1. ECE 456 Computer Architecture Lecture #7 – External (Secondary) Memory Instructor: Dr. Honggang Wang Fall 2013

2. Administrative Issues (11/04/13, Monday) Exam #1 –Keep working on your Project –due Wednesday, Dec. 4 –Report guidelines are available from the class website This lecture –Self-study slide# 6-21 –Focus on RAID Dr. Wang2

3. Review of Lecture #6 In the last lecture, we covered the …... Cache motivation: performance imbalance Basic idea: holds the most recently/frequently used memory words in a small, fast memory Basis: principle of locality - spatial & temporal Cache/main memory structure and organization Cache design issues such as –cache size, mapping functions (3), replacement algorithms (4), write policy (2), cache line size, single-level and multilevel caches, unified and split caches Dr. Wang3

4. CPU CACHECACHE MAIN MEMORY I/O STORAGE DEVICES Registers Internal Memory External Memory plus Virtual Memory Defer to CPU discussion Today’s topic: External Memory Dr. Wang4

5. Memory of Interest Location –external memory Physical types –magnetic / optical memory Physical characteristics –non-volatile Access methods –sequential / direct access memory Dr. Wang5

6. Topics Magnetic surface memory –Magnetic disk –Magnetic tape Redundant Array of Independent Disks (RAID) Optical external memory Dr. Wang6

7. Magnetic Disk Dr. Wang7

8. Magnetic Disk Structure (1) Substrate: a circular platter constructed of non- magnetic material –aluminum or aluminum alloy (traditionally) –glass (now) improved surface uniformity reduction in surface defects better stiffness better shock/damage resistance Disk substrate is coated with magnetizable material Magnetic Disks Dr. Wang8

9. Magnetic Disk Structure (2) A single read/write head or separate ones –A rectangular doughnut shape magnetizable material with a conductive coil –Head is stationary, platter rotates beneath it during R/W –Two directions of current flowing through the coil produce two directions of magnetization on the recording medium, for representing binary 1 and 0 (write) –A magnetic field moving relative to a coil produces a current; when disk surface passes under the head, a current of the same polarity as the recorded one is generated (read) Recording medium Magnetic Disks Dr. Wang9

10. Disk Data Organization Concentric rings/tracks –same width as the head –thousands of tracks per surface –gaps between tracks –divided into sectors –typically hundreds of sectors per track –gaps between sectors Unit of transfer –Block/record –Minimum block size is 1 sector Magnetic Disks Dr. Wang10

11. Disk Formatting To identify start of track and sector Additional information used only by disk drive and not available to user ST506: 30 sector/track, 600 bytes/sector Gap1 17 Id 7 Gap2 41 Data 515 Gap3 20 Gap1 IdGap2DataGap3 Track # 2 Sync Byte 1 Head # 1 Sector # 1 CRC 2 Sync Byte 1 Data 512 CRC 2 600 bytes Dr. Wang11

12. Disk Physical Characteristics Sides Platters Head Motion & Mechanism Disk Portability Dr. Wang12

13. Sides Double-sided disks –Magnetizable coating applied to both sides of platter Single-sided disks –Magnetizable coating applied to one side of platter –Less expensive Magnetic Disk Physical Characteristics (1) Dr. Wang13

14. Magnetic Disk Physical Characteristics (2) Multiple Platters Cylinders Dr. Wang14

15. Head Motion Fixed head –One read-write head per track –Heads mounted on a rigid arm that extends across all tracks Movable head –One read-write head per side –Mounted on a movable arm According to the position of read-write head with respect to the platter –Fixed gap: positioned a fixed distance above the platter –Contact: come into physical contact with the platter medium during a r/w operation –Flying: rest lightly on the platter’s surface when the disk is motionless. The air pressure generated by a spinning disk is enough to make the foil rise above the face. Magnetic Disk Physical Characteristics (3) Head Mechanism Dr. Wang15

16. Disk Portability Non-removable disk –Permanently mounted in the drive –Hard disks Removable disk –Can be removed from drive and replaced with another disk –Provides unlimited storage capacity –Easy data transfer between systems –Floppy disks, ZIP disks Magnetic Disk Physical Characteristics (4) Dr. Wang16

17. Disk Performance Parameters Queuing delays: time to wait for the device, I/O channel Seek time T s : move the disk arm/head to required track Rotational delay T r =1/(2r): wait for data to rotate under head Access time = seek time + rotational delay Transfer time T t =b/(rN) Total average access time: T a = T s + T r + T t Timing of Disk I/O Transfer Access Time Dr. Wang17

18. Hands-On Problem Consider a disk system with an average seek time of 4ms, the rotation speed of the disk in revolutions per minute (rpm) is 15000. Each sector consists of 512 Bytes, and each track consists of 400 sectors. What is the total time to access 20KBytes data? Dr. Wang18

19. Dr. Wang19

20. Magnetic Disk Direct-Access Individual blocks/sectors have a unique address (ID field) Disk drive locates the desired sector within one track, then makes successive accesses Accessing time depends on location of data & previous location Dr. Wang20

21. Magnetic Tape Medium is flexible polyester tape coated with magnetizable material Recording techniques –Parallel recording (old): data are structured as a number of parallel tracks running length wise –Serial recording (temporary, same as magnetic disks): data are laid out as a sequence of bits along each track Tape formatting assists in locating physical records Sequential access Slow, cheap Dr. Wang21

22. Topics Magnetic surface memory –Magnetic disk –Magnetic tape Redundant Array of Independent Disks (RAID) Optical external memory Dr. Wang22

23. RAID Motivation Performance imbalance exists between secondary storage and main memory / processor Using multiple parallel smaller-capacity disk drives to replace a large-capacity disk drive: –Redundant Array of Independent Disks –Redundant Array of Inexpensive Disks Separate I/O requests can be handled in parallel A single I/O request can be executed in parallel if data distributed across multiple disks. Dr. Wang23

24. RAID A standardized scheme: RAID –7 levels: RAID 0  ……  RAID 6 –Not a hierarchy –A set of physical disk drives viewed as a single logical drive by OS –Data distributed across physical drives –Can use redundant disk capacity to store parity information Dr. Wang24

25. RAID 0 (Nonredundant) No redundancy Data striped across all disks Round Robin striping Increased I/O speed Dr. Wang25

26. RAID 1 (Mirrored) Data is striped across disks 2 copies of each stripe on separate disks (mirrored disks) Read from either, write to both (can be done in parallel) Recovery is simple –Swap faulty disk & re-mirror –No down time Expensive Dr. Wang26

27. RAID 2 (Redundancy via Hamming) Data is striped across disks, but using very small stripes (single byte/word) Error correction calculated across corresponding bits on disks Multiple parity disks store error correction Hamming code bits in corresponding positions Read from all, write to all Lots of redundancy  expensive Dr. Wang27

28. RAID 3 (Bit-Interleaved Parity) Similar to RAID 2 Only one redundant disk, no matter how large the array Simple parity bit for each set of corresponding bits Data on failed drive can be reconstructed from surviving data and parity information Dr. Wang28

29. RAID 4 (Block-Level Parity) Each disk operates independently Large stripes Bit by bit parity calculated across stripes on each disk Parity stored in the corresponding strip on parity disk Dr. Wang29

30. RAID 5 (Bock-Level Distributed Parity) Like RAID 4 Parity striped across all disks Round robin allocation for parity stripe Dr. Wang30

31. RAID 6 (Dual-Redundancy) Two parity calculations Stored in separate blocks on different disks User requirement of N disks needs N+2 Dr. Wang31

32. RAID Levels Summary Dr. Wang32

33. Topics Magnetic surface memory –Magnetic disk –Magnetic tape Redundant Array of Independent Disks (RAID) Optical external memory Dr. Wang33

34. Optical Disk Products Compact Disk (CD): non-erasable disk, 12cm diameter, >=60 minutes audio information CD-ROM: non-erasable disk, 12cm diameter, >=650MB capacity, error correction logic CD-Recordable (CD-R): WORM, can write to disk only once CD-Rewritable (CD-RW): can erase and rewrite multiple times Digital Versatile Disk (DVD) ROM: 8/12 cm diameters, >=17GB double-sided capacity, for video & large volumes of digital data DVD-R: can write to disk once, only one-sided disks used DVD-RW: can write multiple times, only one-sided disks used Dr. Wang34

35. Data Organization A single spiral track beginning near disk center and spiraling out to the outer edge Data are organized as blocks/sectors Information packed evenly across the disk Constant linear velocity (CLV): disk rotates more slowly for accesses near the outer edge than for those near the center Dr. Wang35

36. Block Format Mode 0=blank data field Mode 1=2048 byte data+error correction Mode 2=2336 byte data Dr. Wang36

37. Summary of Lecture #7 Magnetic surface memory –Magnetic disk –Magnetic tape Redundant Array of Independent Disks (RAID) –RAID 0 to RAID6 Optical external memory –CD, CD-ROM, CD-R, CD-RW, DVD, DVD-R, DVD-RW Key points: the calculation of total access time for magnetic disks Dr. Wang37

38. Memory Lectures CPU CACHECACHE MAIN MEMORY I/O STORAGE DEVICES Registers Internal Memory External Memory OS level Not real Defer to CPU discussion VIRTUAL MEMORY Next… Next topic: Virtual Memory Dr. Wang38

39. Things To Do Project –Report due Monday, Dec. 4 –Proposal guidelines are available from the class website Check out the class website about –lecture notes –the project Dr. Wang39

40. Accessing Method (Review) Sequential access –access is made in a specific linear sequence –example: tape Direct access –individual blocks have a unique address –access is by jumping to a vicinity plus sequential searching –example: disk – a shared read/write mechanism is used – accessing time depends on location of data & previous location …...... records / blocks read/write mechanism A Dr. Wang40

41. CD vs. DVD DVD has greater capacity due to –Bits are packed more closely on a DVD –Can employ dual-layer –Can be two-sided Dr. Wang41