What Is RAID Write Hole (RWH) Protection in Intel® Virtual RAID on CPU (Intel® VROC)?

Unable to find what the RAID Write Hole protection is used for in Intel VROC RAID 5 volumes.

Intel® Virtual RAID on CPU (Intel® VROC) can protect RAID 5 data even when both unexpected power loss and RAID volume degradation occur at the same time. This double fault condition is, at times, referred to as RAID Write Hole (RWH). Many RAID solutions have dealt with this challenge by acquiring a backup power unit. Intel® VROC addresses this problem by using a journaling drive that can preserve the partial parity and reduce the potential data loss issue.

There are two available modes of Intel® VROC RAID Write Hole protection:

1. Distributed: The RAID Write Hole journal is stored on RAID member drives and there is no need for any additional drivers. This mode provides full protection against the RAID Write Hole but introduces a performance penalty for write-intensive workloads.
2. Journaling Drive: The RAID Write Hole journal is stored on a separate journaling drive. That drive cannot be used for any other purpose. The performance penalty for write-intensive workloads depends on the performance of the journaling drive, but typically the penalty is lower compared to the distributed mode.

RAID Write Hole (RWH) is a fault scenario, related to parity-based RAID. It occurs when a power-failure/crash and a drive-failure (for example: strip write or complete drive crash) occur at the same time or very close to each other. Unfortunately, these system crashes and disk failures are correlated events. This can lead to silent data corruption or irrecoverable data due to lack of atomicity of write operations across member disks in parity-based RAID volumes. Due to the lack of atomicity, the parity of an active stripe during a power-fail may be incorrect and inconsistent with the rest of the strip data; data on such inconsistent stripes does not have the desired protection, and worse, can lead to incorrect corrections (silent data errors).