performance evolution of raid

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The Presentation

Performance Evolution of RAID

Presentation Topic

Member: • Zubair Hossain (13103037)

Group: Processor

1. What is RAID?2. Importance of RAID3. Concept about RAID5. Standard Raid Level6. Nested Raid Level7. Implementation of Raid8. RAID Performance Issue9. Performance and Advantages

Comparison among RAID Levels10. Conclusion11.References

Index

WHAT IS RAID?

RAID MEANS REDUNDANT ARRAY OF INDEPENDENT DISKS.

IT IS ALSO CALLED REDUNDANT ARRAY OF INEXPENSIVE DISKS.

IMPORTANCE OF RAID

1. RELIABILITY

2. REAL-TIME DATA RECOVERY WITH UNINTERRUPTED ACCESS WHEN A HARD DRIVE FAILS

3. SYSTEM UPTIME AND NETWORK AVAILABILITY AND PROTECTION FROM LOSS PROTECTION AGAINST DATA LOSS

4. MULTIPLE DRIVES WORKING TOGETHER INCREASE SYSTEM PERFORMANCE

CONCEPT ABOUT RAID

THE TWO FUNDAMENTAL RAID CONCEPTS :

Data Redundancy

Data striping

DATA REDUNDANCY

REDUNDANCY GIVES US THE ABILITY TO HAVE A DRIVE FAIL WITHOUT LOSING VALUABLE DATA

There are Two Type of Data Redundancy

1. Disk Mirroring2. Data Parity

DISK MIRRORING• KEEP TO COPIES OF DATA ON TWO SEPARATE DISKS

• GIVES GOOD ERROR RECOVERY

• IF SOME DATA IS LOST, GET IT FROM THE OTHER SOURCE

• EXPENSIVE

• REQUIRES TWICE AS MANY DISKS

• WRITE PERFORMANCE CAN BE SLOW

• HAVE TO WRITE DATA TO TWO DIFFERENT SPOTS

• READ PERFORMANCE IS ENHANCED

• CAN READ DATA FROM FILE IN PARALLEL

DATA PARITY

• WAY TO DO ERROR CHECKING AND CORRECTION

• ADD UP ALL THE BITS THAT ARE 1

• IF EVEN NUMBER, SET PARITY BIT TO 0

• IF ODD NUMBER, SET PARITY BIT TO 1

• CONSIDER THE FOLLOWING 2 BYTES

BYTE PARITY

10110011 1

01101010 0

• IF A SINGLE BIT IS BAD, IT IS POSSIBLE TO CORRECT IT

DATA STRIPING

DATA STRIPING IS THE TECHNIQUE OF SEGMENTING LOGICALLY SEQUENTIAL DATA, SUCH AS A FILE, SO THAT CONSECUTIVE SEGMENTS ARE STORED ON DIFFERENT PHYSICAL STORAGE DEVICES.

STANDARD RAID LEVEL RAID – 0 (STRIPING)

RAID – 1(MIRRORING)

RAID – 2(Redundancy through Hamming code)

RAID – 3(Bit Interleaved Parity)

RAID – 4(Block Interleaved parity)

RAID – 5(STRIPING AND PARITY)

RAID-6 (P+Q redundancy)

RAID-0

DATA ARE STRIPPED ON ALL DISKS

OFFER PERFORMANCES

NO REDUNDANCY

2 DISKS MINIMUM, MAXIMUM DEPENDING OF RAID CONTROLLER

DATA ARE SPLIT DEPENDING OF STRIPE SIZE (16/32/64/128KB)

controller

RAID-1

DATA MIRRORED (DUPLICATED) ON SECOND HARD DISK

OFFER REDUNDANCY

EQUIVALENT OF ONE DISK SPACE LOST FOR REDUNDANCY

ONLY ON 2 DISKS

SUPPORT ONE DISK FAILURE

Controller

• STRIPES DATA AT THE BIT LEVEL

• USES A HAMMING CODE FOR ERROR CORRECTION.

• THE DISKS ARE SYNCHRONIZED BY THE CONTROLLER TO SPIN AT THE SAME ANGULAR ORIENTATION

• EXTREMELY HIGH DATA TRANSFER RATES ARE POSSIBLE.

• THIS IS THE ONLY ORIGINAL LEVEL OF RAID THAT IS NOT CURRENTLY USED.

RAID-2

RAID-3

RAID 3 IS CONFIGURED WITH AT LEAST THREE DRIVES

TWO DRIVES CARRY THE DATA AND THE THIRD DRIVE CARRIES PARITY INFORMATION.

USES BYTE-LEVEL STRIPING WITH A DEDICATED PARITY DISK.

GENERALLY CANNOT SERVICE MULTIPLE REQUESTS SIMULTANEOUSLY.

ANY I/O OPERATION REQUIRES ACTIVITY ON EVERY DISK AND USUALLY REQUIRES SYNCHRONIZED SPINDLES

RAID-4

A RAID 4 USES BLOCK-LEVEL STRIPING WITH A DEDICATED PARITY DISK.

OTHER ARE SAME AS RAID 3

Stripes data and parity to generate redundancy. The parity is distributed through the stripe of the disk array. both parity and data are striped across a set of separate disks. Data chunks are much larger than the average I/O size, but are

still resizable. Disks are able to satisfy requests independently

RAID-5

RAID-6

Data is striped across all disks (minimum of four) A two parity blocks for each data block (p and q in the

diagram) is written on the same stripe. If one physical disk fails, the data from the failed disk

can be rebuilt onto a replacement disk. Provides for faster rebuilding of data from a failed disk.

NESTED RAID LEVEL

LEVELS OF NESTED RAID, ALSO KNOWN AS HYBRID RAID COMBINE TWO OR MORE OF THE STANDARD LEVELS OF RAID TO GAIN PERFORMANCE, ADDITIONAL REDUNDANCY, OR BOTH.

Nested Raid Level

RAID-(0+1)

RAID-10 RAID-50 RAID-60

RAID (0+1)

• RAID-01 IS TECHNICALLY A COMBINATION OF RAID-1 AND RAID-0, INCLUDES BOTH MIRRORING AND STRIPING, BUT WITHOUT PARITY.

RAID 10

IT USES RAID-1MIRRORING AND RAID-0 STRIPING, AND HAS BOTH SECURITY AND SEQUENTIAL PERFORMANCE.

IT IS A STRIPED RAID-0 ARRAY WHOSE SEGMENTS ARE MIRRORED RAID-1.

IT IS SIMILAR IN PERFORMANCE TO RAID 0+1, BUT WITH BETTER FAULT TOLERANCE AND REBUILD PERFORMANCE.

IT HAS THE SAME FAULT TOLERANCE AS RAID-1 WITH THE SAME OVERHEAD FOR FAULT TOLERANCE AS MIRRORING ALONE.

RAID-50 • COMBINES MULTIPLE RAID 5

SETS WITH RAID 0 (STRIPING).

• REBUILD TIMES ARE SUBSTANTIALLY LESS THAN A SINGLE LARGE RAID 5 ARRAY.

• USABLE CAPACITY OF RAID 50 IS BETWEEN 67% - 94%, DEPENDING ON THE NUMBER OF DATA DRIVES IN THE RAID SET.

RAID-60

• COMBINES MULTIPLE RAID 6 SETS WITH RAID 0 (STRIPING).

• DUAL PARITY ALLOWS THE FAILURE OF TWO DISKS IN EACH RAID 6 ARRAY.

• STRIPING HELPS TO INCREASE CAPACITY AND PERFORMANCE WITHOUT ADDING DISKS TO EACH RAID 6 ARRAY .

IMPLEMENTATION OF RAID

• THERE ARE TOW TYPES OF RAID IMPLEMENTATION. SoftwareImplementat

ion

Hardware Implementat

ion

SOFTWARE IMPLEMENTATION

• SOFTWARE RAID CAN BE IMPLEMENTED IN A VARIETY OF WAYS: IN THIS CASE, THE RAID IMPLEMENTATION IS AN APPLICATION RUNNING ON THE HOST WITHOUT ANY ADDITIONAL HARDWARE. THIS TYPE OF SOFTWARE RAID USES HARD DISK DRIVES WHICH ARE ATTACHED TO THE COMPUTER SYSTEM VIA A BUILT-IN I/O INTERFACE OR A PROCESSOR-LESS HOST BUS ADAPTER (HBA). THE RAID BECOMES ACTIVE AS SOON AS THE OPERATING SYSTEM HAS LOADED THE RAID DRIVER SOFTWARE.

SOFTWARE IMPLEMENTATION

SOFTWARE BASED RAID:

• SOFTWARE IMPLEMENTATIONS ARE PROVIDED BY MANY OPERATING SYSTEMS.

• A SOFTWARE LAYER SITS ABOVE THE DISK DEVICE DRIVERS AND PROVIDES AN

ABSTRACTION LAYER BETWEEN THE LOGICAL DRIVES(RAIDS) AND PHYSICAL

DRIVES.

• SERVER'S PROCESSOR IS USED TO RUN THE RAID SOFTWARE.

• USED FOR SIMPLER CONFIGURATIONS LIKE RAID0 AND RAID1.

HARDWARE IMPLEMENTATION • A HARDWARE RAID SOLUTION HAS

ITS OWN PROCESSOR AND MEMORY TO RUN THE RAID APPLICATION. IN THIS IMPLEMENTATION, THE RAID SYSTEM IS AN INDEPENDENT SMALL COMPUTER SYSTEM DEDICATED TO THE RAID APPLICATION, OFFLOADING THIS TASK FROM THE HOST SYSTEM. HARDWARE RAID CAN BE FOUND AS AN INTEGRAL PART OF THE SOLUTION

HARDWARE IMPLEMENTATION

HARDWARE BASED RAID:

• A HARDWARE IMPLEMENTATION OF RAID REQUIRES AT LEAST A SPECIAL-

PURPOSE RAID CONTROLLER.

• ON A DESKTOP SYSTEM THIS MAY BE BUILT INTO THE MOTHERBOARD.

• PROCESSOR IS NOT USED FOR RAID CALCULATIONS AS A SEPARATE CONTROLLER

PRESENT.

• The key to performance increases under RAID is parallelism. The ability to access multiple disks simultaneously allows for data to be written to or read from a RAID array faster than would be possible with a single drive.

Performance Issue can be discussed by four points:• Read and Write Performance• Positioning and Transfer Performance• Stripe Width and Stripe Size

RAID Performance Issue

• Mirroring: In Raid every piece of data is duplicated, stored on both drives. There's absolutely no reason to access both drives; the controller, if intelligently programmed, will only ask one of the drives for the data--the other drive can be used to satisfy a different request. This makes RAID significantly faster than a single drive for reads, under most conditions.

• Striping Without Parity: A RAID 0 array has about equal read and write performance . The reason is that the "chopping up" of the data without parity calculation means we must access the same number of drives for reads as we do for writes.

• Striping With Parity: As with mirroring, write performance when striping with parity (RAID levels 3 through 6) is worse than read performance, but unlike mirroring, the "hit" taken on a write when doing striping with parity is much more significant.

Read and Write Performance

• Mirroring: Mirroring improves positioning performance. However, once the data is found, it will be read off one drive.• Striping: Large files that are split into enough blocks to span every drive in

the array require each drive to position to a particular spot, so positioning performance is not improved; once the heads are all in place however, data is read from all the drives at once, greatly improving transfer performance.

On reads, small files that don't require reading from all the disks in the array can allow a smart controller to actually run two or more accesses in parallel. This improves both positioning and transfer performance.

Positioning and Transfer Performance

• Decreasing Stripe Size: As stripe size is decreased, files are broken into smaller and smaller pieces. This increases the number of drives that an average file will use to hold all the blocks containing the data of that file, theoretically increasing transfer performance, but decreasing positioning performance.• Increasing Stripe Size: Increasing the stripe size of the array does

the opposite of decreasing it, of course. Fewer drives are required to store files of a given size, so transfer performance decreases. However, if the controller is optimized to allow it, the requirement for fewer drives allows the drives not needed for a particular access to be used for another one, improving positioning performance.

Stripe Size

Performance and Advantages Comparison among RAID Levels

RAID has been with us for about 20 years, but until only recently it was the domain of big systems and deep pockets. During those 20 years, however, a number of factors have come together to make RAID a reality for both big servers and common desktop systems. Imagine a world where dirt-cheap RAID on every computer means no one ever again losing critical data. The first desktop RAID systems have already appeared on the market. As disk size and cost continue to decline, widespread use of RAID on the desktop is only a matter of time.

Conclusion

• 1. RAID for Enterprise Computing – Copyright @ VERITAS Software Corporation, 1999, 2000 • 2. RAID: High-Performance, Reliable Secondary Storage- • 3. http://en.wikipedia.org/wiki/RAID • 4. http://en.wikipedia.org/wiki/Non-

standard_RAID_levels#RAID_5E.2C_RAID_5EE.2C_and_RAID_6E • 5. http://en.wikipedia.org/wiki/Standard_RAID_levels • 6. http://en.wikipedia.org/wiki/Nested_RAID_levels • 7. http://www.pcguide.com/ • 8. RAID Levels and Components Explained-Developed in Nov. 2007 by Jerry Scott • 9. RAID Storage — Speed and Security (PDF) • 10. RAID Theory: An Overview - Ben Rockwood, Cuddletech • 11. Storage solutions white paper - Copyright 2006 Adaptec, Inc.

References