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

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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

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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