How to Force unrar to Flush Disk Cache

Extracting large archives with unrar can cause the operating system to buffer large amounts of write data in RAM, leading to memory bloat, system stutter, or prolonged delays during unmounting. This guide explains how to use the built-in -ffd switch in unrar to immediately flush file buffers to disk after each file is extracted, guaranteeing that data is physically committed to your drive without overwhelming your system cache.

The -ffd Command-Line Switch

Modern versions of unrar (RAR 6.00 and newer) include a dedicated switch designed specifically for this purpose: -ffd.

When this switch is enabled, unrar calls the operating system's flush routine (such as fsync() on Linux/macOS or FlushFileBuffers() on Windows) immediately after writing each extracted file.

Basic Usage

To extract an archive with full path preservation while forcing a disk cache flush after every file:

unrar x -ffd archive.rar

To extract files directly into the current directory without subfolders:

unrar e -ffd archive.rar

You can place the -ffd flag anywhere alongside other common parameters, such as destination directories or overwrite switches:

unrar x -ffd -o+ archive.rar /path/to/destination/

Why Use -ffd?

• Preventing System Freezes: On systems with large amounts of RAM and slower storage (such as mechanical hard drives, network mounts, or USB flash drives), the operating system's write-cache can fill up rapidly with "dirty" pages. This often leads to system unresponsiveness while the kernel tries to flush the cache.
• Safer Removable Media Transfers: When extracting directly to external storage, flushing every file prevents massive write queues at the end of the process, reducing the risk of data loss if the drive is disconnected prematurely.
• Immediate Space Reconciliation: Operating systems sometimes delay freeing or updating partition metadata until write buffers are cleared; flushing after each file ensures accurate reporting of free space during multi-file jobs.

Performance Considerations

Flushing file buffers prevents caching benefits. By forcing the disk to write and synchronize data continuously, the total extraction time will increase, particularly on mechanical drives (HDDs) where random write operations incur physical seek overhead. On solid-state drives (SSDs) and fast NVMe storage, the performance impact is generally minimal.

Alternative for Older unrar Versions

If you are using a legacy build of unrar that lacks the -ffd switch, the native utility cannot flush per-file writes directly. On Linux or macOS, you can achieve a similar effect by lowering system write-cache thresholds globally or by extracting files sequentially using a shell loop paired with the sync command:

unrar lb archive.rar | while read -r file; do
    unrar x -o+ archive.rar "$file"
    sync
done

For optimal performance and ease of use, upgrading to a current version of unrar and applying the -ffd switch remains the recommended approach.