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