RAID stands for Redundant Array of Independent (originally – Inexpensive) Disks. Redundancy means that the same data is stored in different places of multiple hard drives combined into a single logical unit (ironically, one of the most popular RAID configurations – RAID-0 has no redundancy). This storage technology allows to avoid data loss if one (or even more than one) of the drives fails.

There are at least nine types of RAIDs :

• RAID-0: This technique has striping (chunks of data are spread across the drives) but no redundancy. It offers the best performance but no fault-tolerance.
• RAID-1: This type is also known as disk mirroring and consists of at least two drives that duplicate each other. There is no striping. Read performance is improved since either disk can be read at the same time. Write performance is the same as for single disk storage. RAID-1 provides high performance and best fault-tolerance.
• RAID-2: This type uses striping across disks with some disks storing error checking and correcting information.
• RAID-3: This type uses striping and dedicates one drive to storing parity information.
• RAID-4: This type uses large stripes, which means you can read records from any single drive.
• RAID-5 uses rotating parity, requires at least three drives for the array.
• RAID-6: This type is similar to RAID-5 but includes a second parity scheme that is distributed across different drives and thus offers extremely high fault- and drive-failure tolerance.
• RAID-7: This type implements a real-time embedded operating system as a controller, caching via a high-speed bus.
• RAID-10: Combining RAID-0 and RAID-1 is often referred to as RAID-10, which offers higher performance than RAID-1 but at much higher cost. There are two subtypes: In RAID-0+1, data is organized as stripes across multiple disks, and then the striped disk sets are mirrored. In RAID-1+0, the data is mirrored and the mirrors are striped.
• RAID-50 (or RAID-5+0): This type consists of a series of RAID-5 groups and striped in RAID-0 fashion to improve RAID-5 performance without reducing data protection.
• RAID-53 (or RAID-5+3): This type uses striping (in RAID-0 style) for RAID-3′s virtual disk blocks. This offers higher performance than RAID-3 but at much higher cost.
• RAID-S (also known as Parity RAID): This is a proprietary method for striped parity RAID from EMC Symmetrix that is no longer in use on current equipment. It appears to be similar to RAID-5 with some performance enhancements.

Many people believe that RAID should not fail, as a result of overemphasis of RAID’s fault tolerance and auto rebuilt functions. In reality RAIDs can fail and this happens quite often. Typical reasons for RAID failures are:

• Controller malfunction
• Physical failure of multiple disks
• RAID auto rebuild error
• Human error (most often – reconfiguration of RAID with wrong parameters)
• Logical corruption of RAID volume

Data Recovery from failed RAIDs is performed by proper reconstruction of virtual RAID volumes with the help of professional data recovery tools. Prior to the reconstruction physically failed drives – members of RAID might need to be repaired (sometimes clean room required), sector-by-sector images of all drives must be created.

At Crucial Data Recovery, we have years of experience with all RAID levels, controller cards, configurations and file systems.