Summary

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The performance behavior of the live guest relocation of virtual machines was evaluated.

For each test, a live guest relocation of a 4 GiB z/VM® guest virtual machine was performed while a Java™ workload was executed under Linux™ on System z® running within the z/VM guest virtual machine. For details about the workload, see Test workload.

Depending on the configuration of the ISFC logical link, the live guest relocation took times from 158 down to 14.9 seconds. Furthermore, the quiesce time – the time while the virtual system is stopped during the relocation process and thus is not able to perform work – varied from 21 down to 1.6 seconds.

These variations emphasize the need for carefully configuring the ISFC logical links between the z/VM SSI cluster members, such that high data transfer rates result and minimize the impact of the relocation process on the workload performed by the guest virtual machine.

Given a situation similar to that exhibited by the test environment and the test workload used for the evaluations presented in this paper, the following recommendations should be considered:

  • Prefer ISFC logical configurations with more than one FICON® channel
    • In our tests the observed relocation and quiesce times scaled about inversely linear with the number of FICON channels
    • Consider using FICON channels for ISFC logical links as well as for DASD access if either usage only occurs occasionally; however, the usage of these DASDs should be low such that it does not interfere with z/VM's usage of FCTC connections on the same FICON channels
  • Configure four usable FCTC connections from each FICON channel used for an ISFC logical link
    • In addition, because z/VM designates one or two FCTC connections for "unidirectional use", add one or two FCTC connections, respectively; for details, see Recommendations.
  • For ISFC logical links connecting z/VM members on LPARs residing on the same CEC, we recommend that you use ISFC logical link configurations with "straight" FCTC connections; for details, see ISFC logical link configurations with "straight" FCTC connections.
  • If a virtual system is to be relocated to another z/VM member, with the choice of that target z/VM member being either on the same or on a different CEC, and the same set of FICON channels is configured for the ISFC logical links connecting to either target z/VM member, then relocating to the remote system might be faster because in this case the FICON channels are used only for sending the relocation data (instead sending and receiving in the local case).