[CentOS] SFTP - stalled - on large files

Sean Carolan scarolan at gmail.com
Sun Dec 27 15:32:24 UTC 2009

>> # Turn off SACK
>> net.ipv4.tcp_sack = 0
> and execute "sysctl -p" to apply it.  You can also use "sysctl -w
> net.ipv4.tcp_sack=0" to turn it off temporarily.  Our file transfers worked
> just fine after the change.
> I realize there are differences our situation and yours and this might not
> work in your case.  Given the length of this thread, though, it might be
> worth a try!

It appears that the Netscaler load balancer was the problem.  We
turned off TCP buffering (TCPB) on the netscaler for this particular
virtual server, and I was immediately able to transfer a 95MB file
with no issues.  Citrix has acknowledged that there may be some issues
with the tcp stack on this device, which they think have been resolved
in more recent versions of the Netscaler OS.

Hopefully if anyone else experiences this issue, they'll be able to
Google it via the CentOS list archives.

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