Secondary Storage Devices and Records

1
Secondary Storage Devices and Records

• CSC 402 File Management

2
Make up of Disk Devices

• Bytes
• Capacity
• Disk Pack
• Track
• Sector
• Cylinder

3
Illustrations Disk Surface Disk Pack
4
Accessing the Disk

• Amount of data transferred
• Block address
• Read/Write Head
• Seek
• Rotational delay (latency)
• Block transfer time

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Seagate Cheetah 73
Configuration Number of Discs (physical)
12 Number of Heads (physical) 24 Total
Cylinders 14,100 Bytes Per
Sector 512  
Capacity/Interface Formatted Capacity 73.4
Gbytes Interface Type 80-pin    Performance
   Transfer Rates  Internal Rate (min) 28
Mbits/sec Internal Rate (max) 427
Mbits/sec Formatted Int Rate (min) 26.7
Mbits/sec Formatted Int Rate (max) 40.2
Mbits/sec External (I/O) Rate (max1 60
MBytes/secAvg Formatted Rate 35.5
MBytes/sec   Seek Times  Average Time, Read 5.6
msec typical Average Time, Write 6.2 msec
typical Track-to-Track, Read 0.6 msec
typical Track-to-Track, Write 0.9 msec
typical Average Latency 2.99 msec   Other 
Default Buffer (cache) 4,096 Kbytes Spindle
Speed 10,000 RPM   
6
File Allocation

• Clusters
• Allocation units
• Fragmentation
• File Allocation Table (FAT)
• UNIX
• Super block
• Inodes
• Data blocks

7
UNIX Inode Structure
8
Journey of a Byte - reading

• thefile.read(ch, 1)
• Read sector (or minimum access amount) containing
  info if not already there
• Copy character from buffer to user program
  location

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Journey of a Byte - writing

• thefile.write(ch, 1)
• Read sector where info will be if not already
  there
• Copy character from user program location to
  buffer
• Write buffer to sector on disk

Sector Writing to
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Buffer Management

• Single buffer
• Double buffer
• Buffer pooling
• Least recently used
• Least used

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Sequential Organization

• Sequential access
• Read block
• Take out records one by one
• Direct access by location
• Fixed length records
• Relative record number (rrn)
• Byte offset (rrn-1) sizeof record
• Variable length records
• Byte offset must be known
• Direct access by key

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Sequential Organization (cont)

• Reclaiming Space
• Off line - compaction
• On the fly
• Fixed length records
• Avail stack
• Variable length records
• First fit placement strategy
• Best fit placement strategy
• Worst fit placement strategy

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From Bytes to Fields

• Values that describe attributes
• Fixed length
• Variable length
• Size indicator
• Delimiter

10John Smith9Kutztown2PA
John SmithKutztownPA
14
From Fields to Records

• Collection of fields that describes one entity
• Fixed length
• Fixed length fields
• Variable length fields, always same number of
  bytes

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From Fields to Records (cont)

• Variable length
• Size recorded
• Delimiter

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From Records to Blocks

• Recall file transfer done by block
• Several records per block
• Logical structure pick up entire block
• All in order
• One at a time

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Recall API for File Class

• Open,Create,Close as before
• Other operations do the buffer access
• Write
• Pack record into buffer
• Write buffer to file
• Read
• Read in buffer from file
• Unpack record from buffer
• Append
• Seek to end of file
• Write record with location of -1

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Variable Length Information Files

• Variable length fields and/or records
• Buffer needed to collect data
• Buffer knows how data stored
• Operations for buffer
• Pack a field into a buffer
• Unpack a field from a buffer
• Read from file into buffer
• Write from buffer to file

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API for a Buffer Class
//Copies the size into the buffer then the size
bytes from field to buffer //Return number of
bytes packed int Pack(void field, int
size) //Copies the size from the buffer then
copies that many bytes from buffer to field //If
the size found is gt maxAllowed, Unpack returns
-1 //Return the value of size, the number of
bytes unpacked int Unpack(void field, int
maxAllowed) //Sets marker back to beginning of
buffer to begin packing or unpacking a
record void ResetBuffer()
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API for a Buffer Class (cont)
//If recref ! -1, reads the record at recref
from file into the buffer //If recref -1, reads
the currrent record from file into the
buffer //Return relative record number of the
record read int Read(istream file, int
recref-1) //If recref ! -1, writes the record
in the buffer to relative record number
recref //If recref -1, writes the record in the
buffer to the current location in the
file //Return relative record number of the
record written int Write(ostream file, int
recref-1)
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Using the buffer
class person public ..
private string name int age

• Save
• person gal
• gal.Pack(b) //put bytes from object into buffer
• b.Write(f) //write buffer to file
• Class must provide Pack
• Example
• b.Pack(name.c_str(), name.length() 1)
• b.Pack(age, sizeof(int))

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Using the buffer (cont)

• Retrieve
• person gal
• b.Read(f) //read into buffer from file
• gal.Unpack(b) //put bytes into object
• Class must provide Unpack
• Example
• Char cnameMAX
• b.Unpack(cname, MAX)
• name cname
• b.Unpack(age, sizeof(int))