Understanding Storage Media: Magnetic Disks, Optical, and Magnetic Tape - Prof. David L. T, Study notes of Computer Architecture and Organization

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T and more Study notes Computer Architecture and Organization in PDF only on Docsity! 1 Storage Media – Page 1CSCI 4717 – Computer Architecture CSCI 4717/5717 Computer Architecture Topic: Storage Media Reading: Stallings, Chapter 6 Storage Media – Page 2CSCI 4717 – Computer Architecture Types of External Memory • Magnetic Disk – RAID – Removable • Optical – CD-ROM – CD-Recordable (CD-R) – CD-R/W – DVD • Magnetic Tape • Magnetic Disk Storage Media – Page 3CSCI 4717 – Computer Architecture Physical Disk • Disk substrate coated with magnetizable material (iron oxide…rust) • Substrate used to be aluminium – now glass – Improved surface uniformity -- Increases reliability – Reduction in surface defects -- Reduced read/write errors – Lower fly heights – Better stiffness – Better shock/damage resistance Storage Media – Page 4CSCI 4717 – Computer Architecture Read and Write Mechanisms Storage Media – Page 5CSCI 4717 – Computer Architecture Read and Write Mechanisms (continued) • Recording and retrieval via conductive coil(s) called a head(s) • May be single read/write head or separate ones • During read/write, head is stationary (actually moves radially to platters) and platter rotates beneath head Storage Media – Page 6CSCI 4717 – Computer Architecture Hard Drive Write • Current through coil produces magnetic field • Pulses sent to head • Magnetic pattern recorded on surface below 2 Storage Media – Page 7CSCI 4717 – Computer Architecture Hard Drive Read (traditional) • Magnetic field moving relative to coil produces current – Analogous to a generator or alternator • Coil can be the same for read and write • Used with: – Floppies – Older harddrives Storage Media – Page 8CSCI 4717 – Computer Architecture Hard Drive Read (contemporary) • Separate read head, close to write head • Partially shielded magneto resistive (MR) sensor • Electrical resistance depends on direction of magnetic field – Passing current through it results in different voltage levels for different resistances • High frequency operation -- Higher storage density and speed Storage Media – Page 9CSCI 4717 – Computer Architecture Data Organization and Formatting Storage Media – Page 10CSCI 4717 – Computer Architecture Data Organization and Formatting (continued) • Concentric rings or tracks • Track is same width as head • Thousands of tracks per platter surface • Intertrack gaps – Gaps between tracks protect data integrity • Reduce intertrack gap – increase capacity – possibly increase errors due to misalignment of head or interference from other tracks • Constant angular velocity – Same number of bits per track (variable packing density) Storage Media – Page 11CSCI 4717 – Computer Architecture Tracks divided into sectors • Minimum block size is one sector although may have more than one sector per block • Typically hundreds of sectors per track • May be fixed or variable in length • Contemporary systems are fixed-length with 512 bytes being common • Sectors also have gaps called intratrack or intersector gaps Storage Media – Page 12CSCI 4717 – Computer Architecture Constant Angular Velocity (CAV) • Imagine a matrix with the rows as tracks and the columns as sectors. • Twist matrix into a disk and see how much more packed the center is than the outside. • Creates pie shaped sectors and concentric tracks • Regardless of head position, sectors pass beneath it at the same (constant) speed • Capacity limited by density on inside track • Outer tracks waste with lower data density 5 Storage Media – Page 25CSCI 4717 – Computer Architecture FM Encoding • A magnetic field change at the beginning and middle of a bit time represents a logic one • A a magnetic field change only at the beginning represents a logic zero • Referred to as Frequency Modulation (FM) Storage Media – Page 26CSCI 4717 – Computer Architecture MFM Encoding • Just like FM except that changes at beginning of bit time are removed unless two 0’s are next to each other • Called Modified Frequency Modulation (MFM) Storage Media – Page 27CSCI 4717 – Computer Architecture RLL Encoding Goals of encoding: • to ensure enough polarity changes to maintain bit synchronization; • to ensure enough bit sequences are defined so that any sequence of ones and zeros can be handled; and • to allow for the highest number of bits to be represented with the fewest number of polarity changes Storage Media – Page 28CSCI 4717 – Computer Architecture RLL Encoding (continued) Run Length Limited (RLL) uses polarity changes to represent sequences of bits rather than individual 0’s or 1’s Storage Media – Page 29CSCI 4717 – Computer Architecture RLL Encoding (continued) • Note that the shortest period between polarity changes is one and a half bit periods. • This produces a 50% increased data density over MFM encoding. Storage Media – Page 30CSCI 4717 – Computer Architecture Latest Encoding Technology • Improved encoding methods have been introduced since the development of RLL • Use digital signal processing and other methods to realize better data densities. • These methods include Partial Response, Maximum Likelihood (PRML) and Extended PRML (EPRML) encoding. 6 Storage Media – Page 31CSCI 4717 – Computer Architecture S.M.A.R.T. • Self-Monitoring, Analysis & Reporting Technology System (S.M.A.R.T.) is a method used to predict hard drive failures • Controller monitors hard drive functional parameters • For example, longer spin-up times may indicate that the bearings are going bad • S.M.A.R.T. enabled drives can provide an alert to the computer's BIOS warning of a parameter that is functioning outside of its normal range • Attribute values are stored in the hard drive as an integer in the range from 1 to 253. The lower the value, the worse the condition is. • Depending on the parameter and the manufacturer, different failure thresholds are set for each of the parameters. Storage Media – Page 32CSCI 4717 – Computer Architecture Sample S.M.A.R.T. Parameters • Power On Hours: This indicates the age of the drive. • Spin Up Time: A longer spin up time may indicate a problem with the assembly that spins the platters. • Temperature: Higher temperatures also might indicate a problem with the assembly that spins the platters. • Head Flying Height: A reduction in the flying height of a Winchester head may indicate it is about to crash into the platters. • Doesn’t cover all possible failures: IC failure or a failure caused by a catastrophic event Storage Media – Page 33CSCI 4717 – Computer Architecture Speed • Queuing time – waiting for I/O device to be useable – Waiting for device – if device is serving another request – Waiting for channel – if device shares a channel with other devices (multiplexing) • Disk rotating at a constant speed (energy saver – disk may stop) Storage Media – Page 34CSCI 4717 – Computer Architecture Seek time Process of finding data on a disk • Find correct track by moving head (moveable head) • Selecting head (fixed head) takes no time • Some details cannot be pinned down – Ramping functions – Distance between current track and desired track – Shorter distances and lighter components have reduced seek time Storage Media – Page 35CSCI 4717 – Computer Architecture Rotational Latency Waiting for data to rotate under head – Floppies – 3600 RPM – Hard Drives – up to 15,000 RMP – Average rotational delay is 1/2 time for full rotation • Total Access time = Seek + Latency Storage Media – Page 36CSCI 4717 – Computer Architecture Transfer Time Transfer time = time it takes to retrieve the data as it passes under the head T = b/(rN) where – T = transfer time – b = number of bytes to transfer – N = number of bytes on a track (i.e., bytes per full revolution) – r = rotation speed in RPS (i.e., tracks per second) 7 Storage Media – Page 37CSCI 4717 – Computer Architecture Rotational Position Sensing (RPS) • Allows other devices to use I/O channel while seek is in process. • When seek is complete, device predicts when data will pass under heads • At a fixed time before data is expected to come, tries to re-establish communications with requesting processor – if fails to reconnect, must wait full disk turn before new attempt is made: RPS miss Storage Media – Page 38CSCI 4717 – Computer Architecture Random access • File is arranged in contiguous sectors – only one seek time per track • File is scattered to different sectors or device is shared with multiple processes – seek time increased to once per sector Storage Media – Page 39CSCI 4717 – Computer Architecture Redundant Array of Independent Disks (RAID) • Rate of improvement in secondary storage has not kept up with that of processors or main memory • In many system, gains can be had through parallel systems • In disk systems, multiple requests can be serviced concurrently if there are multiple disks and the data for parallel requests is stored on different disks Storage Media – Page 40CSCI 4717 – Computer Architecture RAID (continued) Standardization of multi-disk arrays • 7 levels (0 through 6) • Not a hierarchy • Common characteristics – Set of physical disks viewed as single logical drive by O/S – Data distributed across multiple physical drives of array – Can use redundant capacity to store parity information to aid in error correction/detection • Third characteristic is needed because multiple mechanisms mean that there are more possibilities for failure Storage Media – Page 41CSCI 4717 – Computer Architecture Striping • User's data and applications see one logical drive • Data is divided into strips – Could be physical blocks, sectors, or some other unit – The strips are then mapped to the different physical drives Storage Media – Page 42CSCI 4717 – Computer Architecture Striping (continued) 10 Storage Media – Page 55CSCI 4717 – Computer Architecture RAID 4 (continued) • Original parity calculation X4(i) = X3(i) ⊕ X2(i) ⊕ X1(i) ⊕ X0(i) • New bit is stored (e.g., X1(i)) – parity is recalculated: X4'(i) = X3(i) ⊕ X2(i) ⊕ X1'(i) ⊕ X0(i) X4'(i) = X3(i) ⊕ X2(i) ⊕ X1'(i) ⊕ X0(i) ⊕ X1(i) ⊕ X1(i) X4'(i) = X4(i) ⊕ X1(i) ⊕ X1'(i) Storage Media – Page 56CSCI 4717 – Computer Architecture RAID 4 (continued) Storage Media – Page 57CSCI 4717 – Computer Architecture RAID 5 • Like RAID 4 except drops parity disk • Parity strips are staggered across all data disks • Round robin allocation for parity stripe • Avoids RAID 4 bottleneck at parity disk • Commonly used in network servers Storage Media – Page 58CSCI 4717 – Computer Architecture RAID 5 (continued) Storage Media – Page 59CSCI 4717 – Computer Architecture RAID 6 • Two parity calculations • XOR parity is one of them • Independent data check algorithm • Stored in separate blocks on different disks User requirement of N disks needs N+2 • High data availability • Three disks need to fail for data loss • Significant write penalty Storage Media – Page 60CSCI 4717 – Computer Architecture RAID 6 (continued) 11 Storage Media – Page 61CSCI 4717 – Computer Architecture RAID Summary