====== Safe Data Migration under High I/O Load ======

Use this method when you need to transfer data from a storage device that is already under heavy load (e.g., RAID resync, slow USB, high latency) to prevent system lockups (high I/O Wait).

==== The Command ====

<code bash>
ionice -c 3 rsync -avxW --progress /source/path/ /destination/path/
</code>

==== Parameter Breakdown ====

  * **ionice -c 3** (Idle Class)
    * **Function:** Sets the I/O scheduling class to "Idle".
    * **Effect:** The process will only read/write to the disk when no other program needs access to it.
    * **Benefit:** Prevents the system from freezing or becoming unresponsive (High Load/IO Wait) during the transfer.

  * **rsync -W** (--whole-file)
    * **Function:** Copies the whole file without calculating delta checksums.
    * **Effect:** Reduces CPU usage and disk seek operations.
    * **Benefit:** Significantly faster for directories with thousands of small files (e.g., `node_modules`, Docker overlays) where calculating diffs is more expensive than the transfer itself.

  * **-avx**
    * **-a**: Archive mode (preserves permissions, timestamps, ownership).
    * **-v**: Verbose (shows progress).
    * **-x**: One File System (prevents rsync from crossing partition boundaries).

==== Typical Use Case ====

Migrating Docker data or large file structures while a RAID array is resyncing or scrubbing:

<code bash>
# Example: Moving Docker data to a new partition
ionice -c 3 rsync -avxW --progress /var/lib/docker/ /mnt/new_drive/docker/
</code>