Data Storage

1
Data Storage

• Secondary Storage Media and Its Performance

2
Physical Media

• Sequential Access Storage Devices (SASD)
• Direct Access Storage Devices (DASD)

3
SASD

• tape paradigm linear in nature

4
SASD

• tape paradigm linear in nature
• data is retrieved according to its physical
  location in the file

5
SASD

• tape paradigm linear in nature
• data is retrieved according to its physical
  location in the file
• all data preceding the desired data must be
  retrieved first

6
SASD

• tape paradigm linear in nature
• data is retrieved according to its physical
  location in the file
• all data preceding the desired data must be
  retrieved first
• file size of n records, requires on the average
  n/2 retrievals

7
Magnetic Tape

• density amount of data record per unit of
  linear measure (bits / in) bytes/in or bpi

8
Magnetic Tape

• density
• gap an unused length of tape permits tape to
  achieve read/write speed or stop, whichever is
  the case

9
Magnetic Tape

• density
• gap
• nominal capacity data capacity if the entire
  tape were to store data

10
Magnetic Tape

• density
• gap
• nominal capacity
• physical record amount of data transferred from
  the tape device to memory

11
Magnetic Tape

• density
• gap
• nominal capacity
• physical record
• logical record a unit of data as viewed by the
  application

12
Magnetic Tape

• density
• gap
• nominal capacity
• physical record
• logical record
• blocking factor number of logical records per
  block, or physical record

13
Magnetic Tape

• density
• gap
• nominal capacity
• physical record
• logical record
• blocking factor
• nominal transfer rate the theoretic maximum
  rate for transferring data (density tape
  transport speed)

14
DASD

• 3-dimensional paradigm

15
Magnetic Tape Parameters
16
Magnetic Tape
Logical Record(s) BF 6
. . .
R R R R R R
G
Z
Physical record Data block
17
Magnetic Tape - Calculations

• Nominal capacity density 12in/ft length of
  reel
• Transfer Rate (density tape transport speed)
• Blocksize BF LCRL
• Time to read a block blocksize/(transfer rate)
  SS
• Time to read a file
• ?(nbr Recs)/BF? (time to read one block)

18
Disk Device
disk pack
individual platters each platter has two
surfaces
19
Disk Platter top view
tracks concentric same capacity
say 200 per surface
20
Disk Pack/platter side view
cylinders
disk surfaces 8 of 10 are used
access arm
spindle
read/write heads
21
DASD

• 3D paradigm
• data is retrieved according to its physical
  location on the device

22
DASD

• 3D paradigm
• data is retrieved according to its physical
  location on the device
• positioning algorithm enables going to the exact
  address of data

23
DASD

• 3D paradigm
• data is retrieved according to its physical
  location on the device
• positioning algorithm enables going to the exact
  address of data
• file size of n records, ideally requires a
  single retrieval

24
Track Format
sectors
25
Direct Access
Hashing algorithm h (primary key) ? (cc, ss,
bb) (x, y, z)
y
x
z
26
Disk Parameters
Latency Seek Time
Home address
27
Access Time

• Latency 3600 rpm 8.3ms
• Seek time device specific
• Average access time vendor supplied
• lt 10 ms
• Latency Seek time Seeking is MUCH more
  expensive than latency a modern interleaf factor
  of 1 means we can (theorectically) read one
  complete track per revolution thereby removing
  latency as a consideration.

28
Access Time contd.

• Caching buffering assuming a buffer of 4Kb,
  grab minimum of 4K per disk read which is 8
  sectors.
• Sector size assumed to be 512 bytes.
• Intermediate cache, if sufficiently large, may
  permit reading entire file into memory.

29
Strategies

• Allocate contiguous cylinders.
• Create file in cylinder-sequence (fill vertically
  then horizontally).
• A file consisting of 5 cylinders requires only 5
  seeks when reading sequentially.
• At creation, order data so that physical and
  logical orders coincide (if possible).