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Understanding Storage Systems: Magnetic Disks, RAID, and Performance, Slides of Computer Aided Design (CAD)

An overview of storage systems, focusing on magnetic disks and their performance characteristics, as well as raid technology for increased availability. Learn about magnetic disk layout, zone bit recording, and the concept of amdahl's law. Discover the advantages and disadvantages of various raid levels, including raid0, raid1, raid2, raid3, raid4, raid5, and raid6.

Typology: Slides

2012/2013

Uploaded on 04/24/2013

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Download Understanding Storage Systems: Magnetic Disks, RAID, and Performance and more Slides Computer Aided Design (CAD) in PDF only on Docsity!

Storage Systems

Why Worry About Storage Systems

  • Response time = CPU time + I/O time
  • Suppose 95% of work is done in the CPU, 5% is I/O
    • If the CPU is improved by a factor of 100, what is the Speedup?
    • Sp = 1 / (1 - 0.95 + (0.95 / 100)) = 16.
    • 83.2% of the improvement is squandered due to poor I/O
  • Future performance gains must consider faster I/O
  • Amdahl’s Law  All parts of a system need to be improved somewhat equally

Magnetic Disk Layout

P l a t t e r

T r a c k

P l a t t e r s

S e c t o r s

T r a c k s

COPYRIGHT 1998 MORGAN KAUFMANN PUBLISHERS, INC. ALL RIGHTS RESERVEDDocsity.com

Zone Bit Recording

  • Originally, all tracks had the same number of sectors
    • Each sector = 512 Bytes
  • Inefficient! Limited by the density of the smallest tracks
  • Outer tracks can hold more sectors (store more data) than inner tracks  called Zone Bit Recording (ZBR)
  • The sequence of information recorded on each sector is
    • sector number
    • gap
    • data including error correction code bits
    • gap

Disk Reliability vs Availability

  • In processing, our main concern is performance (and cost)
  • In I/O, our main concern is reliability (and cost)
  • Reliability  is anything broken?
  • Availability  is the system still available to the user, even if it is broken?
  • RAID technology is designed to provide increased availability for potentially unreliable devices
  • RAID – Redundant Array of Inexpensive Disks
    • Patterson / Katz / Gibson -- 1987

RAID

  • Provide a set of physical disks that appear to be a single logical drive
  • Distribute the data across the drives in the array
  • Allow levels of redundancy to permit recovery of failed disks
  • 7 (basic) levels of RAID (RAID0 – RAID6)
  • RAID is NOT a backup system!
    • Its made to maintain uptime through failures

RAID0 Summary

  • Called striping the data
    • Each stripe is  1 sector
  • Advantage:
    • access to large sections of contiguous data can be done in parallel over all disks
  • Disadvantage:
    • no redundancy

Physical Drive 6 2

6

10

Physical Drive 5 1

5

9

PhysicalDrive 4 0

4

8

Physical Drive 7 3

7

11

RAID

  • Mirrored data
  • Make a complete mirror (i.e. duplicate) of all data

PhysicalDrive 0 0

4

8

Physical Drive 1 1

5

9

Physical Drive 2 2

6

10

Physical Drive 3 3

7

11

mirrors

RAID

  • Redundancy through Hamming codes
  • Store enough extra data in order to detect and correct errors, or in order to provide availability in the case of a failed drive
  • Stripes are small  1 bit (!) per stripe originally  later bytes/words
  • All disk heads are synchronized – does not permit parallel access as in RAID0 and RAID
  • Requires  log 2 (#disks) extra disks to implement

Hamming Codes

  • A technique for enabling error detection & correction (and therefore redundancy as well)
  • Ex: ECC Scheme:
    • store 1110, but suppose an error changes this to 1100

1

1

1 0

0

0

1

Even parity bit

1

1

0 0

0

0

1

Wrong parity bit!

Error in data

Error detected

RAID2 Summary

  • Error correction is done across disks
  • Advantage:
    • Useful in a high failure environment
  • Disadvantage:
    • Expensive
    • Modern disks do not exhibit high failure rate
  • Not used in practice

Physical Drive 4 1xxx xxxx

RAID

  • Bit interleaved parity
  • Like RAID2 but use only 1 parity drive

Physical Drive 0 1xxx xxxx

Physical Drive 1 1xxx xxxx

Physical Drive 2 1xxx xxxx

Physical Drive 3 0xxx xxxx

Stripes of bytes or words Parity drive

Physical Drive 4 P(0-3)

P(4-7)

P(8-11)

RAID

  • Block level parity
  • Same parity scheme as RAID3 but uses large blocks per stripe Physical Drive 0 Block

Block

Block

Physical Drive 1 Block

Block

Block

Physical Drive 2 Block

Block

Block

Physical Drive 3 Block

Block

Block

Stripes of large “blocks” Parity drive

RAID4 Summary

  • Interleaved blocks across disks
  • Advantage:
    • Allows independent access due to large stripes  can support multiple independent reads
    • Error detection / correction supported up to single bit errors
  • Disadvantage:
    • On parallel writes  penalty incurred since all writes require access to the same parity disk to update the parity
  • Not used in practice