What RAID is redundant?

Redundancy is an important concept in data storage and RAID (Redundant Array of Independent Disks) configurations are designed with redundancy in mind. The goal of RAID is to provide continued access to data even when one or more disks fail.

What is RAID?

RAID is a technology that combines multiple disk drives into a logical unit. Data is distributed across the drives according to the specific RAID level being used. This distribution provides various levels of redundancy and performance benefits.

There are several standard RAID levels, each with specific characteristics:

  • RAID 0: Striping without parity or mirroring. Provides improved performance but no redundancy.
  • RAID 1: Mirroring without parity or striping. Provides full redundancy but requires at least two drives.
  • RAID 5: Striping with distributed parity. Provides redundancy while requiring at least three drives.
  • RAID 6: Striping with double distributed parity. Provides redundancy while requiring at least four drives.
  • RAID 10: Mirroring and striping. Provides full redundancy and improved performance but requires at least four drives.

Which RAID levels provide redundancy?

The main RAID levels that provide redundancy are:

  • RAID 1 – Disk mirroring provides fully redundancy by writing identical data to pairs of drives. If one drive fails, the other contains an exact copy of the data.
  • RAID 5 – Distributed parity allows recovery of data if one drive fails. Parity information is spread across all drives.
  • RAID 6 – Double distributed parity provides redundancy for up to two drive failures. Uses dual parity schemes.
  • RAID 10 – A combination of disk mirroring and disk striping provides redundancy through mirroring.

RAID levels without redundancy include RAID 0 and JBOD (Just a Bunch of Disks). These provide better performance but no protection against drive failure.

How does RAID 1 redundancy work?

RAID 1 creates an exact copy of data from one drive to another, producing a mirrored set. All data is duplicated on the RAID 1 array. If one drive fails, the system can instantly switch to the other drive without any loss of data or interruption in service.

The diagram below illustrates a simple RAID 1 array with two mirrored drives:

Drive 1 Drive 2
Data A Data A
Data B Data B

RAID 1 provides complete data redundancy but requires at least two drives. There is no capacity gain since data is duplicated. Write performance may be slowed since writes must go to both drives.

How does RAID 5 redundancy work?

RAID 5 distributes parity information across all the drives. If one drive fails, the missing data can be recreated from the parity data on the remaining drives.

For example, in a 3-drive RAID 5 array, the parity information is spread across all three drives alongside the user data. If one drive fails, the system can reconstruct the data from the parity blocks on the other two functioning drives.

The diagram below shows data A & B stored with parity P across three RAID 5 disks:

Drive 1 Drive 2 Drive 3
Data A Data B Parity P

RAID 5 provides redundancy while requiring a minimum of three drives. Read performance is improved since data is striped across multiple disks. However, write performance may be impacted due to parity calculation.

How does RAID 6 redundancy work?

RAID 6 provides redundancy using double distributed parity. This allows the array to survive the loss of up to two drives.

Like RAID 5, data is striped across multiple disks. However, RAID 6 calculates and writes two independent parity blocks on different drives.

For example, in a 4-drive RAID 6 array, two of the drives contain parity data P and Q. If up to two drives fail, the data can still be rebuilt using the dual parity sets.

The diagram below illustrates RAID 6 with dual parity on a 4-disk array:

Drive 1 Drive 2 Drive 3 Drive 4
Data A Data B Parity P Parity Q

RAID 6 requires a minimum of four drives and provides protection against up to two drive failures. The dual parity calculations can impact write performance.

How does RAID 10 redundancy work?

RAID 10 combines disk mirroring and disk striping to provide redundancy and improved performance.

RAID 10 requires a minimum of four drives, arranged into pairs of mirrored drives. Data is written across the drive pairs in stripes to provide the performance benefits of RAID 0.

If any single drive fails, the complete data set is still accessible on the mirrored drive. RAID 10 can withstand multiple drive losses as long as no more than one drive in a mirrored set is lost.

The diagram below shows data striped across a 4-drive RAID 10 array with two mirrored sets:

Drive 1 Drive 2 Drive 3 Drive 4
Data A Data A Data B Data B

RAID 10 provides full redundancy through mirroring while striping improves performance. The minimum four drive requirement provides usable capacity of only 50% before redundancies.

What are the pros and cons of redundant RAID?

The main advantages of redundant RAID levels include:

  • Fault tolerance – Continue operating if a drive fails.
  • Data protection – Avoid data loss in the event of a drive failure.
  • High availability – Minimum disruption to users and applications.

The disadvantages of redundant RAID can include:

  • Increased cost – More drives required than a non-redundant solution.
  • Complexity – More complex to implement and manage.
  • Lower capacity – Efficiency is reduced since drives store parity data.
  • Slower writes – Parity calculation can impact write performance.

When is RAID redundancy most beneficial?

RAID redundancy provides the most benefit in these situations:

  • Mission critical data that cannot be lost or downtime tolerated.
  • Applications that demand high availability.
  • Insufficient or slow backups that would cause disruption if drives failed.
  • Hardware without hot swap capability where drive replacement causes downtime.
  • Older drives at higher risk of failure.
  • Frequent vibrations or movement that increase drive failure risks.
  • Long rebuild times make uptime more dependent on redundancy.

The costs of redundant RAID may not always outweigh the benefits. Non-critical data or non-essential applications may do fine with simpler disk configurations.

What are the minimum drive requirements for redundant RAID levels?

The minimum number of physical drives required for the main redundant RAID levels are:

  • RAID 1 – 2 drives
  • RAID 5 – 3 drives
  • RAID 6 – 4 drives
  • RAID 10 – 4 drives

Increasing the number of drives beyond the minimums provides additional redundancy and allows for drive failures beyond the guarantees of the RAID level:

  • RAID 1 – Additional mirrored drive pairs
  • RAID 5 – Can withstand 2 drive losses with 8+ drives
  • RAID 6 – Additional 2 drive loss tolerance per 4 drives
  • RAID 10 – More mirrored drive pairs

How is RAID 5 more redundant than RAID 0?

RAID 0 provides no redundancy while RAID 5 uses parity to provide redundant data on a minimum of three drives.

In RAID 0, data is striped across drives for better performance but there is no parity or duplication. If a drive fails, all data on the RAID 0 array will be lost.

RAID 5 stripes data across drives similar to RAID 0. But it also calculates and writes parity information that is distributed evenly across all the drives.

If a single drive fails in a RAID 5 array, the missing data can be recreated from the parity blocks on the remaining drives. This provides full redundancy and protection against drive failure that is not present in RAID 0.

RAID 5 does have slower write performance than RAID 0 since it must calculate and write the parity information. But it provides significantly more redundancy for protection against drive failures.

Is RAID 1 or RAID 10 more redundant?

RAID 1 and RAID 10 both provide full redundancy through drive mirroring. Neither is absolutely superior in redundancy.

RAID 1 mirrors two drives. RAID 10 mirrors sets of drives and stripes data across them. Both fully duplicate all data across drives.

The redundancy of RAID 10 versus RAID 1 depends on the number of drives:

  • 2 drives – RAID 1 and RAID 10 provide the same 1 drive fault tolerance.
  • 4+ drives – RAID 10 can withstand more simultaneous drive losses than RAID 1.

RAID 10 combines mirroring with striping and requires a minimum of 4 drives. This provides the opportunity to build in more redundancy than the 2 drive minimum RAID 1 configuration.

However, both RAID 1 and RAID 10 provide full data copies and complete redundancy. The choice depends on the required number of drives and the need for improved performance from RAID 10 striping.

Is software or hardware RAID better for redundancy?

Software and hardware RAID both provide redundancy capabilities. Hardware RAID is better suited for mission critical systems while software RAID offers more flexibility.

Hardware RAID advantages:

  • Better performance – Dedicated controller improves processing.
  • Lower CPU usage – Doesn’t consume server resources.
  • Automatic rebuilding – Reconstructs data after a failure.
  • Cache backup – Cache protects data during power failures.
  • RAID controller failure protection.

Software RAID advantages:

  • Lower cost – Uses existing drives and CPU.
  • Platform flexibility – Can be implemented on any server OS.
  • Easier to upgrade and migrate to new systems.
  • Support for more RAID levels and configurations.

For critical systems that require maximum redundancy, performance, and uptime, hardware RAID is the better solution. Software RAID provides more economical and flexible redundancy for less demanding applications.

Should I use RAID 5 or RAID 6 for home use?

For home systems, RAID 5 provides a good balance of redundancy and affordability for most users. RAID 6 offers additional redundancy that is likely overkill for home use cases.

Key factors for home use:

  • Cost – RAID 6 requires more drives which increases overall storage expense.
  • Complexity – RAID 6 is harder to setup, manage, and rebuild.
  • Usage – Home media, documents, etc. have more tolerance for downtime.
  • Uptime needs – Home use has less dependence on storage uptime.

The advantages of RAID 5:

  • Minimum 3 drive requirement is more affordable.
  • Single parity provides good redundancy for home use.
  • Easier to implement and manage.
  • Faster rebuilds than RAID 6.

For most home users, RAID 5 offers a cost-effective compromise between performance, capacity, and redundancy. The dual parity protection of RAID 6 offers limited additional benefit over RAID 5 for home media and data storage needs in exchange for higher system costs and complexity.

What RAID level is best for a 4 drive NAS?

For a 4 drive NAS system, the recommended RAID level is RAID 10.

RAID 10 balances performance, capacity, and redundancy by mirroring two drives and striping across sets.

The benefits of RAID 10 for a 4 drive NAS:

  • Provides full redundancy through drive mirroring.
  • Fast read performance from block-level striping.
  • 50% storage efficiency – 2 drive capacity without parity loss.
  • High throughput for demanding applications.
  • Can tolerate up to 2 drive failures by using 4+ drives.

Other options like RAID 5 provide less redundancy on only 4 drives. RAID 6 would require 6+ drives to make use of its dual parity capabilities.

A 4 drive NAS has limited capacity to work with. RAID 10 maximizes redundancy and performance by combining mirroring and striping in an efficient 4 drive configuration.

Conclusion

RAID provides varying levels of redundancy that tradeoff performance, capacity, and fault tolerance. The most redundant RAID levels are RAID 1, RAID 5, RAID 6, and RAID 10 which duplicate or distribute data across multiple drives.

Choosing the right RAID redundancy option requires understanding application needs, uptime requirements, and budget constraints. Hardware RAID provides enhanced redundancy and performance while software RAID offers flexibility and affordability.

Properly implementing a redundant RAID solution provides robust data protection against drive failures and reliability for demanding business applications.