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Chapter 35. Troubleshooting Xen

This chapter covers essential concepts to assist you in troubleshooting problems in Xen. Troubleshooting topics covered in this chapter include:
  • troubleshooting tools for Linux and virtualization.
  • troubleshooting techniques for identifying problems.
  • The location of log files and explanations of the information in logs.
This chapter is to give you, the reader, a background to identify where problems with virtualization technologies are. Troubleshooting takes practice and experience which are difficult to learn from a book. It is recommended that you experiment and test virtualization on Red Hat Enterprise Linux to develop your troubleshooting skills.
If you cannot find the answer in this document there may be an answer online from the virtualization community. See Section A.1, “Online resources” for a list of Linux virtualization websites.

35.1. Debugging and troubleshooting Xen

This section summarizes the System Administrator applications, the networking utilities, and debugging tools. You can employ these standard System administration tools and logs to assist with troubleshooting:

Useful commands and applications for troubleshooting

xentop
xentop displays real-time information about a host system and the guest domains.
xm
Using the dmesg and log
  • vmstat
  • iostat
  • lsof
The iostat, mpstat and sar commands are all provided by the sysstat package.
You can employ these Advanced Debugging Tools and logs to assist with troubleshooting:
  • XenOprofile
  • systemtap
  • crash
  • sysrq
  • sysrq t
  • sysrq w
These networking tools can assist with troubleshooting virtualization networking problems:
  • ifconfig
  • tcpdump
    The tcpdump command 'sniffs' network packets. tcpdump is useful for finding network abnormalities and problems with network authentication. There is a graphical version of tcpdump named wireshark.
  • brctl
    brctl is a networking tool that inspects and configures the Ethernet bridge configuration in the Virtualization linux kernel. You must have root access before performing these example commands:
    # brctl show 
    
    bridge-name    bridge-id          STP  enabled  interfaces  
    -----------------------------------------------------------------------------
    xenbr0             8000.feffffff       no        vif13.0
    xenbr1             8000.ffffefff       yes       pddummy0
    xenbr2             8000.ffffffef       no        vif0.0
    
    # brctl showmacs xenbr0
    
    port-no           mac-addr                  local?       aging timer
    
    1                 fe:ff:ff:ff:ff:           yes            0.00
    2                 fe:ff:ff:fe:ff:           yes            0.00
    
    
    # brctl showstp xenbr0
    xenbr0 
    bridge-id              8000.fefffffffff
    designated-root        8000.fefffffffff
    root-port              0                   path-cost             0
    max-age                20.00               bridge-max-age        20.00
    hello-time             2.00                bridge-hello-time     2.00
    forward-delay          0.00                bridge-forward-delay  0.00
    aging-time            300.01
    hello-timer            1.43                tcn-timer             0.00
    topology-change-timer  0.00                gc-timer              0.02
    
Listed below are some other useful commands for troubleshooting virtualization on Red Hat Enterprise Linux 5. All utilities mentioned can be found in the Server repositories Red Hat Enterprise Linux 5.
  • strace is a command which traces system calls and events received and used by another process.
  • vncviewer: connect to a VNC server running on your server or a virtual machine. Install vncviewer using the yum install vnc command.
  • vncserver: start a remote desktop on your server. Gives you the ability to run graphical user interfaces such as virt-manager via a remote session. Install vncserver using the yum install vnc-server command.