Chapter 34. Troubleshooting

34.1. Debugging and troubleshooting tools
34.2. kvm_stat
34.3. Log files
34.4. Troubleshooting with serial consoles
34.5. Virtualization log files
34.6. Loop device errors
34.7. Enabling Intel VT and AMD-V virtualization hardware extensions in BIOS
34.8. KVM networking performance

This chapter covers common problems and solutions for Red Hat Enterprise Linux 6 virtualization issues.

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 6 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.

34.1. 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:

kvm_stat
kvmtrace
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
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.