How Unrar Prioritizes Original and Recovery Volumes

When extracting multi-part archives, unrar strictly prioritizes original RAR volumes over recovery volumes (.rev files). Recovery volumes are never used as a first-choice data source; instead, the utility only accesses them as a mathematical fallback to reconstruct missing or damaged original parts before proceeding with standard decompression.

The Primary Extraction Process

During a standard extraction command, unrar searches exclusively for the original numbered volumes (such as .part01.rar, .part02.rar, or legacy extensions like .r00, .r01). Because original volumes contain the actual compressed stream, headers, and metadata, unrar streams data directly from these files in sequential order. As long as every original volume is present and passes checksum validation (CRC32 or BLAKE2sp), recovery volumes remain completely untouched.

The Role of Recovery Volumes (.rev)

Recovery volumes do not contain readable archives. Instead, they store parity information generated using Reed-Solomon or Cauchy-based erasure codes. Each individual .rev file contains enough algorithmic redundancy to rebuild exactly one missing or corrupt original volume of the same set, regardless of which specific volume is gone.

Because of this architecture, unrar cannot simply substitute a .rev file in place of a missing .rar file during the live extraction stream. It must use the recovery data to recreate the missing original file on the disk first.

How unrar Decides When to Use Recovery Volumes

The transition from original extraction to recovery processing follows a strict condition check:

  1. Missing or Corrupt Volume Detection: When unrar encounters a missing segment in the sequence or detects data corruption via an integrity check, standard extraction halts.
  2. Parity Assessment: The tool scans the working directory for available .rev files and counts how many original volumes are missing or damaged.
  3. Threshold Evaluation: If the number of missing or damaged volumes is less than or equal to the total number of valid .rev files, unrar initiates the recovery algorithm. If the number of missing files exceeds the available recovery volumes, the process fails because reconstruction is mathematically impossible.
  4. Volume Reconstruction: unrar processes the intact original volumes alongside the necessary number of .rev files to recalculate the missing data blocks. It writes the rebuilt original .rar volumes back to the filesystem.
  5. Resumed Extraction: Once the reconstructed original volumes are written to disk, unrar resumes normal extraction through the now-complete set of standard archive files.