1. File Organizations
What an OS provides
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2. Why files? Computer memory has several problems:
Expensive
Volatile
Persistant data must be stored on disk or tape
The Operating System controls disk access
All disk access is somewhat platform dependent
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3. The Memory Hierarchy CPU Cache Memory Disk Tape
As you move down the cost per byte decreases and the access time increases.
Disk
Tape
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4. Disks Rotating magnetic media
Flat, round platter of metal or plastic and covered with magnetic coating
Information stored as magnetized spots on the magnetic coating called bits
Access arms hold the heads
Move to read different areas
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5. Disk units Organization of disk Cylinders Tracks
Any area that can be read without moving head
Tracks
Concentric circles on surface
Disk rotates under head
Sectors
Pie shaped divisions on surface
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6. Hard Disk Organization
Sector
Track
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7. Disks are DASD Direct Access Storage Device
Any sector may be read directly
Disk address contains
Cylinder address
Forces moving heads
Track address
Selects a head
Sector address
Rotation of sector under head
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8. Hard Disk Drives Copyright © 1998-2013 Curt Hill
The access harm moves all the read/write heads in simultaneously
A cylinder is all the tracks that can be read by one set of heads without moving the access arm. Two on floppies and many hard disks, but can be 20 or more.
The heads fly a few millionths of an inch from the surface and have to be designed aerodynamically so that they will be close to the disk, but never touch it.
A collision is called a head crash
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9. Disk Access Time Seek time Rotational delay
Movement of access arm to correct cylinder
Rotational delay
Movement of disk to position correct sector under read/write head
Activation of appropriate read/write head
Transfer rate of data from disk to main memory
Seek time and rotational delay dominate the access time.
Manufacturers put many read/write heads for each platter to minimize the seek time.
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10. Access Time Again 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
Goal is to reduce these delays
This is done with both hardware and software
RAID is a hardware solution
What are the software solutions?
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11. Reducing Access Time If you access the first block of a track
You have all the access delays
If you then access the second block of a track the only delay is transfer time
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12. Quicker Access Blocks in same track Blocks in same cylinder
Blocks in adjacent cylinders
Pre-fetching
Read the entire track at the time of the request for any block
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13. Disk Reliability How reliable are disk drives?
Reliable but not perfect
The advent of server farms with thousands of consumer disks has allowed for studies that give us some answers
Disk failures tend to make a “bathtub” curve
80% survived four years
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14. Failure Graph
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15. Explanation The initial failures are usually manufacturing defects
They cause an early death
Next comes a period of high reliability
Finally we see a period of drives wearing out
These were from a study of 25,000 drives in a server farm
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16. Failure Rates Again
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17. Common File Organizations
Sequential
Direct
Indexed Sequential
Most others are variations on these basic themes
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18. Sequential Files Records are stored one after another
Accessing 500threcord requires reading 499 prior records
Weakest file organization
Easiest to implement
Even tape drives support
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19. Direct Files AKA Relative files
The file is a linear sequence of equal sized numbered slots
Each slot may be accessed directly
Slots may be empty or used
Key is an integer
Requires DASD 1 2 3 4 5 6 7 8 9 10 11 12 12
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20. Indexed Files AKA Indexed Sequential File
Depending on the OS may actually be two files
An index
The data
The index is a tree of keys
The data is a sequence of records at the bottom of the tree
Requires DASD
Key may be of any type
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21. Index Sequential File Tree
Charles Kline Roberts Zane
Charles Kline Roberts Zane
Abel Bag Casey Charles
Able Bart Calvin Charles Dean Easy Frank Kline Larry Morris Roberts Smith Taylor Vernon Zane
Easy Frank Jam Kline
Leestra May Roberts
Singer Smith Taylor Vernon Zane
Each block should represent one disk block, such as a sector
The top level contains the keys only
The bottom level contains key and data
We can sequentially process the bottom level to get all names
We can randomly enter the tree and find any name with just two accesses
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22. Indexed Files Previous tree only had two levels
One index
One data
Usually many levels of index
One level of data
The index is a tree of keys
The data is a sequence of records at the bottom of the tree
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23. Linkage Indexed files require links between records
The link is a disk address
Cylinder, Track, Sector or
Block number
The links connect the indices with their corresponding data
Different from Sequential or Direct
OS supports these
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24. Access and Organization
There are two typical means to access a file:
Sequential
Random
Sequential access may be applied to any file organization
Random requires a key and may only be applied to Direct or Indexed files
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25. Finally Not every OS provides each of these Always sequential
Usually some form of direct
Indexed sequential seldom
Often provided by external software
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