Storage and Indexes Introduction to Databases Computer Science 557 Instructor: Joe Bockhorst University of Wisconsin - Milwaukee

Announcements Any problems logging in to course accounts? Use grid3, grid5 or weise for course work –Send bugs on weise to Reading Assignment: Chapter 14 in the textbook Program 1 assigned today, due next Friday

Hard Disks DBMS typically store information on “hard” disks I/O operations (“Read” and “Write”) are costly –Should be planned carefully A block is amount of data transferred in one operation (example block size ~ 1024 bytes) Disk Main Memory (RAM) Read Write

Our First Equation seek time rotational delay transfer time

Anatomy of a Hard Disk

Hard Drive Glossary Disks are divided into concentric circular tracks on each disk surface. –Track capacities vary typically from ~ 4 to 50 Kbytes The division of a track into sectors is hard-coded on the disk surface and cannot be changed A block is an integer number of sectors –The block size B is fixed for each system. Typical block sizes range from B=512 bytes to B=4096 bytes. –Whole blocks are transferred between disk and main memory

Typical Disk Parameters (Courtesy of Seagate Technology)

Accessing a Disk Block to Read or Write block give block addr to disk controller –Hardware block address – cylinder #, surface #, block # –Logical block addressing allows higher levels to refer to hardware block address using a block_id Seek time – move head to correct cylinder (~10ms) Rotation time – rotate start of block under head rpm, average rotation time is 4ms Transfer time – transfer entire block Accessing consecutive blocks only need to pay the seek and rotation time once Compare to typical main memory access times which are measured in micro (10 -6 ) or nano (10 -9 ) seconds

Coming soon: Solid State “Disks”? + Random access devices eliminate seek times + faster startup -$$$$ much more expensive than hard disks -($8 / GB vs $0.25 / GB) -Capacities are smaller We will assume hard disk storage in this course

Why not store DB in main memory? $$$$ –cost of RAM is > 100 X hard drive cost main memory is volatile 32 bit addressing

Managing the Hard Disk Query Optimization Relational Operators Files and Access Methods Buffer Management Disk Space Management The DSM provides an abstraction of the block as a unit of data DSM interface includes commands to read and write block commands I/O requests

Operations Supported by DSM allocate_blocks(num_blocks) –Add blocks to DB deallocate block(blockID) –Remove block from DB write_block(blockID, blockPtr) –Write block to disk read_block(blockID, blockPtr) –Read block from disk

Managing the Hard Disk Disk Space Manager 1 yes 23 block ID yes allocated? N-1N no **** hardware addr 6 no 7 **

Example: Managing the Hard Disk allocate_blocks (3) write_block(5,data) read_block(5) deallocate block(2) Disk Space Manager 1 yes 23 block ID yes allocated? N-1N no **** hardware addr 6 no 7 **

allocate_blocks (3) write_block(5,data) read_block(5) deallocate block(2) Example: Managing the Hard Disk Disk Space Manager 1 yes 23 block ID yes allocated? N-1N no 45 yes ** hardware addr 6 yes 7 no 904* //allocate three consecutive blocks

allocate_blocks (3) write_block(5,data) read_block(5) deallocate block(2) Example: Managing the Hard Disk Disk Space Manager 1 yes 23 block ID yes allocated? N-1N no 45 yes ** hardware addr 6 yes 7 no 904* //write blockID 5 to disk write_block(903,data)

allocate_blocks (3) write_block(5,data) read_block(5) deallocate block(2) Example: Managing the Hard Disk Disk Space Manager 1 yes 23 block ID yes allocated? N-1N no 45 yes ** hardware addr 6 yes 7 no 904* // read blockID 5 to buffer read_block(903)

allocate_blocks (3) write_block(5,data) read_block(5) deallocate block(2) Example: Managing the Hard Disk Disk Space Manager 1 yes 23 block ID noyes allocated? N-1N no 45 yes 792*794** hardware addr 6 yes 7 no 904* // read blockID 5 to buffer

Buffer Management Responsible for managing region of main memory called the buffer pool MM pages are called frames (slots that can hold one block) Higher levels of the DBMS need not worry if the page is in memory or not... Just ask for it. Query Optimization Relational Operators Files and Access Methods Disk Space Management Buffer Management

Buffer Manager Operations add_blocks_to_DB(num_blocks) –add new blocks to DB delete_block_from_DB(block_id) –delete block from the DB pin_block(block_id) –bring block from disk to buffer pool if not in BP –increment pin count for block unpin_block(block_id) –decrement pin count for block mark_dirty(block_id) Buffer Manager maintains for each frame –pin count –dirty bit

Buffer Manager Example buffer pool with M frames 123M-1M no block ID pin count dirty initial state of buffer manager 123N-1N

Buffer Manager Example buffer pool with M frames 123M-1M no block ID pin count dirty initial state of buffer manager draw on whiteboard 123N-1N

Buffer Manager Example add_blocks_to_DB(3) pin_block(76) pin_block(13) mark_dirty(13) pin_block(76) unpin_block(13) // now assume all frames are filled and blk 22 // is not in the buffer pool pin_block(22)// BufMgr flushes blk w/ pin count = 0 delete_block(35)