To sync terabytes of data from a remote host to two hard drives with different speeds, use rsync in parallel by launching separate background processes for each destination. This approach bypasses the bottleneck of the slower drive while keeping the faster one fully utilized, ensuring efficient use of your network bandwidth and storage hardware without waiting for the entire dataset to finish on the slowest link first.
Understanding the Bottleneck
When you run a standard command like
rsync -az user@remote:/path /local/fast/drive, the transfer speed is limited by the slowest component in the chain. If you attempt to write to two drives sequentially or use a single process that balances writes poorly, the fast solid-state drive (SSD) will idle while waiting for the mechanical hard disk drive (HDD) to catch up. This results in underutilized network bandwidth and wasted time. The goal is to decouple the destinations so each receives its own stream of data, allowing the fast drive to complete its task independently of the slow drive's physical limitations.
Preparation and Verification
Before initiating large-scale transfers, ensure both local drives are mounted and accessible. Verify the mount points using
df -h to confirm available space. It is critical that both drives have sufficient capacity for the entire dataset. Additionally, check the health of the slower drive. A failing drive can cause timeouts and corruption. For more details on handling drive issues, see
Can Corrupt or Physically Damaged Sectors on a Hard Drive be Written Over?. If the slow drive shows signs of instability, consider running a surface scan before starting the sync.
Parallel Rsync Strategy
The most effective method is to launch two distinct rsync commands simultaneously. Use the ampersand (
&) at the end of each command to run them in the background. This allows the shell to return control immediately while both transfers proceed concurrently.
First, target the fast drive:
rsync -az --progress user@remote:/data /mnt/fast_drive &
Second, target the slow drive:
rsync -az --progress user@remote:/data /mnt/slow_drive &
Using
--progress provides real-time feedback for each job. Without it, you may not know which transfer is lagging. Monitor these jobs using the
jobs command or
ps aux | grep rsync. Ensure that the network connection can handle the aggregate throughput. If your internet speed is 100 Mbps, two streams might saturate the link, causing packet loss. In such cases, limit bandwidth using the
--bwlimit flag.
Handling Interrupts and Errors
If a transfer fails due to network drops or drive disconnections, rsync can resume from where it left off thanks to its checksum algorithm. However, frequent interruptions on the slow drive may indicate hardware issues. Refer to
Unable to Mount HDD: Zipping Files HDD Got Disconnected Suddenly - Tech Support Site if you encounter sudden disconnects during the process. Always verify the integrity of the copied files after completion. Use
md5sum or
sha256sum on the source and both destinations to ensure bit-for-bit accuracy.
Frequently Asked Questions
Will running two rsync processes damage my hard drive?
No, running multiple rsync processes does not inherently damage hardware. However, excessive write operations can generate heat. Ensure your slow drive has adequate cooling and ventilation during long transfers to prevent thermal throttling or premature wear.
How do I stop a specific rsync job without killing the other?
Identify the process ID (PID) of the specific rsync job using
ps aux | grep rsync. Then, send a termination signal using
kill [PID]. This allows the other background transfer to continue uninterrupted, preserving progress on the remaining drive.
Can I limit the bandwidth for the slow drive only?
Yes, add the
--bwlimit=KBPS flag to the command targeting the slow drive. For example,
rsync -az --bwlimit=5000 ... limits that specific stream to 5 MB/s, leaving more bandwidth for the fast drive or other system tasks.
What if the remote host disconnects during the transfer?
Rsync handles partial transfers gracefully. When the connection is restored, simply re-run the same command. Rsync will compare file sizes and timestamps, resuming only the parts that were not successfully transferred, saving significant time compared to starting over.