Raid

RAID

Your Data is Lost?

• Do we have backups of all our data?- The stuff we cannot afford to lose?

• How often do we do backups?- Daily, Weekly or Monthly?

• How long would it take to totally recover from the disaster?

Flow Of Presentation

• JBOD.• What is RAID.• RAID Techniques.• RAID Levels.

HD 1HD 1

HD 2HD 2

HD 3HD 3

ProcessorProcessor

ProcessorProcessor ControllerController

HD 1HD 1

HD 2HD 2

HD 3HD 3

JBODJust A Bunch Of

Disks

Drawbacks for JBOD

• Reliability becomes a big problem as the data in an entire disk may be lost.

• As the number of Disks per component increases, the probability of failure also increases.

Solution?

RAID

R A I D

Redundant Array Of Inexpensive Disks. In 1987, Patterson, Gibson and Katz at the University Of California, Berkeley. Theypublished a paper entitled “A Case for RAID”.•RAID was to combine multiple, inexpensive disks drive into an array of disk drives which yields performance exceeding that of a JBOD.•This array of drives appear to the computer as a single logical storage unit or drive.

Techniques

• Data Striping.• Mirroring.• Parity Check.

Data Striping

• Data – 1,2,3,4.

1313

2424

HD 1 HD 2

Data Mirroring

• Data – 1,2,3.

123

123

123

123

HD 1 HD 2

Parity Bit Check• Data – 1001, 1101.

1111

0101

HD 1 HD 2

0000

1111

HD 3 HD 4

0101

ParityBit

Hot Spare

A B Y0 0 00 1 11 0 11 1 0

Truth TableEX-OR

1 1 0

1111

0101

HD 1 HD 2

0000

1111

HD 3 HD 4

0101

0101

ParityBit

Hot Spare

1 0 0

RAID Levels• RAID 0• RAID 1• RAID 2• RAID 3• RAID 4• RAID 5• RAID 6• RAID 0+1• RAID 1+0

RAID 0

• Striping at the level of blocks.• Data split across in drives resulting in higher data

throughput.• Performance is very good but the failure of any

disk in the array results in data loss.• RAID 0 Commonly referred to as striping.• Minimum Drives required is 2.• Reliability Problems : No Mirroring or Parity Bits

RAID 1

• Uses Mirroring.• Expensive.• Minimum Drives required is 2.• Performance Issues.

No Data loss if either drive fails. Good Read performance. Reasonable Write performance.

• Cost per MB is high.

RAID 2(bit),3(byte),4(block or sector)

• Need to include a dedicated parity hardware.

• Expense of computing and writing parity.

• Performance is good for reads.

• Slow for small writes but fast for large writes.

• Minimum number of drives required is 3.

RAID 5

• Uses Block-Level striping with parity data distributed across all member disks.(it spreads data and parity among all N+1 disks, rather than storing data in N disks and parity in 1 disk.)

• Avoids potential overuse of a single parity disk – improvements over RAID 4.

• Most common parity RAID system.• Minimum drives required is 3.

RAID 6

• Uses Block-Level Stripping with double distributed parity.

• It provides fault tolerance of two drive failures.

• Minimum Drives required is 4.• Provides protection against data loss during

an array rebuild.

RAID 0+1RAID 1+0

• A) RAID 0+1 with a single disk failure.• B) RAID 1+0 with a single disk failure.A) B)

MirrorStripe

Stripe

MirrorStripeMirror