1 / 12

Disks and Files

Disks and Files. DBMS stores information on (“hard”) disks. 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 must be planned carefully!.

paul
Download Presentation

Disks and Files

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Disks and Files • DBMS stores information on (“hard”) disks. • 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 must be planned carefully!

  2. Why Not Store Everything in Main Memory? • Costs too much. $100 will buy you either 512MB of RAM or 50GB of disk today --- that is disk storage 100 times cheaper(but it is approx. 10000 times slower). • Main memory is volatile. We want data to be saved between runs. (Obviously!) • Typical storage hierarchy: • Main memory (RAM) for currently used data. • Disk for the main database (secondary storage). • Tapes for archiving older versions of the data (tertiary storage).

  3. Tracks Arm movement Arm assembly Components of a Disk Spindle Disk head • The platters spin (say, 90rps). • The arm assembly is moved in or out to position a head on a desired track. Tracks under heads make a cylinder(imaginary!). Sector Platters • Only one head reads/writes at any one time. • Block size is a multiple of sector size (which is fixed).

  4. Accessing a Disk Page • Time to access (read/write) a disk block: • seek time (moving arms to position disk head on track) • rotational delay (waiting for block to rotate under head) • transfer time (actually moving data to/from disk surface) • Seek time and rotational delay dominate. • Seek time varies from about 1 to 20msec • Rotational delay varies from 0 to 10msec • Transfer rate is about 1msec per 32KB page

  5. Latency = Queuing Time + Controller time + Seek Time + Rotation Time + Size / Bandwidth { per access + per byte State of the Art: Barracuda 180 • 181.6 GB, 3.5 inch disk • 12 platters, 24 surfaces • 24,247 cylinders • 7,200 RPM; (4.2 ms avg. latency) • 7.4/8.2 ms avg. seek (r/w) • 64 to 35 MB/s (internal) • 0.1 ms controller time • 10.3 watts (idle) Track Sector Cylinder Track Buffer Arm Platter Head source: www.seagate.com

  6. Disk Performance • Calculate time to read 64 KB (128 sectors) for Barracuda 180 X using advertised performance; sector is on outer track Disk latency = average seek time + average rotational delay + transfer time + controller overhead = 7.4 ms + 0.5 * 1/(7200 RPM) + 64 KB / (65 MB/s) + 0.1 ms = 7.4 ms + 0.5 /(7200 RPM/(60000ms/M)) + 64 KB / (65 KB/ms) + 0.1 ms = 7.4 + 4.2 + 1.0 + 0.1 ms = 12.7 ms

  7. 1 inch disk drive! • 2000 IBM MicroDrive: • 1.7” x 1.4” x 0.2” • 1 GB, 3600 RPM, 5 MB/s, 15 ms seek • Digital camera, PalmPC? • 2006 MicroDrive? • 9 GB, 50 MB/s! • Assuming it finds a niche in a successful product • Assuming past trends continue

  8. Disk Characteristics in 2000 $828 $447 $435

  9. Disk Characteristics in 2000

  10. Disk Characteristics in 2000

  11. Disk Characteristics in 2000

  12. RAID Technology • Disk Array: Arrangement of several disks that gives abstraction of a single, large disk. • Goals: Increase performance and reliability. • Two main techniques: • Data striping: Data is partitioned; size of a partition is called the striping unit. Partitions are distributed over several disks. • Redundancy: More disks  more failures. Redundant information allows reconstruction of data if a disk fails.

More Related