than moving the arm to a new radius, which is a mechanical process and thus slow.

37. Accessing a disk • The combination of application, operating system, BIOS and possibly disk interface circuitry determine the location of the data on the hard disk. • The location corresponds to a geometric location on the disk. One needs to know • The cylinder which determines the radius or track. • The head which determines which platter and which side of the platter. • The sector which determines where along the track.

38. Accessing a disk (Cont.) • See if the information is cached before starting the slower process of accessing it on the drive. • If the drive is not already spinning, it must be “spun up” to its working rotational speed. It might have been “spun down” to save energy. • The actuator moves the heads to the appropriate cylinder (track, radial position). • The actuator then selects the appropriate head and waits until the selected sector passes by the head. It then reads.

39. Accessing a disk (Cont.) • The information is read and placed temporarily into a buffer. • The hard disk interface then sends the data to some other part of the PC, in most cases the memory.

40. Cache on the Hard Disk • The hard disk has a cache/buffer. • After requesting data from the hard drive, one does not sit by idly waiting for the result. One carries on with whatever else possible. The result of the read is placed in a buffer and picked up when the processor/memory is ready to take it. • When reading, one also grabs the data in the neighboring sectors and places it in the buffer (pre-fetching). This is the standard idea of caching something you expect to need soon based on locality of reference.

41. Size and Distinction • The cache on the hard disk is typically between 512 KB and 2 MB. • Some SCSI drives may have as much as 16 MB. • Be careful not to confuse the cache on the hard drive with “disk cache” which refers to a section of main memory used to hold data recently read from the hard drive.

42. Seek and ye shall find, but be quick about it • Seek time is the time required to position the head to the selected cylinder. • Typical seek times are in milliseconds (ms) • Recall that processor times are in nanoseconds (ns, approx. a million times smaller) and memory times are in microseconds (s, approx. a thousand times smaller). • Seek time is not access time, but is probably the major part thereof.

43. Versions of Seek Time • Average: from a random track to another random track • This is what is typically reported • 8 – 10 ms • Track-to-track: from one track to the adjacent track • Full stroke: the full range from the innermost to outermost track

44. Settle Time • After the head has reached the appropriate cylinder (track, radius), it must take a short amount of time to stabilize before reading can occur. • This time is called the settle time or settling time. • It is short compared to seek time and does not vary much from manufacturer to manufacturer.

45. Command overhead time • Command overhead time is the time it takes from when the hard drive is given the read instruction to when the actuator starts positioning the head • Since this is an electronic time as opposed to the mechanical seek time, it is much smaller and does not contribute much to the access time.

46. Latency • After the instructions have been interpreted and the actuator begins to move (command overhead) and the head has reached the selected cylinder (seek) and has stabilized (settle), it is still not ready to read. • It must wait until the selected sector rotates past the head. This time is called the latency.

47. Latency and Spindle Speed • The time it must wait for the correct sector to swing by clearly depends on how fast the disks are rotating – the spindle speed. • If the spindles rotates at 10,000 RPM (revolutions per minute), then it rotates at speed of 10,000/60 = 166.7 revolutions per second. • If there are 166.7 revolutions per second, then a revolution takes 1/166.7 seconds = 0.006 s or 6 ms. • The average latency is half of the rotation time or in this case 3 ms.

48. PCGuide table

49. Ordering the to-do list • Because the hard drive is slower than the processor and memory, there may be a back up of tasks for it to perform. The order in which it performs these tasks can greatly affect its efficiency. • One ordering is a simple FIFO (first-in, first-out) ordering. The tasks (reads and writes) are queued up and the first task requested is the first task performed.

50. More Sophisticated Orderings • Seek-Time Optimization (a.k.a. Elevator seeking): • Seek time involves the radial positioning of the head. The tasks are ordered based on their radial positioning to minimize seek time. • Access-Time Optimization (a.k.a. multiple command reordering): • Takes into account both radial and angular positioning to minimize access time which includes seek time and latency.