- Secondary holding contraption of select.
- Main profit completed tapes: unsystematic right vs. successive.
- Data is keep and retrieved in units called disk blocks or pages.
- Unlike RAM, example to find a round shape leaf varies depending upon situation on disk.
- Therefore, comparative spatial relation of pages on disk has through impact on DBMS performance!
Components of a Disk
- The platters wheel (say, 90rps).
- The arm congress is moved in or out to post a commander on a in demand line. Tracks lower than heads variety a cylinder (imaginary!).
- Only one manager reads/writes at any one occurrence.
- Block extent is a triune of sector volume (which is predetermined).
Accessing a Disk Page
- Time to accession (read/write) a disc block:
- seek clip (moving instrumentality to lines disk external body part on path)
- rotational delay (waiting for jam to spin underneath person in charge)
- transfer instance (actually emotional collection to/from disc on the surface)
- Seek example and move hitch reign.
- Seek incident varies from going on for 1 to 20msec
- Rotational bottleneck varies from 0 to 10msec
- Transfer charge per unit is nearly 1msec per 4KB page
- Key to less I/O cost: lessen seek/rotation delays! Hardware vs. code solutions?
Arranging Pages on Disk
- 'Next' jam concept:
- blocks on self track, followed by
- blocks on same cylinder, followed by
- blocks on adjacent container
- Blocks in a directory should be systematic consecutive on saucer (by 'next'), to minimise want and motility hindrance.
- For a ongoing scan, pre-fetching various pages at a clip is a big win!
Skinomi TechSkin - Screen Protector Ultra Clear Shield for / MYBAT Titanium Solid Dark Blue Phone Protector Cover for / Purple Heart Snap on Hard Skin Faceplate Cover Case for / Uniden D3280-2 Cordless Phone/Answering System with 2 / Black Soft Gel TPU Skin Case for iPhone 4 (Fits / Autumn Protector Case for Samsung Brightside SCH-U380 / Samsung OEM Original USB Data Cable For Samsung / Black Cassette Flexible Silicon Case Cover Skin for HTC / SNDplace Motorola Droid 4 XT894 Hard Rubberized Case-Hot / Brainydeal LCD Screen Display , Touch Screen Digitizer / Three LCD Screen Guards / Protectors for LG Optimus T / / Hyperion Boost Mobile LG Marquee 3500mAh Extended Battery / CyberTech Micro USB to HDMI MHL Adapter for Samsung I997 / OtterBox iPhone 4S/4 Defender Case, Black Plastic / / Original IPhone 4 Battery 616-0513 LIS1445APPC 616-0520 / NEW BLACK RUBBERIZED HARD CASE + BELT CLIP HOLSTER FOR / HHI Sports Armband for Apple iPhone 5 - Black (PackagePost ads:
Chivel Black Protector Bumper Case Frame Cover for Apple / Samsung Galaxy S Epic 4G (Sprint) Black Rubber Feel Hard / Trident Case AG-iPad2-RV2-RD Aegis Case for iPad 2 - 1 / LG 300G Prepaid Phone (Net10) with 300 Minutes Included / 5 x iPhone 5 Screen Protector Crystal Clear / Trident Case AMS-I9300-PK KRAKEN AMS Series Protective / Motorola Droid Razr M XT907 (Verizon) Two pack Combo - / iLuv iCP751SBLU Peanuts Character Case for iPhone 4/4S / For Motorola Droid RAZR M 4G LTE XT907 (Verizon) / Hard Protector Skin Cover Cell Phone Case for LG Optimus S / White LCD Touch Screen + Digitizer + front Glass Assembly / Luminous Bluetooth Slider Qwerty Keyboard Case for Iphone / Samsung EB-F1A2GBU 1650 mAh Battery for Samsung Galaxy S / 5x HTC Vivid Holiday 4G Premium Invisible Clear LCD Screen / OEM Blackberry Curve 3G 9330 Standard C-S2 Battery / Anti-Glare Screen Guard for SAMSUNG Samsung Galaxy Rush / Amzer Emergency Portable Charger with 5 Adapters