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Old 03-14-2014, 06:36 PM   #1
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LNA: scp transfer a ten times slower than either the machines or the router. Can

Hi: I want to do
$ scp -rp root@local /home/semoi/source_dir dest_dir >/dev/null 2>/dev/null
The transmission is between two machines both at home. The router is 10/100M, say it uses 100M, and the disk transfer rate in the slower machine is nearly 30MB/s. So, being below 100MB/s, it is this machine that determines the speed, isn't it? But doing a test transfer, scp gave some 30KB/s as mean transfer speed. As I ran scp with the redirection, say scp really performed 100 times slower. Still this is ten times slower than expected! How can this possibly be? The transfer amounted to some 30GB size.

EDIT: measuring time with a clock, it took nearly 2s for a 14MB chunk of files. This gives 4300s = 1.2 hours for 30GB, which is what I want to transfer tonight. Does it sound reasonable?

Last edited by stf92; 03-14-2014 at 06:57 PM.
Old 03-14-2014, 06:51 PM   #2
Didier Spaier
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I don't know the answer, but transferring through an external hard disk would be way faster.
Old 03-14-2014, 07:15 PM   #3
Richard Cranium
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You could run iostat on both machines to see what the disk transfer rates appear to be.

You could also sniff the network with wireshark to see if perhaps you are re-transmitting a lot of packets.
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Old 03-14-2014, 07:21 PM   #4
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Yes, I think I have it. I just ran dd on the hard drive to have a near idea, for I did not have the specs at hand. But with a much larger transfer, I've just got much higher speed. So, specs are mandatory or iostat should be used! I mean, the estimations were too gross. I did not realize hdparm gives me the manufacture specs. With the machine relatively lightly loaded, that would have given me good figures.

Last edited by stf92; 03-14-2014 at 07:24 PM.
Old 03-14-2014, 07:37 PM   #5
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Note that when you router says 10/100M speed, it is actually 10/100Mbits, not 10/100MBytes, so in theory a 100M links could only give 12.5MBytes/s max and actual rate will be slower due to TCP/IP overhead.

Wich scp, you need to take counts of the speed of encryption/decryption on the data also. If you have a slow CPU, you get very low transfer speed.

So 2s for a 14MB data (~7Mb/s) is reasonable speed.

Last edited by qunying; 03-14-2014 at 07:40 PM.
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