3. What is RAID
History
> It first defined by David A. Patterson, Garth A. Gibson, and
Randy Katz at the University of California, Berkeley in 1987 to
describe a redundant array of inexpensive disks.
> What is RAID - It is a technology that allowed computer users
to achieve high levels of storage reliability from low-cost and
RAID allows you to store the same data redundantly (in multiple
paces) in a balanced any to improve overall performance.
4. Benefits of RAID
> Data loss can be very dangerous for an
organization
> RAID technology prevents data loss due to
disk failure
> RAID technology can be implemented in
hardware or software
> Servers make use of RAID technology
5. Hardware vs. Software RAID
> Software RAID
• Software RAID: run on the server’s CPU
• Directly dependent on server CPU performance and load
• Occupies host system memory and CPU operation,
degrading server performance
> Hardware RAID
• Hardware RAID: run on the RAID controller’s CPU
• Does not occupy any host system memory. Is not operating
system dependent
• Host CPU can execute applications while the array adapter's
processor simultaneously executes array functions: true
hardware multi-tasking
6. RAID
• Redundant Arrays of Independent Drives
• Benefits
– Improved data availability
– Improved I/O performance
– Increased scalability
• Levels supported by Array controllers
– RAID 0 – Data striping
– RAID 1 – Drive mirroring
– RAID 4 – Data guarding
– RAID 5 – Distributed data guarding
– RAID 6/ADG - Advanced data guarding
– RAID 0+ 1 – Mirroring and striping
11. RAID Level 0 — Data Striping
File divided into chunks (or segments) and then
written (striped) across multiple drives
12. RAID 0
Uses striping
I/O performance gain
No Data redundancy
Not fault tolerant
Not considered “true” RAID
13. Striping Factor
64KB Host Data
Fixed Striping Factor Based on Drive Sizes
14. RAID Level 1 — Drive Mirroring
Data Written to Two or More Separate Mirrored
Drives
15. RAID 1
Uses mirroring
Also known as duplexing
Fault tolerant
High Disk overhead
Mirroring typically handled system software
Simplest RAID design
16. RAID Level 0+1
Disk 0 is mirrored to disk 2 and disk 1 is mirrored to disk 3.
Then disk 0 is striped with disk 1 and disk 2 is striped to disk 3.
18. RAID 0+1
RAID 1+0 requires an array with four or more physical
disks. The disks are mirrored in pairs and data blocks are
striped across the mirrored pairs
Advantages
Highest read and write performance
No loss of data as long as no failed disks are mirrored to any
other failed disk
Disadvantages
Expensive and Low disk capacity
19. RAID Level 4 — Data Guarding
Data striped across multiple drives and then its parity
sum is calculated and written to a dedicated parity drive
20. RAID 4
Advantages: Disadvantages:
Very high read rates Very slow write rates
Even small writes fill up
Multiple files read at parity write queue
once
Inefficient data
recovery
Uses: Even more Complex
Web Servers, and Controller Design than
other high read, low RAID 3
write situations
21. RAID Level 5 — Distributed Data
Guarding
Data is striped across multiple drives and then its parity sum
calculated and striped across multiple drives. Example of 64KB
striped across five drives using 4KB chunks.
22. RAID 5
RAID 5 uses a parity data formula to create fault tolerance.
In RAID 5 each block of data stripe contains parity data that
is calculated for the other data blocks in that strip.
The blocks of parity data are distributed over the physical
disks that make up the logical drive with each physical
disk containing only one block of parity data
It is referred to as data guarding.
Advantages
High read performance
No loss of data if one physical disk fails
More usable disk capacity
Disadvantages
Relatively low write performance
Data loss occurs if a second disk fails before data from the first
failed disk is rebuilt
23. RAID ADG (Advanced Data Guarding)
> RAID ADG is similar to RAID 5 except this RAID level writes 2 sets of
parity stripped across all drives.
> Protects against failure of ANY 2 drives in the array
A B C P Q
D E P Q F
G P Q H I
P Q J K L
RAID ADG
24. RAID ADG (Advanced Data Guarding)
> P = f1(A, B, C) = RAID 5 Parity
> Q = f2(A, B, C) = new ADG Parity
A B C P Q
D E P Q F
G P Q H I
P Q J K L
RAID ADG
25. RAID ADG (Advanced Data Guarding)
> If 2 parity drives are selected, the system can sustain failure of ANY 2
drives.
X X
A B C P Q
D E P Q F
G P Q H I
P Q J K L
RAID ADG
26. RAID ADG (Advanced Data Guarding) or
RAID 6
RAID advanced data guarding sometimes referred to as RAID
6, is similar to RAID 5 in that parity data is generated and
stored to protect against data loss caused by physical disk
failure
Advantages
High read performance
High data availability
More usable disk capacity
27. RAID 10
Combining RAID 0 and RAID 1 is often referred to as RAID 10
which offers higher performance than RAID 1 but at much
higher cost
Uses multiple (mirrored) RAID 1 in a single array
Data striped across all mirrored sets
Very high fault tolerance
High performance rate
29. Comparing RAID Levels
RAID 0 RAID 1 RAID 5 RAID 10
Read High 2X High High
Write High 1X Medium High
Fault No Yes Yes Yes
tolerance
Disk High Low High Low
utilization
Key Data lost Use double the Lower throughput Very expensive, not
problems when any disk disk space with disk failure scalable
fails
Key High I/O Very high I/O A good overall High reliability with
advantages performance performance balance good performance
30. On-Line Spare
• Replacement for failed drive
• Requires hardware fault tolerance
• Background rebuild process
• Four On-Line Spares maximum (Smart
Array controller)
Mirrored Mirrored
Pair Pair
Mirrored
Pair
On-Line On-Line
Spare Spare
Before During After
Failure Failure Replacement
31. Conclusion
So what have we learned here?
Well we have learned that RAID is not just a
bug spray. RAID is a good solution for
companies or individuals carving more
transfer performance, redundancy and
storage capacity in their data storage
systems.
