Say No to scp
Why you should avoid scp over a WAN
In a Unix environment scp, sftp, and rsync are commonly used to copy data between hosts. While these tools work fine in a local environment, they perform poorly on a WAN. The openssh versions of scp and sftp have a built in 1 MB buffer (previously only 64 KB in openssh older than version 4.7) that severely limits performance on a WAN. Even though rsync is not part of the openssh distribution, rsync typically uses ssh as transport and is therefore subject to the limitations imposed by the underlying ssh implementation. DO NOT USE THESE TOOLS if you need to transfer large data sets across a network path with a RTT of more than around 25ms.
The following results are typical: scp is 10x slower than single stream GridFTP, and 50x slower than parallel GridFTP.
Sample Results
Berkeley, CA to Argonne, IL (near Chicago).
RTT = 53 ms, network capacity = 10Gbps.
| Tool | Throughput |
| scp | 140 Mbps |
| HPN patched scp | 1.2 Gbps |
| GridFTP, 1 stream | 1.4 Gbps |
| GridFTP, 4 streams | 5.4 Gbps |
| GridFTP, 8 streams | 6.6 Gbps |
More information is here.
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