Disks - Detailed Introduction

Disks - Detailed Introduction

• Secondary storage device of choice.
• Main advantage over tapes: random access vs. sequential.
• Data is stored and retrieved in units called disk blocks or pages.
• Unlike RAM, time to retrieve a disk page varies depending upon location on disk.
  
  • Therefore, relative placement of pages on disk has major impact on DBMS performance!
    

• 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!).
• Only one head reads/writes at any one time.
• Block size is a multiple of sector size (which is fixed).
  

• 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 4KB page
    
• Key to lower I/O cost: reduce seek/rotation delays! Hardware vs. software solutions?
  

• `Next’ block concept:
  
  • blocks on same track, followed by
  • blocks on same cylinder, followed by
  • blocks on adjacent cylinder
    
• Blocks in a file should be arranged sequentially on disk (by `next’), to minimize seek and rotational delay.
• For a sequential scan, pre-fetching several pages at a time is a big win!
  

grigorianvlad was there a point to you quoting what ivemns said?

did you forget to write a comment in there or something?

or just trying to up post count? anyways message removed if you had a comment please post one, otherwise consider this a first warning....

don't try to gain post counts, when replying to a post, please put some though into it to continue a discussion or give a good review for others to discusss about, thank you

Interesting article, Ivenms!
I learned about disks too, at college. They are an important part of the computer! For what I care, they are the most important!
In my opinion, the best hard disk producer is Maxtor! And a bad example of a producer is Western Digital. I wouldn't recommend them to anyone. They usually last as long as the guarantee period.
And what do you think is the most important factor? Disk space or rpm?

Thanks Cedik. This article now becomes a good reference for Disks Introduction. I like to see more informations about the Disks on this thread. But I don't get enough time for putting more informations about it.

Both are completely different features of hard disks. Higher RPM helps you to get into the contents faster while Disks space helps you to store more data. It depends according to the user taste..