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Chapter 32. Troubleshooting

32.1. Debugging and troubleshooting tools
32.2. Log files
32.3. Troubleshooting with serial consoles
32.4. Virtualization log files
32.5. Loop device errors
32.6. Enabling Intel VT and AMD-V virtualization hardware extensions in BIOS
32.7. KVM networking performance
This chapter covers common problems and solutions with Fedora virtualization.
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 Fedora to develop your troubleshooting skills.
If you cannot find the answer in this document there may be an answer online from the virtualization community. Refer to Section A.1, “Online resources” for a list of Linux virtualization websites.

32.1. Debugging and troubleshooting tools

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:
  • vmstat
  • iostat
  • lsof
  • 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  
    virtbr0             8000.feffffff       yes        eth0
    # brctl showmacs virtbr0 
    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 virtbr0
    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.
  • 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 vncviwer 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.