2. 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?
5. HD 11
HD
Processor
Processor Controller
Controller
HD 22
HD
HD 33
HD
JBOD
Just A Bunch Of
Disks
6. 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.
8. RAID
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.
12. Parity Bit Check EX-OR
Truth Table
• Data – 1001, 1101. A B Y
0 0 0
0 1 1
1 1 0 1 0 1
1 1 0
11 00 00 11 00
11 11 00 11 11
Parity Hot
HD 1 HD 2 HD 3 HD 4
Bit Spare
13. 1 0 0
11 00 00 11 00 00
11 11 00 11 11 11
Parity Hot
HD 1 HD 2 HD 3 HD 4
Bit Spare
15. 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
16. 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.
17. 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.
18. 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.
19. 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.
20. RAID 0+1
RAID 1+0
• A) RAID 0+1 with a single disk failure.
• B) RAID 1+0 with a single disk failure.
A) B)
Stripe
Mirror Stripe
Mirror Mirror
Stripe
