What is the difference between RAID 5, RAID 5 plus a Hot Spare and RAID 6?

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Question

What is the difference between RAID 5, RAID 5 plus a Hot Spare and RAID 6?

Answer

The RAID combines two or more physical drives into a logical unit presented as a single hard drive to the operating system. There are currently six basic RAID levels: RAID 0, RAID 1, RAID 0+1, RAID 1+0, RAID 3, RAID 4, RAID 5 and RAID 6.

The scope of this article is to provide basic information for the levels RAID 5 and RAID 6 and to compare them from point of view of performance and security.  

Hot spare is a drive that acts as a stand by drive in RAID 1, RAID 5 or RAID 6 volume. It is fully functional drive that contains no data and is not used during normal operation. If a drive from the volume fails, the controller reconstructs the data from the failed drive to the hot spare drive.

• A RAID 5 array is designed to protect against the failure of a single disk within the array. Because of the way that RAID 5 works, the total capacity of one disk is lost to overhead. If, for example, a RAID 5 array contained five 10TB disks, then the array’s usable capacity would be 40TB.
  Requires a minimum of three disk drives. Data is striped across multiple drives with distributed parity. Offers fault tolerance by using parity information to reconstruct data in the event of a single drive failure. Good performance for both read and write operations. Capacity utilization is high since only one drive's worth of space is used for parity. However, RAID 5 does not protect against multiple drive failures. If a second drive fails before the first failed drive is replaced and rebuilt, data loss will occur.
• A RAID 5 (with Hot Spare disk) array can be configured to treat one of the disks as a hot spare. Then one of the disks is reserved as a replacement in the event that a disk fails. For the above example with five 10TB disks, this would decrease the example array’s usable capacity to 30TB.
  Similar to RAID 5 but includes an additional drive designated as a hot spare. The hot spare remains inactive until one of the drives in the RAID array fails. When a drive fails, the hot spare automatically takes its place, rebuilding the data without manual intervention. Provides improved fault tolerance by reducing the risk of data loss during drive rebuilds. However, it still has the same limitations as RAID 5 when it comes to protecting against multiple drive failures.
• A RAID 6 array is designed to protect against two simultaneous disk failures. However, the price for this extra protection is that two disks' worth of capacity is lost to overhead. As such, a RAID 6 array made up of five 10TB disks would have a usable capacity of 30TB because 20 TB is lost to overhead.
  Requires a minimum of four disk drives. Data is striped across multiple drives with double distributed parity. Offers increased fault tolerance by using two sets of parity information to tolerate the simultaneous failure of up to two drives. Can withstand multiple drive failures without data loss.  However, RAID 6 typically has lower write performance compared to RAID 5 due to the additional parity calculations required. It also has a higher capacity overhead compared to RAID 5 since it uses two drives' worth of space for parity.

The performance during Normal Operation is measured in IOPS (Input/output operations per second) and as a sum for all the disks (excluding the Hot Spares and decreased for writing parity data) in the array.  As a rule of the thumb, the higher the overhead associated with writing parity data (in the above example RAID 5 with Hot Spare causes the same overhead like RAID 6) the lower the IOPS.

The reason for implementing RAID arrays is to secure the data. The level of protection does not directly correlate with the overhead. From the above example both RAID 5 with Hot Spare and RAID 6 have same capacity, but offer different level of protection. In case of failure of RAID 5 array with Hot Spare, the Hot Spare is activated and the rebuild process start immediately.

The system can recover from a single disk failure and during the recovery, process is vulnerable to second disk failure.

• Therefore, RAID 5 and RAID 5 with Hot Spare disk offer the same level of protection – single disk failure.  
• In contrast, if a disk fails at RAID 6 array, the recovery will start only after the faulty disk is replaced manually.  However, if during the recovery process second disk fails, the RAID 6 array will stay functional.

In summary, RAID 5 provides basic fault tolerance with good performance, but it cannot protect against multiple drive failures. RAID 5 with a hot spare improves fault tolerance by automatically replacing failed drives, but it still has the same limitation regarding multiple drive failures. RAID 6 offers a higher level of fault tolerance by using double parity, allowing for the simultaneous failure of up to two drives without data loss. However, it has higher write overhead and requires more drives for implementation compared to RAID 5.