What is a RAID Controller & Salient points you need to know about RAID Controllers

By Rajesh K

This post talks about what a RAID Controller is and some salient points you need to know about RAID Controllers and RAID Arrays. If you want a general introduction to RAID and RAID Recovery, head here.

What is a RAID Controller?

RAID stands for Redundant Array of Inexpensive Disks. It is a technique used to store data on two or more disks in order to ensure data redundancy (in case of physical disk failures) and/ or improved storage read-write performance (as the host system can access data stored in multiple disks simultaneously, thereby speeding up the process).

All the physical disks that combined to form a RAID configuration appears as a single disk to the host and is referred to as a RAID Array. One can create, configure and manage RAID Arrays using a RAID Controller.

There are two types of RAID Controllers – Hardware based RAID Controllers and Software based RAID Controllers. Hardware based RAID Controllers are recommended because they come with their own processing/ memory capacity which they use for RAID operations. But Software based RAID Controllers use the processing/ memory capacity of the host system (PC/ Server) they are based on, which reduces the system resources that can otherwise be used by applications.

Hardware RAID Controllers can be added to the PC/ Server (using a PCI-Express slot, for example) or they can come embedded on the system’s motherboard.

Salient points you need to know about RAID Controllers **

1. It is recommended to have similar capacity hard disks within the same RAID array. Multiple sized hard disks can be used within the same array, but the maximum usable capacity per disk would be restricted to the smallest disk on the array.

2. A few RAID Controllers can support both internal and external storage disks.

3. If a system supports hot-swapping of disks, it is possible to replace a failed disk in the RAID array without switching the system off. After the new disk is inserted, the RAID controller begins to rebuild the RAID array using the new drive (automatically, in most cases).

4. It may take up to 15 minutes per GB during the rebuilding process. But this time can vary based on a lot of other factors as well.

5. A spare disk drive can be installed in a hot-standby mode within the host system so that upon failure of one of the disks in a RAID Array, it can automatically (and immediately) be used to reconstruct the data in the failed disk and hence the RAID array is reconstructed without data loss.

6. Even if the RAID Controller fails, it is possible to replace it with a new one without losing data on the disk drives as long as both of them save configuration data in the same way.

7. Some RAID Controllers may have in-built batteries to preserve data in the RAID Controller’s cache during a power failure.

8. Every RAID Controller can support only a certain number of disks, either in the same array or in multiple arrays.

9. RAID Controllers generally have a certain amount of Cache memory to improve the performance by storing and enabling access to frequently required data/ info.

10. A GUI based interface is generally available for the user to configure and maintain the Raid Controller, create and maintain RAID arrays, get notified of individual disk failures, etc.

11. Some RAID Controllers can also issue an audible alarm (like a beeping sound, etc) in case of disk failures/ new disk installations.

12. Additional (limited number of) disk drives can be added to a RAID array with certain RAID Controllers.

13. RAID levels can be optionally and selectively migrated from one level to other supported levels (For example, RAID 1 can be migrated to RAID 10), if required.

** Please note that most of these features are applicable for Computer Servers and a few of them maybe applicable to individual computers as well.

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