CS162 Notes Wednesday 3/14 Topic - Finish I/O device characteristics ----- -Disk drives - Form factors: + 2.5": used in laptops, maximum capacity around 160GB + 3.5": used in desktops and servers, maximum capacity around 750GB + 5.25": obsolete - A hard drive's capacity is directly proportional to the surface area available on its platters and the areal density that the drive was made with. (Drives with smaller form factors have less surface area on its platters.) - Research drives may have faster transfer rates than consumer drives. However, since transfers are directly proportional to the drive's rotational speeds, these research drives are more prone to failure. These drives are enclosed in bulletproof glass or metal for safety reasons (the drive casing can contain a platter explosion). + Laptop drives typically spin at 4200-7200rpm + Consumer drives typically spin at 4200-10000rpm + Server drives typically spin at 10000-15000rpm - Other factors that contribute to transfer rate: + seek time: lower seek times means the head locates the track faster + areal density: more bits per unit area means that the head can read more data at once + queueing algorithms: to handle multiple track requests efficiently + cache size: modern drives have 2-16MB of cache to help prefetch data and as a buffer for writing data to disk. - As technology improves, the MTBF (mean time before failure) of manufactured hard drives has increased and has exceeded 1 million hours in some cases. Unfortunately, these claims may not be realistic and recent studies have shown that MTBF ratings are exaggerated for both consumer and server grade drives. - Power concerns + Drives that spin faster and move their heads faster drain more power + Drives consume different amounts of power, depending what state they are in (eg. idle, seeking, reading/writing) + Laptop drives tradeoff between speed and power consumed, which is a function of the required voltage to run the drive - Shock tolerance + During normal operation, a hard drive's read/write head hovers just microns or nanometers above the platter. A crashed head will physically destroy the data recorded on the disk. + To further guard against crashing heads, hard drive manufacturers design hard drives to withstand a certain amount of acceleration. Laptop drives are rated to withstand approximately 150G during operation and 1000G when not running. Desktop drives have a lower tolerance since they aren't meant to be moved often -- approximately 60G during operation and 350G when not running. - Platter defects + All the sectors of a hard drive are tested by a manufacturer before shipping + Bad sectors are remapped by the hard drive controller to alternative tracks, remapped to alternative sectors, or just skipped over by renumbering the sectors on that track - Hard drives now use a logical, linear address space, which is mapped by a controller. - Modern hard drives have a varying amount of bits on each track, with more bits on the outer tracks since there is more track space. -Alternative drives - Solid State Disk (SSD) + Instead of using magnetic platters to record data, solid state disks record data using MOS transistors with floating gates. By applying a high voltage to either terminals of the transistor, charge can be forced onto or off of the floating gate. + Sandisk recently announced a 32GB 2.5" SSD targeted for laptops + Since there are no moving parts in a SSD, they are expected to last longer than hard drives and have lower random access times than hard drives. - Drums rotates | | ------- |------|- |------|- read/write heads |------|- |______|- | | + Idea behind drums was to eliminate the seek time from hard drives. + They could do this by making as many read/write heads as there were tracks. + As the drum rotated, the read/write heads would work in parallel to read data. + Drums were very expensive and lagged behind regular hard drives in capacity. So they never quite caught on. -Optical drives - Idea: bits are stored as a series of pits and lands in a spiral (or concentric circles) on a reflective material. A laser aimed at the reflective material distinguishes between 1's and 0's depending on the diffraction it picks up. (Magneto-optical uses weak magnetic fields though.) - Original drives were meant to read audio CDs and read data at 150Kb/s (1x) - The physical construction of CDs do not allow for speeds faster than 12x without having the CD shatter. However, drives may achieve faster transfer rates by using multiple read lasers. - Optical drives have similar access patterns as hard drives: seek time, rotational latency, and data transfer - Magneto-optical + A weak laser first heated up the disc before a stronger write laser actually went through to write the bits. + Naturally rewritable - DVD + Similar to existing CDs, but with a much denser pattern of bits. + Capacities of 4.5GB, 9GB, or 18GB depending on the number of layers used and whether the layers are single-sided or double-sided + Problem with red laser is that you can't focus on anything smaller than the wavelength. - Blue Ray + Tried to improve on DVD's data density by switching to a blue laser. + Blue has a shorter wavelength than red, so the bits recorded onto a Blue Ray disc are smaller. - HD DVD + Improves on DVD's capacity, but doesn't reach Blue Ray's storage capacity. + However, easier to convert existing DVD construction layers to HD DVD lines - Network Drives - NAS - Network Attached Storage + Box that connectors by a link-layer medium (eg. Ethernet) to a network + Users on the network see the NAS as another computer with a file system - SAN - Storage Area Network + More sophisticated than a NAS + A separate network containing storage, linked together with fast link-layer mediums such as fiber optic + Users send their data off to a storage network, rather than a computer