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CMSC 691X: UNIX Systems CMSC 691X: UNIX Systems AdministrationAdministration

What is RAID?• Redundant Array of

Inexpensive/Independent Disks• RAID can improve availability and

throughput (although actually reliability – whether anything is broken – suffers because of the larger number of disks)

• Data is stored on several disks instead of a single disk

RAID Levels

RAID LevelFailures Survived

Data Disks

Check Disks

0 Nonredundant 0 8 0

1 Mirrored 1 8 8

2 Memory-style ECC 1 8 4

3 Bit-interleaved parity 1 8 1

4 Block-interleaved parity 1 8 1

5 Block-interleaved distributed parity

1 8 1

6 P+Q redundancy 2 8 2

RAID 0: Striping

• This level offers no redundancy – no extra data is kept.

• The performance is the best of any level. Throughput is increased by striping data across several disks.

RAID 1: Mirroring

• Uses twice as many disks• Whenever data is written to one

disk, that data is also written to a redundant disk so that there are always two copies of the information

• When a disk fails, the system merely goes to its mirror for the data

RAID 3: Bit-Interleaved Parity

• Reads and writes go to all disks in a group, with one extra disk to hold the check information in case there is a failure.

• Parity is simply the sum of the data in all the disks modulo 2. Lost data can be reconstructed by examining the parity.

• Every access goes to all disks.

RAID 4: Block-Interleaved Parity

• Allows applications to do smaller accesses than RAID 3, allowing independent accesses to occur in parallel.

• Small, independent reads are easy – simply read the data and then check for error detection.

• Writes are harder – old data is read, new data is compared, and only those parity bits whose values change are updated.

• The parity disk becomes a bottleneck, since the parity disk must be updated on every write.

RAID 5: Block-Interleaved Distributed Parity

• This is a way to get rid of the bottleneck of RAID 4 – distribute the parity information across all disks.

0

3

6

9

1

4

7

10

2

5

8

11

P0

P1

P2

P3

0

3

6

P3

1

4

P2

9

2

P1

7

10

P0

5

8

11

RAID 4 RAID 5

Higher Levels• RAID 6: Adds a second parity scheme that

is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance

• RAID 7: Includes a real-time embedded operating system as a controller, caching via a high-speed bus, and other characteristics of a stand-alone computer.

Higher Levels (cont.)• RAID 10: Offers an array of stripes in

which each stripe is a RAID-1 array of drives. This offers higher performance than RAID-1 but at much higher cost.

• RAID 53: Offers an array of stripes in which each stripe is a RAID 3 array of disks. This offers higher performance than RAID 3 but at much higher cost.

RAID in Linux

• Linux offers built-in software RAID capabilities

• Advantages of Linux Software RAID– Threaded rebuild process– Fully kernel-based configuration– Backgrounded array reconstruction– Hot-swappable drive support– Automatic CPU detection to use CPU

Optimizations

Options for Software RAID

• You can create a software RAID in Linux using only one hard disk!

• You can create a software RAID in Linux using multiple drives.

• Specifying the type of RAID you wish to install on your systems, depends on how they are used; refer back to RAID Levels.

Creating RAID Partitions

• Using Disk Druid, Select software RAID from the Filesystem Type

• Select the drive on which the RAID is to be created

• Enter the size of the RAID partition

• Choose other options as needed for your RAID

Software RAID Configuration

• Once RAID partitions have been created, select the Make RAID option on the Disk Druid main partitioning screen.

• Enter a mount point, select a filesystem, and choose your RAID Level.

• A spare partition can be specified for RAID 1 and RAID 5

RAID Drive Summary

• You have now created a software RAID in Linux.

• Try it at home!

Rules of Thumb

• When setting up a RAID with different sized hard drives, configure partitions on each drive to be the same size.

• Whenever possible, use the same hardware specifications for multiple hard drives

• Creating a software RAID on a single hard drive will slow performance because data must be written twice using only one head.

Do-It-At-Home RAID• Build your own home RAID using

– Soyo KT-SY333 Dragon Ultra Motherboard– AMD Athlon XP 2200+ CPU– 1024MB DDR PC2700 RAM– (4) 160GB Maxtor DiamondMax DX540 Hard

drives– Multiple cooling fans– Large Tower Case with a 400W power supply

Soyo KT-SY333 Motherboard

• Embedded Hipoint IDE-RAID chip, providing ATA-133 IDE-RAID 0,1,0+1

• Setup both Hardware and Software RAIDs simultaneously!

Setting up your RAID• Enter the BIOS set-up of your Soyo

motherboard.• Enable the desired hardware RAID

Level.• Install Linux on your machine with or

without the software RAID option.• Selecting hardware RAID Level 0 and

software RAID Level 0 will provide the best level of performance.

• This setup provides for a fast 640GB RAID.

Additional Resources

• Patterson, David A. and John L. Hennessy. Computer Architecture: A Quantitative Approach. San Francisco, CA: Morgan Kaufmann Publishers, Inc., 1996.

• http://whatis.techtarget.com/definition/0,289893,sid9_gci214332,00.html

Additional Resources …

• http://www.redhat.com/docs/manuals/linux/RHL-7.3-Manual/custom-guide/

• http://www.soyotek.com/products/proddesc.php?id=46

• http://www.maxtor.com/• http://www.amd.com/