CPSC 404 Unit 1: Disk Storage

Transcription

1 CPSC 404 Unit 1: Disk Storage Based on: Ramakrishnan & Gehrke, Database Management Systems (our textbook), Chapter9, pp ) Bryant & O Hallaron, Computer Systems (pp ) Some ideas also come frpm: Garcia-Molina, Ullman, & Widom, Database Systems: The Complete Book Original slides by Ed Knorr; Updates and changes by George Tsiknis

2 Learning Goals Explain the impact that disk activity has on DBMS query performance. Draw the memory hierarchy. Show where database bottlenecks are most likely to occur and where extensive caching takes place. Compare and contrast: cost, capacity, and speed of access, in the levels of the memory hierarchy. Identify the components of a disk drive. Given disk geometry figures, calculate the amount of time that it takes to read or write a number of bytes, blocks/pages, tracks, or cylinders of data onto a disk. Given disk geometry figures, compute the minimum, average, and maximum seek, rotation, and transfer times to/from a disk. Compare and contrast the relative speeds of seek, rotation, and transfer times when accessing a given size of data on disk. Unit 1 2

3 Learning Goals (cont.) Explain how a large file, broken up into pages, can be optimally placed on a disk to improve performance. Compare and contrast hard disk drives (HDDs) to solid state disks (SSDs). Discuss performance implications. Defend the ongoing role of hard disk drives in the DBMS world (e.g., explain why we can t eliminate spinning, hard disk drives anytime soon). Explain the difference between a random read and a sequential read, and argue why one is preferable over the other. Argue for or against: It is worth spending a fair bit of DBA time on: analyzing disk geometries/usage, table/index design, and I/O costs of database transactions/utilities. Unit 1 3

4 Disks and DBMS Files A DBMS stores information on hard disk drives, and uses the disks extensively. This has major implications for DBMS design. READ: transfer data from disk to main memory (RAM) WRITE: transfer data from RAM to disk Both are high-cost operations (relative to in-memory operations); so, I/Os must be planned carefully! Data is stored and retrieved in units called blocks or pages. Unlike RAM, the time to retrieve a disk page varies depending upon its location on disk Therefore, relative placement of pages on disk has major impact on DBMS performance! Unit 1 4

5 Relationships among Files, Disks, RAM, Buffer Pool(s), DBMS, OS RAM CPU OS User Program and Data DB DBMS Buffer Pool page or block Unit 1 5

6 Compare Disk to RAM Cost: Compare the cost of a hard disk drive to the cost of RAM : Hard Disk: $100 for 2 TB RAM: $100 for 4-32 GB We may have many GB of data, but not all of it needs to be in memory. It s very time-consuming to load it all. Volatility: RAM is volatile, but we want data to be saved between runs. Storage Hierarchy Main storage = RAM for currently used data SRAM is faster than DRAM, but more expensive Secondary storage = Disk for most data Tertiary storage = Disk, Tape, etc. (for archives) Unit 1 6

7 Typical Memory Hierarchy (source: B&O 11) L0: registers CPU registers hold words retrieved from L1 cache Smaller, faster, costlier per byte Larger, slower, cheaper per byte L4: L3: L2: L1: on-chip L1 cache (SRAM) off-chip L2 cache (SRAM) main memory (DRAM) local secondary storage (local disks) L1 cache holds cache lines retrieved from L2 cache L2 cache holds cache lines retrieved from main memory Main memory holds disk blocks retrieved from local disks Local disks hold files retrieved from disks on remote network servers L5: remote secondary storage (tapes, distributed file systems, Web servers) Unit 1 7

8 Components of a Disk Drive The platters spin (e.g., rpm) The whole arm assembly is moved in, or out, to position a head on a desired track, k. All tracks with the same track # (e.g., track k) make up an imaginary cylinder, k. Only one head reads/writes at any one time. Arm assembly Disk Head Arm Movement Spindle Tracks Platters A sector is minimum unit of data the disk can read or write. A (logical) block is a set of consecutive sectors; minimum transfer unit for a DBMS. Sector

9 Accessing a Disk Block The access time (i.e., time to read or write a disk block (sometimes called a page)) is typically made up of 3 components: 1. Seek time: move arm to position the disk head at a cyl/track 2. Rotational delay or latency: wait for block to rotate under head 3. Transfer time: move data between disk and RAM) Seek time and rotational delay dominate. Seek time varies from about 1-20 ms Rotational delay varies from about 0-10 ms The transfer rate is usually no more than 1 ms per 4K page, and often much less. It depends on the rotation rate The time to transfer a piece of data is the time needed for the piece of data to pass under the head. Unit 1 9

10 Accessing a Disk Block (cont ) Consider a disk with the following parameters: RPM = rotation rate in rotations per minute f = number of surfaces = platters x surfaces per platter c = number of cylinders (or tracks per surface) s = average sectors per track b = number of bytes per sector AST = average seek time = time to cross 1/3 of cylinders disk capacity = c f s b bytes rotation time = 1/RPM min = 60,000/RPM ms rotational delay = ½ rotation time transfer time per sector = rotation time / s access time = seek time + rotational delay + transfer time Unit 1 10

11 Example of a Disk Geometry Megatron 747: rpm = platters of 2 surfaces each c = 2 13 = 8192 cylinders s = average # sectors/track = 2 8 = 256 b = #bytes/sector = 2 9 = 512 (0.5K) B = block size = 4096 bytes (4K) moving head assembly between cylinders = 1 ms (setup time) + 1 ms/500 cylinders 1 Unit 1 11

12 Example (cont.) What is the max/avg seek time? What is the max/avg rotational latency time? Unit 1 12

13 Example (cont.) What is the block transfer time (i.e., time to read/write a block)? What is the disk capacity? Unit 1 13

14 Arranging Pages on Disk Intelligently Key to lowering I/O costs? Reduce seek and rotation time! The next block concept. We prefer to access: Blocks on same track, followed by Blocks on same cylinder, followed by Blocks on adjacent cylinders In other words, blocks in a file should be arranged sequentially on disk (by next order), so as to minimize seek and rotational delays. Unit ,3

15 Arranging Pages on Disk (cont.) For a sequential scan of data pages, prefetching additional pages helps performance. e.g., read 32 or more pages at a time For some queries, a user may want to see only the first few rows of a table; but maybe after seeing the first few rows DBMS may be able to predict what the user wants to do next hence, the term anticipatory prefetch. Unit 1 15

16 Solid State Disks (SSDs) Flash memory is based on EEPROMs (Electrically Erasable Programmable ROM) e.g., USB memory sticks, MP3 players, cameras, cell phones, PDAs Can be reprogrammed about 10 5 times SSD is a storage technology based on flash memory Advantages No spinning disks or moving parts Less likely to be damaged Lower power requirements Less heat; therefore, less cooling needed Quieter Faster reads Unit 1 16

17 Solid State Disks (cont.) Disadvantages: Cost (e.g., 100x more expensive per byte; prices decrease fast) Smaller Capacity (typically 100x less, due to cost) Random writes are slower than reads because you can t just write a single, changed page p to disk; you have to write a whole bank (chunk) X of data. e.g., block X is 0.5 MB; you have to copy X and identify changes, then erase the whole original block (e.g., 1 ms) before re-writing Limited number of write cycles (about 100,000), and then the block of memory can t be used anymore SSDs probably only get us 1 order of magnitude closer to the speed of RAM. Recall: 5 orders of magnitude difference between RAM and HDD A big improvement, but there s still a big gap! Unit 1 17

18 Summary RAM vs. Disk: About 5 orders of magnitude difference in speeds (nanoseconds vs. milliseconds) Disk: Much cheaper and more plentiful Memory is getting faster and more affordable, but data grows at an even faster rate! SSDs Solve some problems, but have some of their own HDDs will be with us for a long time yet. We see more computers/devices with SSDs these days. Organizing the data on the disk to take advantage of how disk works can speed up certain data operations Can make a big difference between a random read and a sequential read. Unit 1 18