Redundant Arrays of Inexpensive Disks (RAID)

What a cool idea!

Andrew RobinsonUniversity of Michigan

<androbin@umich.edu>

Authors

• David A Patterson• Garth Gibson• Randy H Katz

Officially published in 1988.

Overview

• What is RAID?• Why bother?• What is RAID, really?• How well does it work?• How’s it holding up?

What is RAID?

• Take a bunch of disks and make them appear as one disk.

• Put data on all of them• Use all at once to gain performance• Duplicate data to gain reliability• Buy cheap disks to gain dollars

why bother?This seems like a lot of work… ???

What is RAID?Why bother?What is RAID, really?How well does it work?How’s it holding up?

Let’s go back to 1987

CPUs and Memory kept getting faster…

• Exponential growth everywhere!• CPU Performance: 1.4X increase per year– More transistors– Better architecture

• Memory Performance: 1.4-2X increase per year– Invention of caches– SRAM technology

… but disks did not.

• It’s hard to make things spin exponentially faster every year (they tend to fly apart).

• Disk seek time improved at a rate of approximately 7% a year.

• Caching had been employed to buffer I/O activity, this works reasonably well for predictable workloads.

Slow I/O Makes Slow Computers

• Amdahl’s Law describes the impact of only improving some pieces, while leaving others.

S – The effective speedupF – Fraction of work in faster modeK – Speedup while in faster mode

…really slow.

• If applications spend 10% of their time in I/O, when computers are 10 times faster, they will only appear 5% faster.

Something needed to be done.

What should we do?

• Single Large Expensive Disks (SLED) are not improving fast enough.

• Larger memory or solid state drives weren’t practical

• Small personal hard drives are emerging… can we do something with those?

Inexpensive Disks Rock

Visual Comparison

Why didn’t someone do this before?

• Standards like SCSI have finally allowed drive makers to integrate features seen in traditional mainframe controllers.

There is a problem…

• A hundredfold increase in number of disks means a hundredfold increase decrease in total reliability

what is RAID, really?that’s all really nice, but ???

What is RAID?Why bother?What is RAID, really?How well does it work?How’s it holding up?

A couple levels… a single idea

• RAID manages the tradeoff between performance and reliability

• RAID comes in levels (RAID1 to RAID5)• These levels represent points in the

performance reliability space

Groups, Disks, and Check Disks

• RAID organizes disks into groups of reliability• Some of the disks in a group store error

correcting data

D = Total disks with dataG = Disks in a groupC = Number of check disks in a group

Metrics

• Useable Storage – Percent of storage that holds data, excluding parity information

• Performance – Tough to make one number:– Reads, Writes, and Read-Modify-Write Access

Patterns– Sequential and Random Data Distribution

RAID1 – The Naive Approach

• Mirroring of all data• To read:– Use either disk

• To write:– Send to both disks

simultaneously

• Minor read performance increase.

Evaluation

Pros• Reads can occur

simultaneously• Seek times can improve

with special controllers• Predictable performance

Cons• Useable storage is cut in

half• All other performance

metrics are left the same

Alright for large sequential jobs and transaction processing jobs

RAID2 – Bit Level Striping

• Uses Hamming Code for Error Detection• Requires many check disks– For 10 data disks, 4 check disks– For 25 data disks, 5 check disks

• Can detect errors, and determine the at-fault disk

RAID2 - Visually

Evaluation

Pros• Better useable storage, 71%

for G=10, 83% for G=25

Cons• Dismal small random data

access performance: 3-9% of RAID1 or SLED

Good for large sequential jobs, bad for transaction processing systems.

RAID3 – Byte Level Striping

• Simpler parity error correction• Only a single check disk required for error

detection• Cannot determine which disk failed, but that’s

usually pretty obvious• Transfers of large continuous blocks is good

RAID3

Evaluation

Pros• Even better useable

storage, 91% for G=10, 96% for G=25

Cons• Small random data access

performance: Just as bad as RAID2

Even better for large sequential jobs, bad for transaction processing systems.

What is parity?

• Parity is calculated as an XOR of the data blocks.

• XOR is reversible:– 1011 (A1) XOR 1100 (A2) => 0111 (AP) “parity”

– 0111 (AP) XOR 1011 (A1) => 1100 (A2)

– 0111 (AP) XOR 1100 (A2) => 1011 (A1)

• This makes error detection and reconstruction possible!

RAID4 - Block Level Striping

• Like RAID3, but more parallelly• Interleave data at sector level rather than bit

level• Allows for servicing of multiple block requests

by different drives• Still keeps all the parity information on a single

drive

RAID4

Evaluation

Pros• Finally better small random

access. Reads are fast!

Cons• Small writes, and read-

write-modifies are still slow.

Good for large sequential jobs, still not great for transaction processing systems.

RAID5 – Block Level Striping with Distributed Parity

• Instead of checksums on a single disk, we distribute them across all disks.

• Allows us to support multiple writes per group

RAID5

Evaluation

Pros• Really good usable storage• Finally decent small random

data access performance across the board!

Cons• Slightly worse write

performance, data must be written to two disks simultaneously

Finally, a system that works well for both applications!

how well does it work?sounds complicated, ???

What is RAID?Why bother?What is RAID, really?How well does it work?How’s it holding up?

As a Whole

• RAID has many different levels that achieve different tradeoffs in reliability and performance

• Almost all of them, for some (or many) use cases will outperform a SLED for the same cost.

Read-Modify-Write Per Disk Performance

how’s it holding up?wow, raid sounds awesome, ???

What is RAID?Why bother?What is RAID, really?How well does it work?How’s it holding up?

Arriving back in 2012 now…

RAID has held up remarkably well

• Data centers around the world use RAID technology.

• The small, inexpensive disk is the de facto standard of storage

• The ideas developed for RAID have been applied to many not-RAID things

Some open questions

• What will become of RAID as new, super fast storage mediums start to become cost effective?

• How does it fit in with massive internet-scale storage farms?

Take Aways

• RAID offers significant advantage over SLED for the same cost– RAID5 offers 10x improvement in performance,

reliability, and power consumption while reducing size of array.

• RAID allows for modular growth (add more disks)• Cost effective option to meet challenge of

exponential growth in processor and memory speeds

References

• “A Case for Redundant Arrays of Inexpensive Disks” by David A Patterson, Garth Gibson, and Randy H Katz

• “RAID: A Personal Recollection of How Storage Became a System” by Randy H Katz

• Slides by David Luo and Ramasubramanian K.• Images generously borrowed from Wikipedia

<http://en.wikipedia.org/wiki/RAID>

Thank you!