ssh / scp slow on 10GBE

Stefan Priebe s.priebe at
Mon Jan 7 06:00:13 EST 2013

Hi Tomas,

 >Just to make sure, could you please measure round-trip-time on the
 >connection? Simply running ping will do...

While running:
#~: dd if=/dev/zero bs=1048576 count=8192 | ssh -c aes128-cbc -m 
umac-64 at dd of=/dev/null
8192+0 records in
8192+0 records out
8589934592 bytes (8,6 GB) copied, 21,8169 s, 394 MB/s
16777216+0 records in
16777216+0 records out
8589934592 bytes (8,6 GB) copied, 21,1476 s, 406 MB/s

These are the ping stats:
22 packets transmitted, 22 received, 0% packet loss, time 20996ms
rtt min/avg/max/mdev = 0.060/0.103/0.176/0.026 ms

 >If the result is not 2 ms or higher, ssh window side is most probably
 >not the culprit.
OK thanks for this info.

 >Part of the gap between encryption speed and the dd measurement can be
 >accounted to checksumming, some to initial key-exchange and rekeying.
 >The gap still feels quite wide though.
Now it's 400MB/s using ssh and 1230MB/s when using a direct TCP connection.

 > don't know, how your initial measurements (76 MBps) were done. But if
 >it involved copying multiple smaller files, this could very well be scp
 >protocol overhead. In that case tar could help.
Yes sorry scp and a disk was involved. I'm now talking about 400MB/s vs. 


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