[CentOS] SFTP - stalled - on large files

Wed Dec 23 00:41:36 UTC 2009
Jay Leafey <jay.leafey at mindless.com>

We had a similar problem copying files between servers on two of our 
campuses via SCP.  After a while the connection just stalled out and 
hung.  The problem turned out to be SCP and SFTP interacting a bug in 
the SACK (Selective Acknowledgment) algorithm used in Linux.  We turned 
it off on the two endpoints using the following addition to 

> # 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!
Jay Leafey - Memphis, TN
jay.leafey at mindless.com
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