topic Re: SSH Lag in Operating System - Linux

SSH Lag

wobbe — Mon, 17 Jan 2011 19:19:49 GMT

Whenever a linux machine is under high disk utilization the console (ssh with putty) becomes very slow. Almost unresponsive.
I've seen this with RHEL5.x on older machines and now again with Ubuntu 10.04.1 LTS on a brand new DL380G7 with 24 cores.
Is there a way I can tweak the ssh daemon to get it to respond bit better?

Re: SSH Lag

Matti_Kurkela — Tue, 18 Jan 2011 09:51:51 GMT

Do you mean SSH is slow to connect, or is an already-established SSH session slow too?

Is a local console session slow too?

If your system disk is very busy with other tasks, the system's response to commands is likely to slow down, no matter what login method is used.

This is one of the reasons why applications that are expected to have a high disk utilization should have their own disks/RAID sets, separate from the system disk(s).

It might be possible you're experiencing an I/O bottleneck: the system attempts to transfer more data than the bus on the system board can handle, resulting in a situation that is somewhat similar to a traffic jam.

MK

Re: SSH Lag

wobbe — Thu, 20 Jan 2011 16:39:16 GMT

It's slow to connect and slow executing commands. It's also slow at the console (ILO) but still a bit more responsive than using SSH. My root is on the same LUN at the volume that is beeing written to, so this could be part of the reason.
Also, I'm using SATA disks in RAID 5 and the source volume is on RAID 10 SAS volume. (not on the same server)

But still, I expected more from a brand new DL380G7 performance model.

Re: SSH Lag

Timothy M Carr — Thu, 20 Jan 2011 21:27:24 GMT

This sound like an I/O issue to me. What's your average system iowait during the high disk utilization?

In looking to resolve this issue the first step would be to separate your root partition from you working partition.

Re: SSH Lag

Alzhy — Mon, 24 Jan 2011 14:18:28 GMT

"Also, I'm using SATA disks in RAID 5 and the source volume is on RAID 10 SAS volume. (not on the same server)"

Are you syaing your OS/Boot disk is carved out of the same RAID5 set as the filesystem being written to on the same Smart Array Controller?

If Yes - then that is likely your problem. Our standard is either OS is on a SAN Boot disk or if local -- the OS uses the RAID disks exlcievekly on its own -- no data filesystems...