This document discusses disk reliability and RAID levels. It begins by defining disk reliability measures like mean time between failures and annual failure rate. It then explains that using multiple disks in an array can improve reliability but decreases the mean time to failure of each disk. Different RAID levels are introduced as ways to improve reliability through redundancy. RAID levels use techniques like striping data across multiple disks, mirroring data on two disks, and calculating parity data. The document discusses several common RAID levels and how they distribute and protect data differently to provide redundancy while improving performance.

1. BITS Pilani
Pilani Campus
Data Storage Technologies
& Networks
Dr. Virendra Singh Shekhawat
Department of Computer Science and Information Systems

2. BITS Pilani, Pilani Campus
Topics
• Disk reliability measures
• Improving Disk Reliability
– RAID Levels
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Image source: msdn.microsoft.com

3. BITS Pilani, Pilani Campus
Disk Performance issues[1]
• Reliability
– Mean Time-Between-Failure (MTBF)
• e.g. 1.2 TB SAS drive states a MTBF value of 2 million
hours
– Annual Failure Rate (AFR)
• To estimate the likelihood that a disk drive will fail during
a year of full use
• Individual Disk Reliability (as claimed in
manufacturer’s warranties) is often very high
– E.g. Rated: 30,000 hours In Practice: 100,000 for an
IBM disk in 80s
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4. BITS Pilani, Pilani Campus
Disk Performance issues[2]
• Access Speed
– Access Speed of a pathway = Minimum speed among all
components in the path
– e.g. CPU and Memory Speeds vs. Disk Access Speeds
• Solution:
– Multiple Disks i.e. array of disks
– Issue: Reliability
• MTTF of an array = MTTF of a single disk / # disks in the array
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5. BITS Pilani, Pilani Campus
Disk Reliability
• Redundancy may be used to improve Reliability
– Device Level Reliability
• Improved by redundant disks
– This of course implies redundant data
– Data Level Reliability
• Improved by redundant data
– This of course implies additional disks
• (RAID) Redundant Array of Inexpensive Disks
– or Redundant Array of Independent Disks
• Different Levels / Modes of Redundancy
• Referred to as RAID levels
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How to achieve reliability?
• Use more number of small sized disks !!
– What should be the number of disks?
– How small should be the disks?
– How should they be structured and used?
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7. BITS Pilani, Pilani Campus
Performance Improvement in
Secondary Storage
• In general multiple components improves the
performance
• Similarly multiple disks should reduce access time?
– Arrays of disks operates independently and in parallel
• Justification
– With multiple disks separate I/O requests can be
handled in parallel
– A single I/O request can be executed in parallel, if the
requested data is distributed across multiple disks
• Researchers @ University of California-Berkeley
proposed the RAID (1988)
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8. BITS Pilani, Pilani Campus
RAID
• Redundant Array of Inexpensive Disks
– Connect multiple disks together to
• Increase storage
• Reduce access time
• Increase data redundancy
• Provide fault tolerance
• Many different levels of RAID systems
• differing levels of redundancy,
• error checking,
• capacity, and cost
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9. BITS Pilani, Pilani Campus
RAID Fundamentals
• Striping
– Map data to different disks
– Advantage…?
• Mirroring
– Replicate data
– What are the implications…?
• Parity
– Loss recovery/Error correction / detection
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RAID
• Characteristics
1. Set of physical disks viewed as single logical drive
by operating system
2. Data distributed across physical drives
3. Can use redundant capacity to store parity
information
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Data Mapping in RAID 0
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No redundancy or error correction
Data striped across all disks
Round Robin striping

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RAID 1
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Mirrored Disks
Data is striped across disks
2 copies of each stripe on separate disks
Read from either and Write to both

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Data Mapping in RAID 2
Bit interleaved data
Lots of redundancy
Use parallel access technique
Very small size strips
Expensive: Good for erroneous disk
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Data Mapping in RAID 3
• Similar to RAID 2
• Only one redundant disk, no matter how large the array
• Simple parity bit for each set of corresponding bits
• Data on failed drive can be reconstructed from surviving data
and parity information
• Question:
• Can achieve very high transfer rates. How?
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15. BITS Pilani, Pilani Campus
RAID 4
• Make use of independent access with block level striping
• Good for high I/O request rate due to large strips
• Bit by bit parity calculated across stripes on each disk
• Parity stored on parity disk
• Drawback???
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16. BITS Pilani, Pilani Campus
RAID 5
• Round robin allocation for parity stripe
• It avoids RAID 4 bottleneck at parity disk
• Commonly used in network servers
• Drawback
– Disk failure has a medium impact on throughput
– Difficult to rebuild in the event of a disk failure (as
compared to RAID level 1)
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17. BITS Pilani, Pilani Campus
RAID 6
• Two parity calculations
• Stored in separate blocks on different disks
• High data availability
– Three disks need to fail for data loss
– Significant write penalty
• Drawback
– Controller overhead to compute parity is very high
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18. BITS Pilani, Pilani Campus
Nesting of RAID Levels:
RAID(1+0)
• RAID 1 (mirror) arrays are built first,
then combined to form a RAID 0
(stripe) array.
• Provides high levels of:
– I/O performance
– Data redundancy
– Disk fault tolerance.
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19. BITS Pilani, Pilani Campus
Nesting of RAID Levels:
RAID(0+1)
• RAID 0 (stripe) arrays are built first, then
combined to form a RAID 1 (mirror) array
• Provides high levels of I/O performance
and data redundancy
• Slightly less fault tolerance than a 1+0
– How…?
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RAID Implementations
• 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 RAID 0 and
RAID 1.
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21. BITS Pilani, Pilani Campus
Hardware Implementation
• 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.
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22. BITS Pilani, Pilani Campus
Thank You!
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