RAIDSECTION (2.3.5)

ASHLEY BAILEY

SEYEDFARAZ YASROBI

GOKUL SHANKAR

OVERVIEW

• History of RAID

• Importance of RAID

• RAID Levels and Pros/Cons

HISTORY OF RAID

• CPU performance has been increasing exponentially over the past decade. Not so with disk performance, the gap between CPU performance and disk performance has become much larger over time. 1

• Rapid improvements in capacity of large disks was not only target of disk designers, also price-performance and reliability cost. 2

• Unreliability of disks forced computer system managers to make backup version frequently. The Mean Time To Failure(MTTF) of an array of disks could be greater-than 3 years (30,000 hr MTTF IBM 3380). 2

1 Structured Computer Organization, 6th ed. , Austin, Tanenbaum2 Patterson et al., 1988

HISTORY OF RAID

• To overcome this unreliability in inexpensive large disks in 1988 Patterson et al. described a new way to make use of the extra disks containing redundant information = Redundant Arrays of Inexpensive Disks (RAID) *Changed in 1980 to: Redundant Arrays of Independent Disks.

• RAID = A disk subsystem that increases performance or provides fault tolerance or both. 2

• Several different schemes for RAIDs to look like a single disk were defined and are known as RAID level 0 through RAID level 5. RAID levels are six different organizations (not hierarchy), each with a different mix of reliability and performance characteristics. 1

1 Structured Computer Organization, 6th ed. , Austin, Tanenbaum2 http://www.pcmag.com/encyclopedia/term/50148/raid

• IMPORTANCE OF RAID

• Many RAIDs consist of a RAID SCSI controller plus a box of SCSI disks that appear to the operating system as a single large disk. No software changes are required to use the RAID. 1

• RAIDs appear like a single disk to the software, all RAIDs have the property that the data are distributed over the drives, to allow parallel operation. 1

• RAIDs offer a cost effective option to meet the exponential growth in the processor and memory speeds. 2

• Today, most motherboards have built-in RAID but not necessarily every RAID configuration. 3

1 Structured Computer Organization, 6th ed. , Austin, Tanenbaum2 Patterson et al., 19883 http://www.pcmag.com/encyclopedia/term/50148/raid

• IMPORTANCE OF RAID

RAID allows:

• the ability to change disks while the system continues to run.

• for faster data transfers.

• a loss of a physical disk will not mean the loss of your data, because data is written on more than one physical disk.

All info on this slide from: http://www.elnexus.com/articles/raid.aspx

LEVELS OF RAID

• In RAIDs, data is distributed over different drives, to allow parallel operation.

• There are several schemas for doing this, known as RAID levels (increasing levels are not necessarily better, they are just different)

• Most RAIDs have fault tolerance: If one component fails, then the design will ensure the request is still successful. This minimizes data loss

RAID 0

• Utilizes striping: distributing data over multiple drives

• Drives consist of strips and drives are accessed in a round robin fashion to maximize parallelization

• Each strip has k sectors (the first having 0 to k-1)

• Very fast, but only used when data loss is not a concern

Pros: • Works best for large data requests. They will be split

up among all disks, to run in parallel

Cons: • No fault tolerance provided in this design• Is less useful with operating systems that ask for data

one sector at a time. This would fail to utilize any parallelization. A single drive in a RAID is much smaller than a SLED, so a RAID would not be recommended in this scenario

All info on this slide from: Structured Computer Organization, 6th ed. , Austin, Tanenbaum

RAID 1

• Disk mirroring: Similar in structure to RAID 0 (striping), but each drive is duplicated. When writing data, each duplicate drive is also written with the same data. This means when reading data, either drive can be utilized for faster read times.

Pros: • Up to twice as fast read times.• If a drive fails, then the duplicate can be used with no

problems.

Cons: • Increased write times compared to RAID 0• Higher costs to maintain the duplicate drives

All info on this slide from: Structured Computer Organization, 6th ed. , Austin, Tanenbaum

RAID 2

• Bit level parity: works on a word (or even byte) basis. For example, if you had a 7-bit Hamming coded word (4-bit word with 3 parity words), then a 7 drive RAID could be used to distribute each bit.

• A 38-bit Hamming word (32-bit word plus 6 parity bits and 1 bit for word parity) would need 39 drives.

Pros: • The total throughput is immense: 32 sectors worth of

data can be written in one sector time.• If one bit/drive fails, then the Hamming code will

repair the data right away

Cons: • The number of drives needed means costs will be

greater and all the drives must be rotationally synchronized. Overhead (for cost and performance) will be increased

All info on this slide from: Structured Computer Organization, 6th ed. , Austin, Tanenbaum

RAID 3

• Byte level parity: Very similar to RAID 2, except only 1 parity bit is used. This seems more error prone than RAID 2, which is true for some random errors, but for most errors, if a single drive fails, then the design will know which bit is causing the error and will know how to correct it.

Pros: • Just like RAID 2, offers high data rates

Cons: • Like RAID 2, the number of separate I/O requests per

second is no better than a single drive• The drives must be rotationally synchronized

All info on this slide from: Structured Computer Organization, 6th ed. , Austin, Tanenbaum

ADDITIONAL NOTES ON RAID 0-3

RAID 0: Used where speed is prioritized over data integrity. Mostly for intensive applications, like graphics/video editing/engineering apps. Also used by applications that store a large amount of temporary uncompressed image data (like Photoshop) for quick data retrieval. 1

RAID 1: Commonly used in consumer desktops; also entry servers 1

RAID 2: Rarely used because of the complexity of an external Hamming code and because of the costs to implement a large number of disks 2

RAID 3: Very uncommon because of the need for synchronous disk spin and a dedicated parity drive. Sometimes used in specialized database or processing environments 2

1: http://www.elnexus.com/articles/raid.aspx2: http://www.pcmag.com/article2/0,2817,2370235,00.asp

RAID 4

• Uses block level striping

• RAID level 4 is like RAID level 0, with a strip-for-strip parity written onto an extra drive. If a drive crashes, the lost bytes can be recomputed from the parity

• Uses multiple data disks, and a dedicated disk to store parity

• This is not commonly used

Pros: • Good random reads, as the data blocks are striped.

Cons:

• Bad random writes, as for every write, it has to write to the single parity disk.

RAID 5

• RAID level 5 by distributes the parity bits uniformly over all the drives in round robin fashion

• the most common RAID configuration for business servers and enterprise NAS devices

Pros: • You can lose any one disk and not lose your backup

data. Just replace the disk with a new one.

Cons: The downside to RAID 5 is the performance hit to servers that perform a lot of write operations

RAID 6

• It's identical to RAID 5, except it's an even more robust solution because it uses one more parity block than RAID 5

• RAID 6 is also used frequently in enterprises

Pros: • It allows for the failure of two disks simultaneously

with no data loss.

Cons: Slightly slower than RAID 5 on writes but there is no added delay for reads.

RAID 10 (HYBRID)

• RAID 10 = RAID 1 + RAID 0

• RAID 10 works by striping and mirroring your data across at least two disks.

Pros: • RAID 10 is secure because mirroring duplicates all

your data. • It’s fast because the data is striped across two or

more disks

Cons: • expensive

RAID 50 (HYBRID)

• RAID 50 = RAID 5 + RAID 0

• A RAID 50 combines the straight block-level striping of RAID 0 with the distributed parity of RAID 5

• This is a RAID 0 array striped across RAID 5 elements. It requires at least 6 drives

Pros: • Provides great balance between storage

performance, storage capacity, and data integrity that’s not necessarily found in other RAID levels.

• One drive from each of the RAID 5 sets could fail without loss of data.

Cons: • The time spent in recovery (detecting and responding

to a drive failure, and the rebuild process to the newly inserted drive) represents a period of vulnerability to the RAID set.

REFERENCES

• http://www.elnexus.com/articles/raid.aspx

• http://www.pcmag.com/article2/0,2817,2370235,00.asp

• http://www.cru-inc.com/data-protection-topics/understanding-raid/

• http://www.thegeekstuff.com/2011/11/raid2-raid3-raid4-raid6/

CREDITS

• Ashley Bailey: History of RAID and Importance of RAID.

• Gokul Shankar: Levels of RAID, RAID 0, RAID 1, RAID 2, RAID 3, Additional Notes on RAID 0-3

• Seyedfaraz Yasrobi: RAID 4, RAID 5, RAID 6, RAID 10, RAID 50