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Setup is a Centos 6 x64 server and a Windows 7 client running putty. Multiple sessions are opened to this same server from the same client. Periodically, one of these sessions slows to a crawl and the other remains quite quick. If a new session is established, it sets up quite quickly and works well. Have checked server NICs, iostats and top and see no apparent issues. The problem appears to show up most often when large files are being listed on the screen or a large cut and paste operation is in progress. ie 'cat messages'

The server has a "twin" setup at the same time with exact same hw/sw config and sessions to that server never exhibit the same issue.

I was able to ssh into another server and then ssh over to the affected server while I had a putty session open that was experiencing a problem.

I did a tcpdump from there and there is virtually nothing being sent to the putty client. The only traffic I see is when it is updating. Today the issue showed up during a yum install. This is what my putty session looks like:

Installing : ncurses-devel-5.7-3.20090208.el6.x86_64 [#####
Installing : ncurses-devel-5.7-3.20090208.el6.x86_64 [#######
Installing : ncurses-devel-5.7-3.20090208.el6.x86_64 [########
Installing : ncurses-devel-5.7-3.20090208.el6.x86_64 [##########
Installing : ncurses-devel-5.7-3.20090208.el6.x86_64 [############

This is what was captured in about a 10 minute period when this slows down. TCPdump showed no errors at all and very little traffic to the putty client. I just left it run and 30 minutes later, yum is done installing and the session has returned to normal. The issue in this case just showed up during a yum install, yesterday is was catting the maillog and the day before editing a file in vi.

I am suspecting that this might be a hw issue on the box itself but I can't seem to put my finger on anything specific.

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Try to find a reproducible path of operations. I'm suspecting a network issue here. Use Wireshark to see what is going on. In case you're seeing TCP Previous segment lost lines there, you know it's a networking issue. Either way, please provide a minimal tcpdump/pcap file while it occured. –  gertvdijk Sep 19 '12 at 21:26

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