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I'm periodically establishing short SSH connections to one of the local servers. I observe intermittent timeouts. After investigating the issue, I found that SSH timeouts happen when there is a lot of IO write activity to the server's RAID array. What happens is that SSH connection "hangs up" for the duration of that write burst.

The server has plenty of available user memory (18 out of 64GB), swap space, and its CPUs are definitely not overloaded.

Is it:

  1. a normal behaviour

  2. indicates some RAID hardware problem

  3. something is not configured properly

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Could you attach your sshd config? – Sergei Lomakov Jun 20 '12 at 4:50
It's a default sshd_config. Here you go: pastebin.com/jNhssqu3 – OutputLogic Jun 20 '12 at 17:37

I do not claim to be an expert, but I would imagine that this is "normal behavior" if read/write to the RAID array is configured to be synchronous, in which case reads/writes kind of end up becoming blocking events, especially if SSH needs to reference anything on said array as the user tries to log in.

With synchronous behavior, it wouldn't really matter about available ram or "spare" CPU cycles, because everything would be waiting on disk activity.

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