RDP: Row-Diagonal Parity for Double Disk Failure Correction
RDP is an algorithm that protects against double disk failures. RDP can be applied to RAID systems. RDP is also known as RAID-DP/RAID-6 (There are other RAID-6 approaches to handle two disk failures, but RDP is the most intuitive).
- 2.Related Work
- 3.Double Disk Failure Models and Analysis
- 4.Row-Diagonal Parity Algorithm
- 5.Proof of Correctness
- 6.Performance Analysis
- 7.Algorithm Extensions
- 8.Implementation Experience
- 9.Measured Performance
There are two types of disk failures: Individual disks can fail by whole-disk failure, whereby all the data on the disk becomes temporarily or permanently inaccessible, or by media failure, whereby a small portion of the data on a disk becomes temporarily inaccessible. The previous RAID only considers whole-disk failures.
Multiple disk errors are likely: the authors gave a detailed analysis of why this is the case in section 3 (which I'm not going to get into).
RDP is built on RAID-4 or RAID-5. In this paper, we will focus on RAID-4.
In this case, p = 5. We have (p+1) disks and (p-1) data disks.
XOR is still used for parity. The figure shows the diagonal of each block. In the example above, if we have whole-disk failures on data disks 1 and 3, the data can be easily recovered in many ways.
RDP can also be extended to encompass multiple RAID-4 or RAID-5 disk arrays in a single RDP disk array.
- Read performance is unaffected.
- Sequential write: Write p-1 stripes at once for best performance (update row and diagonal parity at the same time).
- Partial stripe writes: Writing d blocks by subtraction requires 2d+4 I/Os (d+2 for read, d+2 for write), and writing d blocks by additive requires n I/Os (n-d-2 for read, d+2 for write). Thus, we use a combination of additive and subtractive.
- Proof of correctness and optimality is covered in the paper.
Write performance measured: RDP gives a much better reliability for the same cost and performance.
- G: number of separate RAID groups connected to the filer
- d: number of data disks per RAID group
- p: number of parity disks per RAID group
- NetApp: A cloud data services and data management company.
Prof. Andrea's slides on RAID and RDP