How Unrar Handles Corrupted Recovery Volumes

When extracting multi-part RAR archives, recovery volumes (.rev files) are designed to reconstruct missing or damaged archive parts using parity-based error correction. If the recovery volumes themselves are corrupted, unrar evaluates the integrity of each individual volume, discards unusable parity blocks, and only proceeds if enough healthy recovery files remain to satisfy the mathematical threshold required for reconstruction.

Initial Integrity Verification

Before initiating any data reconstruction, unrar reads the header and block checksums (typically CRC32 for RAR4 or BLAKE2sp for RAR5) of every available .rev file. Recovery volumes do not possess a self-healing mechanism; they cannot repair other damaged recovery volumes. If unrar detects a checksum mismatch or structural truncation within a .rev file, it immediately flags that specific volume as invalid and discards it from the recovery pool.

The Parity Threshold Requirement

RAR archives use erasure coding (such as Reed-Solomon algorithms) to recover damaged sets. The fundamental rule of this mechanism is simple: to reconstruct N missing or corrupted data volumes (.rar, .partXX.rar), the utility requires at least N fully intact recovery volumes (.rev).

When one or more recovery volumes are damaged:

• Sufficient Valid Volumes: If you have more .rev files than missing archive volumes, a corrupted .rev file has no negative impact on the extraction. For example, if two archive parts are missing and you have four .rev files, but one .rev file is corrupt, unrar still possesses three valid .rev files. Because three is greater than or equal to two, the extraction and reconstruction proceed successfully.
• Insufficient Valid Volumes: If the corruption of recovery volumes reduces the count of valid .rev files below the number of missing main volumes, the mathematical threshold cannot be met. In this state, unrar cannot reconstruct the missing data.

Extraction Failure and Error Handling

When the valid recovery volume count falls below the required threshold, unrar aborts the reconstruction process. It typically reports a checksum error or outputs a message indicating that the archive is corrupt or that there are not enough recovery volumes to rebuild the dataset.

If the extraction is executed using standard commands, unrar deletes any partially unpacked or incomplete files to avoid leaving corrupt data on the disk. However, if invoked with the keep broken files flag (-kb), unrar will extract whatever intact data it can read up to the point of corruption, though any files spanning the damaged or missing volumes will remain incomplete.