Project Phase I

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Chapter 5 Record Storage and Primary File
Organizations - Computer storage
hierarchies and how they are used in database
systems. - Magnetic disk storage and their
characteristics.
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• Data stored physically on a computer storage
  medium.
• Two main categories
• Primary storage includes storage media that can
  be operated on directly by the CPU, such as the
  computer main memory, and cache memory.eg. Cache
  memory(RAM), DRAM
• Secondary storage includes magnetic disks,
  optical disks, and tapes. These devices usually
  have a larger capacity, cost less, and provide
  slower access to data than do primary storage
  devices. e.g. CD,tape drives, RAIDS, magnetic
  disks.

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Storage of Databases In some cases, entire
databases can be kept in main memory (with a
backup copy on magnetic disk). These databases
are called main memory databases. These are
particularly useful in real-time applications
that require extremely fast response times. Most
Databases are not stored in main memory for many
reasons
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• Reasons Databases are usually stored on secondary
  storage mediums
• Generally databases are too large to fit entirely
  in main memory.
•  Databases typically store large amounts of data
  that must persist over long periods of time.
• The circumstances that cause permanent loss of
  stored data arise less frequently for disk
  secondary storage than for primary storage.
• The cost of storage per unit of data is an order
  of magnitude less for disk then for primary
  storage.

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The data on disk is organized as files of
records. Each record is a collection of data
values that can be interpreted as facts about
entities, their attributes, and their
relationships. 
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Secondary Storage Devices The most basic unit of
data on the disk is a single bit of information.
A bit value can be 0 or 1. Bits are grouped
into bytes (or characters). ( 8 bits 1Byte) The
capacity of a disk is the number of bytes it can
store. Disks are all made of magnetic material
shaped as a thin circular disk and protected by a
cover. single-sided (if it stores information on
only one of its surfaces) and double-sided (if
both surfaces are used.). To increase capacity,
disks can be assembled into a disk pack, which
includes many disks being grouped together into
one unit.
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• Hardware Description of Disk Devices
• - Magnetic disks used for storing amounts of
  data
• data bits and bytes (characters)
• - Two kinds of disks floppy disks and hard disks
• floppy disk 400 Kbytes - 1.5 Mbytes
• hard disk hundred Mbytes - Gbytes
• - single-sided or double-sided
• - disk pack disks assembled together
• - track, cylinder and sector

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Sector Each track is divided into sectors by the
disk management system. It is hard-coded and
cannot be changed. Block Each track is
divided into blocks by the operating system
during disk formatting (or initialization).
Block size is fixed during formatting and
cannot be changed dynamically. General block
size 512 - 4096 bytes. A disk with hard-coded
sectors has the sectors subdivided into
blocks. Blocks are separated by fixed-size
inter-block gaps.
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Information is stored on a disk surface in
circles of very small width. Each circle is
called a track. For disk packs, the tracks with
the same diameter on the various surfaces are
called a cylinder because of the shape they would
form if connected in space.
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The concept of a cylinder is important because
data stored on one cylinder can be retrieved much
faster than if it were distributed among
different cylinders. The actual hardware
mechanism that reads or writes data is a disk
read/write head, which is part of a system called
a disk drive. A read/write head includes an
electronic component attached to a mechanical
arm. All arms are connected to an actuator
attached to another electrical motor, which moves
the read/write heads in unison and positions them
precisely over the cylinder of tracks
specified. Some disk units have fixed read/write
heads, with as many as there are tracks. These
are called fixed-head disks, whereas disk units
with an actuator are called moveable-head disks.
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The division of a track into sectors is
hard-coded on the disk surface and cannot be
changed.
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To transfer a disk block, given its address, the
drive controller must first mechanically position
the read/write head on the correct track. The
time required to do this is called the seek time.
Following that, there is another delay called
the rotational delay or latency the time it
takes for the desired block to rotate into
position under the read/write head. Finally,
some additional time is needed to locate and
transfer the data this is called the transfer
time. Disk Access Time seek time latency
transfer time The seek time and rotational delay
are usually much larger than the block transfer
time.
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• Seeking can involve many operations, but only
  three will be considered here
• 1) Start up time (s)
• 2) The number of cylinders traversed (n).
• 3) Time to traverse 1 cylinder (t).
• Seek time F(t) t n s

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• Rotational Delay refers to the time it takes the
  platters to rotate once (360 degrees). Generally
  given in RPM.
• X rev 1 min X rev
• ------- -------- --------
• 1min 60 sec 60 sec
• X/60 tells us the number of revolutions per
  second.

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• Rotational Delay Example
• A hard drive rotates at 3600 RPMs. How long does
  it take to rotate one time?
• 60/3600 1/60 16.7 msec
• When we look for data on a drive, the worst case
  occurs when the data we want is just behind the
  read/write arm. In the best case the data is just
  in front. On average, the data we are looking for
  will be directly opposite the side we are on.
  Therefore on average, we only need to rotate one
  half a revolution to read the sector we are
  looking for.
• If our drive is 3600 RPMs, it will take 16.7/2
  or 8.35 msecs to rotate the platter one half a
  revolution.

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• Transfer time is the time it takes to physically
  read the data from the platter.
• of bytes transferred
• -------------------------- time for 1
  rotation.
• of bytes on a track
• Time to read one sector is 1/( of a sectors
  per track) rotation time.
• For example, if there are 32 sectors per track,
  the time to transfer one sector is 1/32 of a
  revolution times the amount of time to rotate
  once.

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• Transfer Time Example
• If we have a drive which rotates at 3600 RPMs,
  how long does it take to read a sector if we have
  32 sectors per track?
• First we need to find out how long it takes to
  rotate one revolution. From the last example, we
  know that a 3600RPM drive takes 16.7 msecs to
  rotate.
• Now lets multiply the time for one revolution
  and multiply it by the amount we want to rotate.
• (1/32) 16.7 msec 0.52 msec
• 0.52 msec is the transfer time.