Course Progress

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Course Progress

• Data Placement Methods
• Server architecture and zoned disk
• Data Placement on disks

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A Simple Multimedia Server System
Processor subsystem
Application server
Control or scheduling server
Clients
Storage subsystem
Network
Data server
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Distributed Multimedia Servers
Simple MM server
Simple MM server
Simple MM server
network
client
client
client
client
client
client
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Multimedia Server System
query Q
indexes
objects
X, Y, Z,
X, Y, Z,
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Input/Output Processors
I/O devices are slow. IOP connects I/O bus to
memory bus and CPU.
CPU
Memory bus
Memory
I/O Processor
I/O bus
Disk 1
Disk 2
Disk 3
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Input/Output Processors
target device
where cmnds are
I/O Instruction
Step 1 CPU issues I/O Instruction to IOP
CPU
OP Device Address
looks in memory for commands
I/O instruction
Memory
I/O Processor
command
I/O bus
Disk 1
Disk 2
Disk 3
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Input/Output Processors
Step 2 IOP reads command from memory
CPU
Memory bus
I/O instruction
Memory
I/O Processor
command
I/O bus
Command
OP Addr Cnt Other
Disk 1
Disk 2
Disk 3
what to do
where to put data
how much
special requests
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Input/Output Processors
Step 3 IOP transfers data to/from memory
directly
CPU
Memory bus
I/O instruction
Memory
I/O Processor
command
I/O bus
Disk 1
Disk 3
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Input/Output Processors
Step 4 IOP sends interrupt to CPU when done
CPU
Memory bus
I/O interrupt
I/O instruction
Memory
I/O Processor
command
I/O bus
Disk 1
Disk 3
Disk 2
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Magnetic Disk Structure
Read/write heads
Platters
Tracks
Platter
Track
Sectors
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Modern Zoned Disk Layout

• Fixed sector size in number of bytes
• Fixed number of sectors within each zone
• More sectors per track in outer zones
• Number of tracks in each zone may vary

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CD DVD Layout

• Single spiral track running from inside to the
  rim
• Fixed sector size in number of bytes
• Dual layer DVD may have a second spiral track
  that is in the same or opposite direction

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IBM Millipede Project

• Silicon tip punch holes in postage-stamp-sized
  polymer
• Holes are 10 nanometers or 50 atoms
• Density 1 trillion bits per square inch (20 X
  disks)
• Rewriteable Write with tip at 400?C, read with
  tip at 300?C, erase at hot tips
• 1,000 times slower than hard disk
• 1,024 tips working in parallel

Source L.D. Paulson, Tiny Punch Cards boost
storage capacity, Computer, Sept 2002, p.22.
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Nanteros NRAM

• The first 10 Gbit nano-technology RAM
• Differing electrical charges swing tubes into 1
  of 2 positions (0 1)
• Small size 10-Gbit (10 billion carbon nanotubes)
  in a billionth of a metre
• Short distance to move gt fast read/write
• Static without power gt Non-volatile
• 50 times stronger than steel gt many write cycles
• Quality control by selecting only nanotubes
  growing properly

Source L.D. Paulson, Nanotech RAM Holds Promise
for Universal Memory, Computer, Sept 2003, p.15.
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Disk Access Operations

• Disk accesses data in sectors of 512 to 2048
  bytes. Each disk I/O command executes a number of
  basic steps
• Obtain I/O channel to memory
• Seek the required cylinder
• Switch to the selected head within the cylinder
• Wait for the start of the required sector to meet
  the head
• Transfer the sector via I/O channel to memory
• Send interrupt to the CPU for I/O completion

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Disk Access Time

• The major mechanical steps are
• Seek time move the read/write heads to the track
• Rotational latency wait for the start of the
  required sector to come under the head.
• Optional RPS miss additional cycle if I/O path
  fails to establish before transfer
• Dependent on the duration between consecutive
  seeks
• Transfer time transfer the sector via I/O path
  to memory
• Other steps are electronic and contribute to less
  than 5 of the disk access time
• Seek time and rotational latency are overheads

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Performance Model

• Continuous model - A close approximation of the
  discrete performance with small inter-track gap
  considering each track as a zone.
• If each sector has the same probability of being
  accessed, then the probability of a head at track
  of radius x is

Ring Area 2?xdx
Disk Area
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Seek Time

• Seek time is the time required for the seek
  action which consists of the following components
  
• 1.  The arm is accelerated until it reaches the
  maximum speed.
• 2.   The arm is traveling at the maximum speed.
• 3.   The arm is decelerated until it stops.
• 4.   The head is settled on the required track.
• The seek time, s, relates to the seek distance,
  D, below

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Seek time curve

• Almost linear, with curve at short seek distance.

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Seek Distance

• Mean seek distance of completely random seeks is
  by integrating all possible absolute seek
  distances
• Thus, mean seek distance, is

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Seek Distance

• The variance of seek distance. VarD is
• Random seeks can be modeled easily, but most
  seeks are not random. Consecutive requests
  usually access consecutive sectors.
• Place data that are retrieved together in the
  same track and cylinder would reduce seek time

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Rotational Latency

• The disk rotates at fixed angular speed.
• Let T be the revolution time.
• Let Qt be probability that rotational latency is
  between time t and tdt, where 0 lt t ? T, and
  dt?0. We have
• The mean rotational latency is

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Rotational Latency
t
dt
t lt L lt tdt
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Rotational Latency

• The disk rotates at fixed angular speed.
• Let T be the revolution time.
• Let Qt be probability that rotational latency is
  between time t and tdt, where 0 lt t ? T, and
  dt?0. We have
• The mean rotational latency is T/2.
• The variance of rotational latency, VarL is

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Rotational Latency

• Sectors do not starts anywhere, rotational
  latency also depends on the seek distance
• Seeklatency increase stepwise with seek distance

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Data Transfer Time

• Data transfer time is the time to read the
  required data from the current track. Maximum
  areal density is the maximum recording density on
  the disk surface.
• On a track of radius x, if the amount of data in
  this track is given by 2?xk, where k is recording
  density in bytes/unit length, then the data
  transfer rate at this track is 2?xk/T.
• To transfer R bytes of data at a track of radius
  x, the data transfer time is

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Data Transfer Time

• Mean data transfer time is

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Data Transfer Time

• The variance of data transfer time is
• Var(t)
• Var(t)

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Summary

• In magnetic disks, data are recorded on
  concentric circles on disk platters. Each zone
  has a different number of sectors.
• Major component of disk access time are seek
  time, rotational latency, and data transfer time
• For completely random accesses,
• the mean seek distance is
• the mean rotational latency is
• the mean data transfer time is