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I am transferring a bunch of gigabyte-sized files using SFTP over a fairly fast connection, between two servers running Ubuntu linux, with not much CPU load.

I am usually able to reach 5.0 MB transfer speeds (around 40 megabit/sec), though we have 90 megabit of bandwidth available and it doesn't seem to want to go any higher.

Running at 5.0 MB:

  • The source machine has a CPU load of no more than 2.7% for ssh
  • The target machine has a CPU load of no more than 13.3% for sshd and 3.0% for sftp-server

I can't get it to go any faster. If I open a second SFTP session, the two never exceed 5.0 MB combined.

 

And now for the actual question. Occasionally SFTP will transfer incredibly slowly for no apparent reason. Both machines are not busy, and the Internet connection is empty, yet the transfer plods along at around 120 KB/s.

  • Interrupt SFTP, reconnect. Still 120 or so KB/s
  • Interrupt SFTP, reconnect. Still 120 or so KB/s
  • Interrupt SFTP, reconnect. Still 120 or so KB/s
  • Interrupt SFTP, reconnect. This time it starts out slow but works its way back up to 5.0 MB/s

What the heck is going on?

I know there is some sort of mechanism to keep SFTP from overloading a slow SSH connection, but that mechanism appears to be broken and unreliable.

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  • What about I/O on disks?
    – Paolo
    Jan 31, 2017 at 6:01
  • have you checked whether you can initiate another connection during this time and achieve additional throughput? Seems like a packet drop / bandwidth limitation. Maybe your routers in between treat this as low priority traffic and working something else which has higher priority during this time. P.S. Holy cow.! a three year old question. How in the world did this pop up in my feed? :/
    – Tharaka Devinda
    Aug 13, 2020 at 6:11

1 Answer 1

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Most likely is a bandwidth issue ; Most hosting providers will restrict your bandwidth after certain amount of used bandwidth. Explain to your providers what issues are you facing so they can help you out.

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2
  • we have no bandwidth restrictions. after hitting such slowness, I am able to reconnect and achieve 5 MB/sec for gigabytes of additional data.
    – Dale Mahalko
    Jan 31, 2017 at 5:23
  • TCP can slow down for many other reasons than using up all the available bandwidth.
    – kasperd
    Jan 31, 2017 at 6:08

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