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What is the limit on sshd connection? How much data can it process?

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closed as not a real question by Zoredache, Chris S, John Gardeniers, theotherreceive, splattne Aug 3 '10 at 13:01

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2 Answers

If the question you're trying to ask is "why is my SCP slow" or "how can I make SCP/SFTP transfers faster", you'll want to look into the HPN-SSH patch here: http://www.psc.edu/networking/projects/hpn-ssh/

Newer versions of OpenSSH do have larger buffers that fix some of the speed issue on high-latency high-throughput connections, but not all of it. The HPN-SSH patches also add the 'NoneSwitch' option, which allows you to encrypt the authentication but not the transfer itself. This will save keep you from bottlenecking on the CPU for high-speed transfers. Of course, you'll only want to use that switch if you don't need to encrypt the data.

Also, if you're using the -C (compression), don't. It tends to bottleneck rapidly on a single CPU over anything but a pretty slow link. If you have multiple cores, it may be worth using something like pigz (multithreaded gzipper) or tamp (multithreaded lzo.. a bit easier on the CPU than gzip).

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The bandwidth of your connection is going to be the primary determining factor on the rate of data transfer. Otherwise, you could do some comparative testing. If you mean some limit on the total, I don't think there is one.

What is it that you're trying to do? What problem are you trying to solve?

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contiguous data is sent over sshd, its taking up a lot of CPU. CPU idle goes to 0% while xfer happens Top output: last pid: 4037; load averages: 0.75, 0.40, 0.24 up 0+02:26:11 17:39:04 82 processes: 3 running, 78 sleeping, 1 zombie CPU states: 3.5% user, 32.8% nice, 59.0% system, 4.7% interrupt, 0.0% idle Mem: 212M Active, 68M Inact, 80M Wired, 451M Cache, 69M Buf, 2696M Free Swap: 2048M Total, 2048M Free PID USERNAME THR PRI NICE SIZE RES STATE TIME WCPU COMMAND 248 root 1 98 0 6072K 2524K select 0:24 6.69% sshd –  hari Aug 3 '10 at 1:00
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Your CPU (or crypto accelerator) will also limit the speed. I notice this primarily with gigabit LAN transfers. –  Chris S Aug 3 '10 at 1:04
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