Home | Projects | Notes > Operating Systems > RAID Storage

RAID Storage

What is RAID?

• Commonly stands for Redundant Array of Inexpensive Disks (or Redundant Array of Independent Drives)

• A RAID puts together two or more separate drives that appear to behave as a single drive.

• A RAID distributes data across volumes.

Why RAID?

• There are several forms of RAID that are designed to:

  • Provide faster access to data

  • Keep data safe

  • Or a combination of both

Common Implemented RAID Levels

• RAID 0

  • Blocks are striped, no mirror, no parity

  • Fastest

  • No protection from disk failure

  • Requires 2 or more disks

• RAID 1

  • Blocks mirrored, no stripe, no parity

  • Safest

  • Most disk failure protection

  • Best for critical data where access speed is not an issue

  • Requires 2 or more disks

• RAID 4

  • Blocks are striped, dedicated parity

  • Fast and safe

  • Best for general use on SSDs

  • Fast read/write of large files

  • Requires 3 or more disks

• RAID 5

  • Blocks stribed, distributed parity

  • Best for general use on HDDs

  • Fast read/write use on HDDs

  • Fast read/write of large files

  • Requires 3 or more disks

• RAID 6

  • Similar to RAID 5 with extra parity recovery information

  • Allows for the failure of 2 disks

  • Slightly slower writes

  • Requires 4 or more disks

• RAID 1+0 (10)

  • Blocks mirrored and striped

  • Fast and safe

  • Requires 4 or more disks

Stripted Volume

• A striped volume spreads data from a single volume over two or more drives.

• Each drive contains only a portion of a file.

• When reading or writing, the request gets sent to all drives at the same time and each drive works only on its portion of the file.

• This system FAILS if one drive fails!

Mirrored Volume

• Mirrored volumes each contain a copy of the entire drive.

• Provides protection of data through reducndancy.

• If one drive fails, the other(s) still have a copy of the data.

Parity

• Creates Fault Tolerance in striped drives.

• Calculates info about data stored in two drives and stores on a third.

• If a drive fails, then the data can be rebuilt for it from the parity blocks.

• Safer, but slower and requires more storage.

• Distributed parity distributes parity data across all drives in the array.

References

Stallings, W. (2018). Operating Systems: Internals and Design Principles (9th ed.). Pearson Education, Inc.