[HOWTO] A big discovery for restore performance issues!

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PREDATAR Control23

Been a long time since I've posted on here, but I'm still alive and doing TSM :)

We had a TSM client that was getting pathetic restore times (3-5mb/s for a single large file). Then to my horror I discovered it wasn't just that client, there was lots of them with the same issue. Anyway, what followed was lots of tests with instrumentation, then a probe and wireshark. And we found an issue.

If ...

  • your TSM server runs on Windows.
  • and your TSM client's OS has tcp delayed acknowledgements on (most do by default)
  • and your client tuning parameters are set to certain values
  • ... your performance will be bad
This is due to the way delayed tcp acknowledgements are handled by the Windows TCPIP stack, and consequently the way the TSM Server interacts with the stack. This is documented under
https://support.microsoft.com/en-us/kb/823764

I found the issue could be worked around somewhat by playing with the client parameters. Am yet to test disabling tcp delayed acknowledgements at the OS level, but will do soon. I have not tested this on all combinations of tsm client&server versions, or on the OS versions.

Below summarises the tuning options. We had servers in the Zone of Horror section, which is where the tcp delayed acknowledgement problem was hurting us.

The tests were done of a single 300MB .zip file with no client compression, on a 1GbE link.

LjYBErV.png



 

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PREDATAR Control23

I experienced a similar issue. The difference being, my TSM server was running on zLinux. Many of my Windows clients had horrible restore performance. Very good information.
 
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