How to Test RAR Archives with Multithreaded Unrar

Testing large RAR archives for file integrity can be a time-consuming process on modern systems if restricted to a single CPU core. This guide explains how to use the official RARLAB unrar command-line utility with the -mt switch to run multithreaded integrity tests, significantly speeding up decompression checks on multi-core processors.

The Basic Command Syntax

To test an archive using multiple threads, use the t (test) command combined with the -mt<threads> switch:

unrar t -mt<threads> archive.rar
  • t: Instructs unrar to test archive integrity without extracting files to the disk.
  • -mt<threads>: Specifies the number of CPU threads to allocate to the operation (from 1 to 64).

Practical Examples

1. Test Using a Specific Number of Threads

If you want to allocate 4 CPU threads to the integrity check:

unrar t -mt4 backup.rar

2. Test Using All Available CPU Cores

By default, modern versions of the official unrar utility automatically detect and use all available logical CPU cores if you specify the switch without a number or omit it entirely in recent releases. To explicitly ensure automatic thread management across all cores, run:

unrar t -mt backup.rar

3. Test a Multi-Volume (Spanned) Archive

When working with split RAR files (such as .part1.rar, .part2.rar), you only need to run the command against the first volume. The multithreading parameter will apply across all consecutive parts:

unrar t -mt8 archive.part1.rar

Important Considerations

  • Utility Version: The -mt switch requires the proprietary freeware version of unrar distributed by RARLAB. The open-source GNU alternative (unrar-free) found in some Linux distributions does not support multithreading or the modern RAR5 format. Ensure you have installed the non-free unrar package via your system's package manager.
  • Storage Bottlenecks: While multi-threading distributes decompression workloads across multiple CPU cores, testing speed may still be limited by drive read speeds, particularly on mechanical hard drives or slow external storage devices.