Methods for RAID 5 Data Recovery

RAID or Redundant Array of Inexpensive Disks is a popular data storage solution that offers increased reliability and reduces downtime. At Precision Data Recovery Services, we handle two main implementations of RAID - RAID 1 (mirror) and RAID 5 (distributed parity). In the case of a RAID 1 or RAID 5, there is spare capacity within the storage array that enables it to continue functioning even if a single disk fails. If you are unfamiliar with how data is stored in a RAID 1 or RAID 5 array, please visit our dedicated RAID and Server Data Recovery website for more information.

Common Causes of RAID 5 Data Loss

The primary cause of data loss in a RAID 5 array is often multiple disk failure. It is rare for multiple disks to fail at the same time, and usually, there has been a previous disk failure that has gone unnoticed or unaddressed by the system administrator. This leads to a second disk failure that results in the failure of the RAID. In such cases, it can be challenging to determine the order in which the disks have failed.

Determining the Order of Disk Failure

The first step in the data recovery process is to create a full disk image of each disk in the RAID set. This not only ensures the safe storage of data and reduces the risk of additional disk failure but also allows for efficient comparison of data across the disk set. The next step is to compare the data structures across the disk images. If we have access to all the disks in the RAID set, we can run an XOR evaluation to determine if the data is consistent across the RAID array. This evaluation provides an indication of which disk failed first.

Understanding RAID 5 and XOR

RAID 5 uses the bitwise “exclusive OR” (XOR) function to calculate parity values from the array data. The XOR function satisfies two crucial conditions:

If X xor Y = Z, then X = Z xor Y and Y = Z xor X 

If X and Y occupy the same number of bits, Z also occupies that number of bits X xor Y = Z

X xor Y = Z

Using the properties of the XOR function, one can calculate one of the missing values if all the others are known.

Hexadecimal Editor Inspection

The XOR evaluation only indicates if there is consistency across the disk set and does not specify which disk is inconsistent. This must be determined by the technician working on the RAID array. In the example of a 4-disk RAID 5 opened in WinHex, it becomes clear that one disk had failed some time ago and had not received any data writes. Large areas of the disk were filled with “00” data, while the other three disks had consistent data.

By examining the raw data in a Hexadecimal editor, it becomes apparent which disk is not fully integrated into the RAID array.

Increased Complications with More Disks

In the example above, it was straightforward to determine the cause of failure and establish the order of disk failure because there were only four disks in the RAID array. However, Precision Data Recovery Services regularly receives RAID arrays with more than 10 disks. The number of variables increases as the number of disks in the array grows, making the recovery process increasingly challenging. Additionally, the order of disk failure can be harder to establish if the first disk failed only hours before the second. If data was being written to the array during this period, it is essential to determine the order to ensure a successful recovery.

Working with disk sets formatted with a non-standard number of bytes-per-sector can also present additional complications. Enterprise HDDs with a 520+ bps formatting will be discussed in a separate article.