Effective memory access time:

EA = HC + (1-H)S
Where H = Hit ratio
Hit ratio = references satisfied by cache / total references
C = cache access time; S = memory access time

Effective disk buffer cache access time:

EA = HC + (1-H)S
Where H=Hit ratio C = cache access time; S = memory access time
Disk access time = seek time (tracks)+ rotational time (section) + transferlength / bandwidth

Storage System

• MTBF (Mean Time Between Failure):
  Describes the expected time between two failures for a repairable system
  

  Time between failures.svg
  • Time between failures = {down time - up time}
  • C12pture.PNG

• Mean Time to Repair
  (Total down time) / (number of breakdowns)

• Mean Time to Fail:
  For the product that cannot be repaired.

RAID (Redundant Array of Independent)

• RAID 0

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• Block Level Striping(4k or 8k bytes of storage)

• Provides balanced I/O of disk, throughput approximately doubles

• Any one of the disk failure could cause the MTTF reduces by a factor of 2

• RAID 1

Capddure.PNG

• Block Level Mirroring

• Provides higher read throughput but lower write throughput

• MTTF increases substantially (MTTF^2)

• RAID 2

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• Bit Level Mirroring(4k or 8k bytes of storage)

• Provides higher transfer rate

• MTTF increases substantially (MTTF^2)

• RAID 3

Cadfure.PNG

• Byte Level Striping (Parity)

• Provides higher transfer rate as RAID 0

• MTTF increases substantially (1/3 of RAID 1 = MTTF^2/3)

• P0 is parity (exclusive-or) for b0 and b1. Knowing any two of the disk data could recover the third data.

• RAID 4

Cadture.PNG

• Block Level Striping (Parity)

• Similar to RAID 3

• Disk 3 has a lot more stress because it has more writes than either disk 1 and disk 2 as the parity has to be updated on every write

• RAID 5

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• Block Level Striping (Parity)

• Single disk failure tolerant array

• No dedicated disk for parity blocks, parity blocks are also striped

• MTTF is same as RAID 3

  
  
  Capture.PNG

• RAID 6

• At least 5 disks including two parity blocks used

• Reliability is of the order of MTTF^3/10

• p0 and p1 are the parity blocks for blocks A0,A1,A2