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Disk Scheduling

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Raid 0. Characteristics/Advantages ... Not a True RAID: not fault tolerant ... Organized as RAID 4 only RAID 5 distributes parity strips across all disk ... – PowerPoint PPT presentation

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Title: Disk Scheduling


1
Disk Scheduling
  • CS3450
  • Chapter 11.5 - 11.6

2
Memory Hierarchy
3
Magnetic disk
Track
Sector
Read write head
Spindle
4
Disk Performance Parameters
  • Access time
  • sum of seek time and rotational delay
  • the time it takes to get in position to read or
    write
  • Data transfer occurs as the sector moves under
    the head
  • Data transfer for an entire file is faster when
    the file is stored in the same cylinder and in
    adjacent sectors

5
Disk Scheduling Policies
  • Seek time is the reason for differences in
    performance
  • For a single disk there will be a number of I/O
    requests
  • If requests are selected randomly, we will get
    the worst possible performance

6
Disk Scheduling Policies
  • First-in, first-out (FIFO)
  • process request sequentially
  • fair to all processes
  • approaches random scheduling in performance if
    there are many processes

7
FCFS
Illustration shows total head movement of 640
cylinders.
8
Disk Scheduling Policies
  • Priority
  • goal is not to optimize disk use but to meet
    other objectives
  • short batch jobs may have higher priority
  • provide good interactive response time

9
Disk Scheduling Policies
  • Last-in, first-out
  • good for transaction processing systems
  • the device is given to the most recent user so
    there should be little arm movement
  • possibility of starvation since a job may never
    regain the head of the line

10
Disk Scheduling Policies
  • Shortest Service Time First
  • select the disk I/O request that requires the
    least movement of the disk arm from its current
    position
  • always choose the minimum Seek time

11
SSTF (Cont.)
12
Disk Scheduling Policies
  • SCAN
  • arm moves in one direction only, satisfying all
    outstanding requests until it reaches the last
    track in that direction
  • direction is reversed

13
SCAN (Cont.)
14
Disk Scheduling Policies
  • C-SCAN
  • restricts scanning to one direction only
  • when the last track has been visited in one
    direction, the arm is returned to the opposite
    end of the disk and the scan begins again

15
C-SCAN (Cont.)
16
Disk Scheduling Policies
  • N-step-SCAN
  • segments the disk request queue into subqueues of
    length N
  • subqueues are process one at a time, using SCAN
  • new requests added to other queue when queue is
    processed
  • FSCAN
  • two queues
  • one queue is empty for new request

17
Stable-Storage Implementation
  • Write-ahead log scheme requires stable storage.
  • To implement stable storage
  • Replicate information on more than one
    nonvolatile storage media with independent
    failure modes.
  • Update information in a controlled manner to
    ensure that we can recover the stable data after
    any failure during data transfer or recovery.

18
RAIDRedundant Array of Independent Disks
  • Set of physical disk drives viewed by the os as a
    single logical drive
  • Data are distributed across the physical drives
    of an array
  • Redundant disk capacity is used to store parity
    information, which guarantees data recoverability
    in case of a disk failure.

19
RAID 0 (non-redundant)
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 9
strip 10
strip 11
strip 8
strip 13
strip 15
strip 12
strip 14
20
Raid 0
  • Characteristics/Advantages
  • Implements a striped disk array, the data is
    broken down into blocks and each block written to
    a separate disk drive
  • High Transaction and transfer rate
  • No parity calculation overhead
  • simple design/ easy to implement
  • Disadvantages
  • Not a True RAID not fault tolerant
  • failure of one drive will result in all data in
    an array being lost
  • should not be used for critical environments

Recommended uses Non-critical applications
requiring high bandwidth Image Editing,
Pre-press Applications
21
Data Mapping for RAID Level 0 Array
Physical Disk 0
Physical Disk 1
Physical Disk 2
Physical Disk 3
strip 0
strip 1
strip 2
strip 3
strip 4
strip 5
strip 6
strip 7
strip 8
strip 9
strip 10
strip11
strip 12
strip 13
strip 14
strip 15
22
RAID 1 (mirrored)
23
Raid 1Mirroring and Duplexing
  • Characteristics/Advantages
  • Each strip is mapped to two separate drives
    (mirrored)
  • Read request serviced by disk with minimum seek
    time
  • Write request can be done to both disk in
    parallel. No Parity computation penalty
  • 100 redundancy of data
  • Medium Transaction and transfer rate
  • Simplest true RAID design
  • Disadvantages
  • COST (twice the disk space)
  • High I/0 Rate if mostly read request. No
    improvement for write request.
  • Software implementation can load the CPU/Server
    degrading throughput at high activity levels
    hardware implementation preferred

Recommended uses Accounting, Payroll, Any
application requiring very high availability
24
RAID 2 (redundancy through Hamming code)
f2(b)
f1(b)
f0(b)
b2
b1
b0
b2
25
Raid 2
  • Disadvantages
  • Very High ratio of ECC disk to data disk
  • low transaction rate
  • Only effective choice in an environment of high
    disk errors. Considered overkill and not
    implemented
  • Characteristics/Advantages
  • All member disks participate in every I/0
    request. Strips are very small (work or byte)
  • Hamming code used to store bits across all disk
  • High Transfer rate

Recommended uses Not Implemented
26
RAID 3 (bit-interleaved parity)
P(b)
b2
b1
b0
b2
27
Raid 3
  • Disadvantages
  • Only one I/0 request can be executed at a time.
  • Low Transaction rates
  • Characteristics/Advantages
  • All member disks participate in every I/0
    request. Strips are very small (work or byte)
  • Simple parity used to store bits across all disk
    (requires only a single redundant disk.)
  • High Data transfer rates. Parallel transfer of
    data from all data disk. Good for moving very
    large sequential files in a timely manner

Recommended uses Video production, Image/video
Editing. Any application requiring very high
throughput
28
RAID 4 (block-level parity)
block 0
block 1
block 2
block 3
P(0-3)
block 4
block 5
block 6
block 7
P(4-7)
block 9
block 10
block 11
block 8
P(8-11)
block 13
block 15
block 12
block 14
P(12-15)
29
Raid 4
  • Disadvantages
  • Parity disk becomes a bottleneck for write
    operations
  • Medium/low Data transfer Rate
  • Characteristics/Advantages
  • Each member disk operates independently. So
    separate I/0 request can be done in parallel.
  • Strips are large
  • Each entire block is written on a data disk.
    Parity for same
  • High transaction rate

Recommended uses Not Implemented
30
RAID 5 (block-level distributed parity)
block 0
block 1
block 2
block 3
P(0-3)
block 5
block 4
block 6
P(4-7)
block 7
block 9
block 10
block 11
P(8-11)
block 8
block 12
P(12-15)
block 13
block 14
block 15
P(16-19)
block 16
block 17
block 18
block 19
31
Raid 5
  • Characteristics/Advantages
  • Each member disk operates independently. So
    separate I/0 request can be done in parallel.
  • Organized as RAID 4 only RAID 5 distributes
    parity strips across all disk
  • Avoids the bottleneck of RAID 4
  • High Transaction rate
  • Disadvantages
  • Disk failure time consuming/difficult compared to
    RAID level1
  • medium/low Transfer Rates

Recommended uses High request rate,
read-intensive, data lookupInternet servers,
Database servers WWW, file , application servers
32
RAID LEVELS
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