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 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 20ms.

The following results are typical: scp is 18x slower than single stream GridFTP, and 25x slower than parallel GridFTP.

Sample Results
Berkeley, CA to Argonne, IL (near Chicago).
RTT = 53 ms, network capacity = 10Gbps.

Tool Throughput
scp 330 Mbps
HPN patched scp 1.2 Gbps
GridFTP, HTTP (e.g.:wget), 1 stream 6 Gbps
GridFTP, 4 streams 8 Gbps (disk limited)

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