Linux mdadm software RAID 6 - does it support bit corruption recovery?

Solution 1:

Linux software RAID is not going to protect you from bit corruption and silent data corruption is a well known issue with it. In fact, if the kernel is able to read the data from one disk it would never know that it is bad. The RAID only kicks in if there is an I/O error when reading the data.

If you are worried about data integrity you should consider using a file system like Btrfs or ZFS that ensure data integrity by storing and verifying checksums. These file systems also take care of the RAID functionality, so you don't need the kernel software raid if you go that way.

Solution 2:

RAID5 and RAID6 can detect and usually correct bit corruption if you verify parity of the entire drive. This is called "scrubbing" or "parity checking" and typically takes 24-48 hours on most production RAID system. During that time performance may be significantly degraded. (Some systems allow the operator to prioritize scrubbing over read/write access or below it.) RAID6 has a higher chance of correcting it, because it can correct it if you have two drive failures, whereas RAID5 can only handle 1 drive failure, and drive failures are more likely when you are scrubbing because of the increased activity.

Solution 3:

I would have added this as a comment but I don't have sufficient reputation; I wanted to clarify: RAID5 can DETECT bit corruption but it doesn't know which drive has the corruption without a read error. As a result, a scrub couldn't fix this without a read error - it would most likely just log it and update the parity bit to match. RAID6's algorithm is position-dependant so it can detect which drive contained the error and correct the bit corruption.

Solution 4:

All the answers above are incorrect regarding the capabilities of RAID 6. RAID 6 algorithms operate byte-by-byte just as RAID 5, and if a single byte on any one drive is corrupt, even with no error indicated by the drive, it can be detected AND CORRECTED. The algorithm for doing so is completely explained in

In order to perform this check, the parity P and Q drives must also be read along with the data drives. If the computed parity P' and Q' differs with no drive errors, an analysis can pinpoint which of the drives is incorrect, and correct the data.

In addition, if the drive identification is to a drive that is not present (such as drive 137 if there are only 15 drives), more than one drive is providing corrupted data FOR THAT BYTE, signaling an uncorrectable error error. When there are much fewer than 256 drives in the set, this is detected with high probability per byte, and since there are many bytes in a block, with extremely high probability per block. If the drive identification is not consistent for all bytes within the RAID block, again, more than one drive is providing corrupted data, and generally one might reject the condition, but so long as all the drive identifications are valid, the block need not necessarily be rejected.

It takes longer than the usual verification time to perform this correction, but it only needs to be performed with the syndrome (P and Q) calculation shows an error.

All this being said, however, I have not examined the mdadm code to determine whether single-byte corruption is handled. I am aware that mdadm reports RAID6 syndrome errors on the monthly scan, but from the error message it is not clear whether they are being corrected - it does not stop the drive array nor identify any particular drive in the message.