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<chapter id="Troubleshooting">
<title>Troubleshooting</title>
<para>This chapter provides answers to commonly asked questions. In order to
improve your user experience with VirtualBox, it is recommended to read this
section to learn more about common pitfalls and get recommendations on how
to use the product.</para>
<sect1>
<title>Procedures and tools</title>
<sect2>
<title>Categorizing and isolating problems</title>
<para>More often than not, a virtualized guest behaves like a physical
system. Any problems that a physical machine would encounter, a virtual
machine will encounter as well. If, for example, Internet connectivity
is lost due to external issues, virtual machines will be affected just
as much as physical ones.</para>
<para>If a true VirtualBox problem is encountered, it helps to
categorize and isolate the problem first. Here are some of the questions
that should be answered before reporting a problem:<orderedlist>
<listitem>
<para>Is the problem specific to a certain guest OS? Specific
release of a guest OS? Especially with Linux guest related
problems, the issue may be specific to a certain distribution and
version of Linux.</para>
</listitem>
<listitem>
<para>Is the problem specific to a certain host OS? Problems are
usually not host OS specific (because most of the VirtualBox code
base is shared across all supported platforms), but especially in
the areas of networking and USB support, there are significant
differences between host platforms. Some GUI related issues are
also host specific.</para>
</listitem>
<listitem>
<para>Is the problem specific to certain host hardware? This
category of issues is typically related to the host CPU. Because
of significant differences between VT-x and AMD-V, problems may be
specific to one or the other technology. The exact CPU model may
also make a difference (even for software virtualization) because
different CPUs support different features, which may affect
certain aspects of guest CPU operation.</para>
</listitem>
<listitem>
<para>Is the problem specific to a certain virtualization mode?
Some problems may only occur in software virtualization mode,
others may be specific to hardware virtualization.</para>
</listitem>
<listitem>
<para>Is the problem specific to guest SMP? That is, is it related
to the number of virtual CPUs (VCPUs) in the guest? Using more
than one CPU usually significantly affects the internal operation
of a guest OS.</para>
</listitem>
<listitem>
<para>Is the problem specific to the Guest Additions? In some
cases, this is a given (e.g., a shared folders problem), in other
cases it may be less obvious (for example, display problems). And
if the problem is Guest Additions specific, is it also specific to
a certain version of the Additions?</para>
</listitem>
<listitem>
<para>Is the problem specific to a certain environment? Some
problems are related to a particular environment external to the
VM; this usually involves network setup. Certain configurations of
external servers such as DHCP or PXE may expose problems which do
not occur with other, similar servers.</para>
</listitem>
<listitem>
<para>Is the problem a regression? Knowing that an issue is a
regression usually makes it significantly easier to find the
solution. In this case, it is crucial to know which version is
affected and which is not.</para>
</listitem>
</orderedlist></para>
</sect2>
<sect2>
<title>Collecting debugging information</title>
<para>For problem determination, it is often important to collect
debugging information which can be analyzed by VirtualBox support. This
section contains information about what kind of information can be
obtained.</para>
<para>Every time VirtualBox starts up a VM, a so-called <emphasis
role="bold">"release log file"</emphasis> is created containing lots of
information about the VM configuration and runtime events. The log file
and resides in the VM log file folder. Typically this will be a
starting a VM, the configuration file of the last run will be renamed to
<computeroutput>.1</computeroutput>, up to
<computeroutput>.3</computeroutput>. Sometimes when there is a problem,
it is useful to have a look at the logs. Also when requesting support
for VirtualBox, supplying the corresponding log file is
mandatory.</para>
<para>For convenience, for each virtual machine, the VirtualBox main
window can show these logs in a window. To access it, select a virtual
machine from the list on the left and select "Show logs..." from the
"Machine" window.</para>
<para>The release log file (VBox.log) contains a wealth of diagnostic
information, such as Host OS type and version, VirtualBox version and
build (32-bit or 64-bit), a complete dump of the guest's configuration
(CFGM), detailed information about the host CPU type and supported
features, whether hardware virtualization is enabled, information about
stopping, etc.), guest BIOS messages, guest Additions messages, device
specific log entries and at the end of execution, final guest state and
condensed statistics.</para>
<para>In case of crashes, it is very important to collect <emphasis
role="bold">crash dumps</emphasis>. This is true for both host and guest
crashes. For information about enabling core dumps on Linux, Solaris,
and OS X systems, refer to the core dump article on the VirtualBox
website.<footnote>
<para><ulink
url="http://www.virtualbox.org/wiki/Core_dump">http://www.virtualbox.org/wiki/Core_dump</ulink>.</para>
</footnote></para>
<para>For network related problems, it is often helpful to capture a
trace of network traffic. If the traffic is routed through an adapter on
the host, it is possible to use Wireshark or a similar tool to capture
the traffic there. However, this often also includes a lot of traffic
unrelated to the VM.</para>
<para>VirtualBox provides an ability to capture network traffic only on
a specific VM's network adapter. Refer to the network tracing article on
the VirtualBox website<footnote>
<para><ulink
url="http://www.virtualbox.org/wiki/Network_tips">http://www.virtualbox.org/wiki/Network_tips</ulink>.</para>
</footnote> for information on enabling this capture. Note that the
trace files created by VirtualBox are in .pcap format and can be easily
analyzed with Wireshark.</para>
</sect2>
<sect2>
<title>The built-in VM debugger</title>
<para>VirtualBox includes a built-in VM debugger, which advanced users
may find useful. This debugger allows the user to examine, and to some
extent, control, the VM state.<note>
<para>Use the VM debugger at your own risk. There is no support for
it, and the following documentation is only made available for
advanced users with a very high level of familiarity with the
the PC architecture. A degree of familiarity with the internals of
the guest OS in question is not required, but may be very
helpful.</para>
</note></para>
<para>The VM debugger is available in all regular production versions of
VirtualBox, but it is disabled by default because the average user will
have little use for it. There are two ways to access the
debugger:<itemizedlist>
<listitem>
<para>A debugger console window displayed alongside the VM</para>
</listitem>
<listitem>
<para>Via the <computeroutput>telnet</computeroutput> protocol at
port 5000</para>
</listitem>
</itemizedlist></para>
<para>The debugger can be enabled in two ways:<itemizedlist>
<listitem>
<para>Start the <computeroutput>VirtualBox</computeroutput>
process with a <computeroutput>--dbg</computeroutput>,
<computeroutput>--debug</computeroutput>, or
<computeroutput>--debug-command-line</computeroutput> argument.
See the VirtualBox usage help for details. Note that these
arguments are only useful when a VM is started immediately, using
the <computeroutput>--startvm</computeroutput> argument.</para>
</listitem>
<listitem>
<para>Set the
<computeroutput>VBOX_GUI_DBG_ENABLED</computeroutput> or
<computeroutput>VBOX_GUI_DBG_AUTO_SHOW</computeroutput>
environment variable to an arbitrary value before launching the
VirtualBox process. Setting these variables (only their presence
is checked) is effective even when the first VirtualBox process is
the VM selector window. VMs subsequently launched from the
selector will have the debugger enabled.</para>
</listitem>
</itemizedlist></para>
<para>A new 'Debug' menu entry will be added to the VirtualBox
application. This menu allows the user to open the debugger
console.</para>
<para>The VM debugger command syntax is loosely modeled on Microsoft and
IBM debuggers used on DOS, OS/2 and Windows. Users familiar with symdeb,
CodeView, or the OS/2 kernel debugger will find the VirtualBox VM
debugger familiar.</para>
<para>The most important command is
<computeroutput>help</computeroutput>. This will print brief usage help
for all debugger commands. The set of commands supported by the VM
debugger changes frequently and the
<computeroutput>help</computeroutput> command is always
up-to-date.</para>
<para>A brief summary of frequently used commands follows:<itemizedlist>
<listitem>
<para><computeroutput>stop</computeroutput> -- stops the VM
execution and enables single stepping</para>
</listitem>
<listitem>
<para><computeroutput>g</computeroutput> -- continue VM
execution</para>
</listitem>
<listitem>
<para><computeroutput>t</computeroutput> -- single step an
instruction</para>
</listitem>
<listitem>
</listitem>
<listitem>
</listitem>
<listitem>
</listitem>
<listitem>
<para><computeroutput>u</computeroutput> -- unassemble
memory</para>
</listitem>
<listitem>
<para><computeroutput>dg</computeroutput> -- print the guest's
GDT</para>
</listitem>
<listitem>
<para><computeroutput>di</computeroutput> -- print the guest's
IDT</para>
</listitem>
<listitem>
<para><computeroutput>dl</computeroutput> -- print the guest's
LDT</para>
</listitem>
<listitem>
<para><computeroutput>dt</computeroutput> -- print the guest's
TSS</para>
</listitem>
<listitem>
<para><computeroutput>dp*</computeroutput> -- print the guest's
page table structures</para>
</listitem>
<listitem>
</listitem>
<listitem>
<para><computeroutput>bl</computeroutput> -- list
breakpoints</para>
</listitem>
<listitem>
<para><computeroutput>bc</computeroutput> -- clear a
breakpoint</para>
</listitem>
</itemizedlist></para>
<para>See the built-in <computeroutput>help</computeroutput> for other
available commands.</para>
<para>The VM debugger supports symbolic debugging, although symbols for
guest code are often not available. For Solaris guests, the
<computeroutput>detect</computeroutput> command automatically determines
the guest OS version and locates kernel symbols in guest's memory.
Symbolic debugging is then available. For Linux guests, the
<computeroutput>detect</computeroutput> commands also determines the
guest OS version, but there are no symbols in the guest's memory. Kernel
symbols are available in the file
file must be copied to the host, for example using
<computeroutput>scp</computeroutput>. The
<computeroutput>loadmap</computeroutput> debugger command can be used to
make the symbol information available to the VM debugger. Note that the
<computeroutput>kallsyms</computeroutput> file contains the symbols for
the currently loaded modules; if the guest's configuration changes, the
symbols will change as well and must be updated.</para>
<para>For all guests, a simple way to verify that the correct symbols
are loaded is the <computeroutput>k</computeroutput> command. The guest
is normally idling and it should be clear from the symbolic information
that the guest operating system's idle loop is being executed.</para>
<para>Another group of debugger commands is the set of
<computeroutput>info</computeroutput> commands. Running
<computeroutput>info help</computeroutput> provides complete usage
information. The information commands provide ad-hoc data pertinent to
various emulated devices and aspects of the VMM. There is no general
guideline for using the <computeroutput>info</computeroutput> commands,
the right command to use depends entirely on the problem being
investigated. Some of the info commands are:<itemizedlist>
<listitem>
<para><computeroutput>cfgm</computeroutput> -- print a branch of
the configuration tree</para>
</listitem>
<listitem>
<para><computeroutput>cpuid</computeroutput> -- display the guest
CPUID leaves</para>
</listitem>
<listitem>
<para><computeroutput>ioport</computeroutput> -- print registered
I/O port ranges</para>
</listitem>
<listitem>
<para><computeroutput>mmio</computeroutput> -- print registered
MMIO ranges</para>
</listitem>
<listitem>
<para><computeroutput>mode</computeroutput> -- print the current
paging mode</para>
</listitem>
<listitem>
<para><computeroutput>pit</computeroutput> -- print the i8254 PIT
state</para>
</listitem>
<listitem>
<para><computeroutput>pic</computeroutput> -- print the i8259A PIC
state</para>
</listitem>
<listitem>
</listitem>
<listitem>
<para><computeroutput>pcnet0</computeroutput> -- print the PCnet
state</para>
</listitem>
<listitem>
<para><computeroutput>vgatext</computeroutput> -- print the
contents of the VGA framebuffer formatted as standard text
mode</para>
</listitem>
<listitem>
<para><computeroutput>timers</computeroutput> -- print all VM
timers</para>
</listitem>
</itemizedlist></para>
<para>The output of the <computeroutput>info</computeroutput> commands
VirtualBox VMM internals. However, when used properly, the information
provided can be invaluable.</para>
</sect2>
</sect1>
<sect1>
<title>General</title>
<sect2 id="configPeriodicFlush">
file system</title>
<para>Occasionally, some host file systems provide very poor writing
commands. This is normal behavior and should normally cause no real
problems, as the guest should repeat commands that have timed out.
However some guests (e.g. some Linux versions) have severe problems if a
write to an image file takes longer than about 15 seconds. Some file
systems however require more than a minute to complete a single write,
if the host cache contains a large amount of data that needs to be
written.</para>
<para>The symptom for this problem is that the guest can no longer
access its files during large write or copying operations, usually
leading to an immediate hang of the guest.</para>
<para>In order to work around this problem (the true fix is to use a
faster file system that doesn't exhibit such unacceptable write
performance), it is possible to flush the image file after a certain
amount of data has been written. This interval is normally infinite, but
can be configured individually for each disk of a VM.</para>
<para>For IDE disks use the following command:</para>
<screen>VBoxManage setextradata "VM name"
<para>For SATA disks use the following command:</para>
<screen>VBoxManage setextradata "VM name"
<para>The value [x] that selects the disk for IDE is 0 for the master
device on the first channel, 1 for the slave device on the first
channel, 2 for the master device on the second channel or 3 for the
master device on the second channel. For SATA use values between 0 and
29. Only disks support this configuration option; it must not be set for
<para>The unit of the interval [b] is the number of bytes written since
the last flush. The value for it must be selected so that the occasional
long write delays do not occur. Since the proper flush interval depends
on the performance of the host and the host filesystem, finding the
optimal value that makes the problem disappear requires some
experimentation. Values between 1000000 and 10000000 (1 to 10 megabytes)
are a good starting point. Decreasing the interval both decreases the
probability of the problem and the write performance of the guest.
Setting the value unnecessarily low will cost performance without
providing any benefits. An interval of 1 will cause a flush for each
write operation and should solve the problem in any case, but has a
severe write performance penalty.</para>
<para>Providing a value of 0 for [b] is treated as an infinite flush
interval, effectively disabling this workaround. Removing the extra data
key by specifying no value for [b] has the same effect.</para>
</sect2>
<sect2>
<para>If desired, the virtual disk images can be flushed when the guest
issues the IDE FLUSH CACHE command. Normally these requests are ignored
for improved performance. The parameters below are only accepted for
disk drives. They must not be set for DVD drives.</para>
<para>To enable flushing for IDE disks, issue the following
command:</para>
<screen>VBoxManage setextradata "VM name"
<para>The value [x] that selects the disk is 0 for the master device on
the first channel, 1 for the slave device on the first channel, 2 for
the master device on the second channel or 3 for the master device on
the second channel.</para>
<para>To enable flushing for SATA disks, issue the following
command:</para>
<screen>VBoxManage setextradata "VM name"
<para>The value [x] that selects the disk can be a value between 0 and
29.</para>
<para>Note that this doesn't affect the flushes performed according to
the configuration described in <xref linkend="configPeriodicFlush"
xrefstyle="template: %n" />. Restoring the default of ignoring flush
commands is possible by setting the value to 1 or by removing the
key.</para>
</sect2>
</sect1>
<sect1>
<title>Windows guests</title>
<sect2>
<title>Windows bluescreens after changing VM configuration</title>
<para>Changing certain virtual machine settings can cause Windows guests
to fail during start up with a bluescreen. This may happen if you change
VM settings after installing Windows, or if you copy a disk image with
an already installed Windows to a newly created VM which has settings
that differ from the original machine.</para>
<para>This applies in particular to the following settings:<itemizedlist>
<listitem>
<para>The ACPI and I/O APIC settings should never be changed after
installing Windows. Depending on the presence of these hardware
features, the Windows installation program chooses special kernel
and device driver versions and will fail to startup should these
hardware features be removed. (Enabling them for a Windows VM
which was installed without them does not cause any harm. However,
Windows will not use these features in this case.)</para>
</listitem>
<listitem>
<para>Changing the storage controller hardware will cause bootup
failures as well. This might also apply to you if you copy a disk
image from an older version of VirtualBox to a virtual machine
created with a newer VirtualBox version; the default subtype of
IDE controller hardware was changed from PIIX3 to PIIX4 with
VirtualBox 2.2. Make sure these settings are identical.</para>
</listitem>
</itemizedlist></para>
</sect2>
<sect2>
<title>Windows 0x101 bluescreens with SMP enabled (IPI timeout)</title>
<para>If a VM is configured to have more than one processor (symmetrical
multiprocessing, SMP), some configurations of Windows guests crash with
an 0x101 error message, indicating a timeout for inter-processor
interrupts (IPIs). These interrupts synchronize memory management
between processors.</para>
<para>According to Microsoft, this is due to a race condition in
Windows. A hotfix is available.<footnote>
<para>See <ulink
</footnote> If this does not help, please reduce the number of virtual
processors to 1.</para>
</sect2>
<sect2>
<title>Windows 2000 installation failures</title>
<para>When installing Windows 2000 guests, you might run into one of the
following issues:</para>
<itemizedlist>
<listitem>
<para>Installation reboots, usually during component
registration.</para>
</listitem>
<listitem>
<para>Installation fills the whole hard disk with empty log
files.</para>
</listitem>
<listitem>
<para>Installation complains about a failure installing
</listitem>
</itemizedlist>
<para>These problems are all caused by a bug in the hard disk driver of
Windows 2000. After issuing a hard disk request, there is a race
condition in the Windows driver code which leads to corruption if the
operation completes too fast, i.e. the hardware interrupt from the IDE
controller arrives too soon. With physical hardware, there is a
guaranteed delay in most systems so the problem is usually hidden there
(however it should be possible to reproduce it on physical hardware as
well). In a virtual environment, it is possible for the operation to be
done immediately (especially on very fast systems with multiple CPUs)
and the interrupt is signaled sooner than on a physical system. The
solution is to introduce an artificial delay before delivering such
interrupts. This delay can be configured for a VM using the following
command:</para>
<screen>VBoxManage setextradata "VM name"
<para>This sets the delay to one millisecond. In case this doesn't help,
increase it to a value between 1 and 5 milliseconds. Please note that
this slows down disk performance. After installation, you should be able
to remove the key (or set it to 0).</para>
</sect2>
<sect2>
<title>How to record bluescreen information from Windows guests</title>
<para>When Windows guests run into a kernel crash, they display the
infamous bluescreen. Depending on how Windows is configured, the
information will remain on the screen until the machine is restarted or
it will reboot automatically. During installation, Windows is usually
configured to reboot automatically. With automatic reboots, there is no
chance to record the bluescreen information which might be important for
problem determination.</para>
<para>VirtualBox provides a method of halting a guest when it wants to
perform a reset. In order to enable this feature, issue the following
command:</para>
</sect2>
<sect2>
<title>No networking in Windows Vista guests</title>
<para>Unfortunately, with Vista, Microsoft dropped support for the
virtual AMD PCNet card that we are providing to virtual machines. As a
result, after installation, Vista guests initially have no networking.
VirtualBox therefore ships a driver for that card with the Windows Guest
Additions; see <xref linkend="vista_networking" />.</para>
<para>Starting with version 1.6.0 VirtualBox can emulate an Intel E1000
network device which is supported by Vista without any third-party
drivers.</para>
</sect2>
<sect2>
<title>Windows guests may cause a high CPU load</title>
<para>Several background applications of Windows guests, especially
virus scanners, are known to increases the CPU load notably even if the
guest appears to be idle. We recommend to deactivate virus scanners
within virtualized guests if possible.</para>
</sect2>
<sect2 id="win7_audio">
<title>No audio in Windows Vista (64-bit) and Windows 7 guests</title>
<para>32-bit Windows 7 does not ship with drivers for our emulated audio
hardware (AC'97). However, running Windows Update should solve the
problem by getting an appropriate driver for it automatically. After
that update followed by a reboot you should have working audio.</para>
<para>For the 64-bit versions of Windows Vista and 7 you have to
download the Realtek AC'97 drivers to enable audio.</para>
<para>See <ulink
for download instructions.</para>
</sect2>
<sect2>
<title>Long delays when accessing shared folders</title>
<para>The performance for accesses to shared folders from a Windows guest
might be decreased due to delays during the resolution of the VirtualBox
shared folders name service. To fix these delays, add the following entries
to the file
<computeroutput>\windows\system32\drivers\etc\lmhosts</computeroutput>
of the Windows guest:</para>
<screen>255.255.255.255 VBOXSVR #PRE
255.255.255.255 VBOXSRV #PRE</screen>
<para>After doing this change, a reboot of the guest is required.</para>
</sect2>
</sect1>
<sect1>
<title>Linux and X11 guests</title>
<sect2>
<title>Linux guests may cause a high CPU load</title>
<para>Some Linux guests may cause a high CPU load even if the guest
system appears to be idle. This can be caused by a high timer frequency
of the guest kernel. Some Linux distributions, for example Fedora, ship
a Linux kernel configured for a timer frequency of <emphasis
role="bold"> 1000Hz</emphasis>. We recommend to recompile the guest
kernel and to select a timer frequency of 100Hz.</para>
<para>Linux kernels shipped with Red Hat Enterprise Linux (RHEL) as of
release 4.7 and 5.1 as well as kernels of related Linux distributions
(for instance CentOS and Oracle Enterprise Linux) support a kernel
parameter <emphasis>divider=N</emphasis>. Hence, such kernels support a
lower timer frequency without recompilation. We suggest to add the
kernel parameter <emphasis>divider=10</emphasis> to select a guest
kernel timer frequency of 100Hz.</para>
</sect2>
<sect2>
<title>AMD Barcelona CPUs</title>
<para>Most Linux-based guests will fail with AMD Phenoms or
Barcelona-level Opterons due to a bug in the Linux kernel. Enable the
linkend="settings-general-advanced" />).</para>
</sect2>
<sect2 id="trouble-linux-buggy">
<title>Buggy Linux 2.6 kernel versions</title>
<para>The following bugs in Linux kernels prevent them from executing
correctly in VirtualBox, causing VM boot crashes:<itemizedlist>
<listitem>
<para>The Linux kernel version 2.6.18 (and some 2.6.17 versions)
introduced a race condition that can cause boot crashes in
VirtualBox. Please use a kernel version 2.6.19 or later.</para>
</listitem>
<listitem>
<para>With hardware virtualization and the I/O APIC enabled,
kernels before 2.6.24-rc6 may panic on boot with the following
message:<screen>Kernel panic - not syncing: IO-APIC + timer doesn't work! Boot with
apic=debug and send a report. Then try booting with the 'noapic' option</screen></para>
<para>If you see this message, either disable hardware
virtualization or the I/O APIC (see <xref
linkend="settings-system" />), or upgrade the guest to a newer
kernel.<footnote>
<para>See <ulink
url="http://www.mail-archive.com/git-commits-head@vger.kernel.org/msg30813.html">http://www.mail-archive.com/git-commits-head@vger.kernel.org/msg30813.html</ulink>
for details about the kernel fix.</para>
</footnote></para>
</listitem>
</itemizedlist></para>
</sect2>
<sect2>
<title>Shared clipboard, auto-resizing and seamless desktop in X11
guests</title>
<para>Guest desktop services in guests running the X11 window system
(Solaris, Linux and others) are provided by a guest service called
<computeroutput>VBoxClient</computeroutput>, which runs under the ID of
the user who started the desktop session and is automatically started
using the following command lines <screen>VBoxClient --clipboard
VBoxClient --display
VBoxClient --seamless</screen> when your X11 user session is started
if you are using a common desktop environment (Gnome, KDE and others).
If a particular desktop service is not working correctly, it is worth
checking whether the process which should provide it is running.</para>
<para>The <computeroutput>VBoxClient</computeroutput> processes create
files in the user's home directory with names of the form
<computeroutput>.vboxclient-*.pid</computeroutput> when they are running
in order to prevent a given service from being started twice. It can
happen due to misconfiguration that these files are created owned by
root and not deleted when the services are stopped, which will prevent
them from being started in future sessions. If the services cannot be
started, you may wish to check whether these files still exist.</para>
</sect2>
</sect1>
<sect1>
<title>Windows hosts</title>
<sect2>
<title>VBoxSVC out-of-process COM server issues</title>
<para>VirtualBox makes use of the Microsoft Component Object Model (COM)
for inter- and intra-process communication. This allows VirtualBox to
share a common configuration among different virtual machine processes
and provide several user interface options based on a common
architecture. All global status information and configuration is
which is an out-of-process COM server. Whenever a VirtualBox process is
started, it requests access to the COM server and Windows automatically
starts the process. Note that it should never be started by the end
user.</para>
<para>When the last process disconnects from the COM server, it will
terminate itself after some seconds. The VirtualBox configuration (XML
files) is maintained and owned by the COM server and the files are
locked whenever the server runs.</para>
<para>In some cases - such as when a virtual machine is terminated
unexpectedly - the COM server will not notice that the client is
disconnected and stay active for a longer period (10 minutes or so)
keeping the configuration files locked. In other rare cases the COM
server might experience an internal error and subsequently other
processes fail to initialize it. In these situations, it is recommended
to use the Windows task manager to kill the process
</sect2>
<sect2>
the guest does not notice when the medium changes, make sure that the
Windows media change notification (MCN) feature is not turned off. This
is represented by the following key in the Windows registry:<screen><literal>HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Cdrom\Autorun</literal></screen>Certain
applications may disable this key against Microsoft's advice. If it is
set to 0, change it to 1 and reboot your system. VirtualBox relies on
Windows notifying it of media changes.</para>
</sect2>
<sect2>
<title>Sluggish response when using Microsoft RDP client</title>
<para>If connecting to a Virtual Machine via the Microsoft RDP client
(called Remote Desktop Connection), there can be large delays between
input (moving the mouse over a menu is the most obvious situation) and
output. This is because this RDP client collects input for a certain
time before sending it to the VRDP server built into VirtualBox.</para>
<para>The interval can be decreased by setting a Windows registry key to
smaller values than the default of 100. The key does not exist initially
and must be of type DWORD. The unit for its values is milliseconds.
Values around 20 are suitable for low-bandwidth connections between the
RDP client and server. Values around 4 can be used for a gigabit
Ethernet connection. Generally values below 10 achieve a performance
that is very close to that of the local input devices and screen of the
host on which the Virtual Machine is running.</para>
<para>Depending whether the setting should be changed for an individual
user or for the system, either</para>
<screen>HKEY_CURRENT_USER\Software\Microsoft\Terminal Server
Client\Min Send Interval</screen>
<para>or</para>
<screen>HKEY_LOCAL_MACHINE\Software\Microsoft\Terminal Server
Client\Min Send Interval</screen>
<para>can be set appropriately.</para>
</sect2>
<sect2>
<title>Running an iSCSI initiator and target on a single system</title>
<para>Deadlocks can occur on a Windows host when attempting to access an
iSCSI target running in a guest virtual machine with an iSCSI initiator
(e.g. Microsoft iSCSI Initiator) that is running on the host. This is
caused by a flaw in the Windows cache manager component, and causes
sluggish host system response for several minutes, followed by a
"Delayed Write Failed" error message in the system tray or in a separate
message window. The guest is blocked during that period and may show
error messages or become unstable.</para>
<para>Setting the environment variable
<computeroutput>VBOX_DISABLE_HOST_DISK_CACHE</computeroutput> to 1 will
enable a workaround for this problem until Microsoft addresses the
issue. For example, open a command prompt window and start VirtualBox
like this:</para>
<screen>set VBOX_DISABLE_HOST_DISK_CACHE=1
VirtualBox</screen>
<para>While this will decrease guest disk performance (especially
writes), it does not affect the performance of other applications
running on the host.</para>
</sect2>
</sect1>
<sect1>
<title>Linux hosts</title>
<sect2 id="linuxkernelmodulefailstoload">
<title>Linux kernel module refuses to load</title>
<para>If the VirtualBox kernel module
an "Error inserting vboxdrv: Invalid argument", check (as root) the
output of the <computeroutput>dmesg</computeroutput> command to find out
why the load failed. The most common reasons are:</para>
<itemizedlist>
<listitem>
<para>With Linux 2.6.19 and higher, the NMI watchdog may be active.
Add <computeroutput>nmi_watchdog=0</computeroutput> to the kernel
command line (e.g. in your grub configuration) and reboot. With the
Debian and Ubuntu installation modules, execute <computeroutput>sudo
dpkg-reconfigure virtualbox</computeroutput> again.</para>
</listitem>
<listitem>
<para>The kernel disagrees about the version of the gcc used to
compile the module. Make sure that you use the same compiler as used
to build the kernel.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2>
drive, but this does not appear to work, make sure that the current user
has permission to access the corresponding Linux device file
distributions, the user must be added to a corresponding group (usually
called <computeroutput>cdrom</computeroutput> or
<computeroutput>cdrw</computeroutput>).</para>
</sect2>
<sect2>
different name, VirtualBox may be unable to find it. On older Linux
drives:</para>
<para><orderedlist>
<listitem>
<para>VirtualBox examines if the environment variable
<computeroutput>VBOX_CDROM</computeroutput> is defined (see
below). If so, VirtualBox omits all the following checks.</para>
</listitem>
<listitem>
<para>VirtualBox tests if
</listitem>
<listitem>
currently mounted by checking
</listitem>
<listitem>
<para>In addition, VirtualBox checks if any of the entries in
devices.</para>
</listitem>
</orderedlist></para>
<para>In other words, you can try to set VBOX_CDROM to contain a list of
modern Linux distributions, VirtualBox uses the hardware abstraction
layer (hal) to locate CD and DVD hardware.</para>
</sect2>
<sect2>
<title>Linux host floppy not found</title>
<para>The previous instructions (for CD and DVD drives) apply
accordingly to floppy disks, except that on older distributions
by default, and this can be overridden with the
<computeroutput>VBOX_FLOPPY</computeroutput> environment
variable.</para>
</sect2>
<sect2>
incorrect VirtualBox, host or guest configuration, it is possible that
guest kernel error messages (for Linux guests) or application error
messages (for Windows guests). VirtualBox performs the usual consistency
checks when a VM is powered up (in particular it aborts with an error
starting the VM), but it cannot detect all misconfigurations. The
necessary host and guest OS configuration is not specific for
VirtualBox, but a few frequent problems are listed here which occurred
in connection with VirtualBox.</para>
<para>Special care must be taken to use the correct device. The
controller or to a IDE controller that interfaces to the Linux SCSI
subsystem (common for some SATA controllers), this must refer to the
<literal>ide-scsi</literal> kernel module is loaded. This module is
early 2.6 kernels. Many Linux distributions load this module whenever a
kernel supports this and the <literal>ide-scsi</literal> module is not
loaded.</para>
always an IDE device) apply to the guest configuration. Since this setup
is very common, it is likely that the default configuration of the guest
works as expected.</para>
</sect2>
<sect2>
<title>VBoxSVC IPC issues</title>
<para>On Linux, VirtualBox makes use of a custom version of Mozilla
XPCOM (cross platform component object model) for inter- and
intra-process communication (IPC). The process
<computeroutput>VBoxSVC</computeroutput> serves as a communication hub
between different VirtualBox processes and maintains the global
configuration, i.e. the XML database. When starting a VirtualBox
component, the processes <computeroutput>VBoxSVC</computeroutput> and
<computeroutput>VirtualBoxXPCOMIPCD</computeroutput> are started
automatically. They are only accessible from the user account they are
running under. <computeroutput>VBoxSVC</computeroutput> owns the
VirtualBox configuration database which normally resides in
<computeroutput>~/.VirtualBox</computeroutput>. While it is running, the
configuration files are locked. Communication between the various
VirtualBox components and <computeroutput>VBoxSVC</computeroutput> is
performed through a local domain socket residing in
case there are communication problems (i.e. a VirtualBox application
cannot communicate with <computeroutput>VBoxSVC</computeroutput>),
terminate the daemons and remove the local domain socket
directory.</para>
</sect2>
<sect2 id="usb_linux">
<title>USB not working</title>
<para>If USB is not working on your Linux host, make sure that the
current user is a member of the
<computeroutput>vboxusers</computeroutput> group. On older hosts, you
need to make sure that the user has permission to access the USB
filesystem (<computeroutput>usbfs</computeroutput>), which VirtualBox
relies on to retrieve valid information about your host's USB devices.
The rest of this section only applies to those older systems.</para>
<note>
<para>The current rdesktop-vrdp implementation does not support
accessing USB devices through the
<computeroutput>sysfs</computeroutput>!</para>
</note>
<para>As <computeroutput>usbfs</computeroutput> is a virtual filesystem,
a <computeroutput>chmod</computeroutput> on
permissions for <computeroutput>usbfs</computeroutput> can therefore
<emphasis>only</emphasis> be changed by editing the
<para>For example, most Linux distributions have a user group called
<computeroutput>usb</computeroutput> or similar, of which the current
user must be a member. To give all users of that group access to usbfs,
make sure the following line is present:<screen># 85 is the USB group
85 with the group ID that matches your system (search
Alternatively, if you don't mind the security hole, give all users
access to USB by changing "664" to "666".</para>
<para>The various distributions are very creative from which script the
<computeroutput>usbfs</computeroutput> filesystem is mounted. Sometimes
the command is hidden in unexpected places. For SuSE 10.0 the mount
command is part of the udev configuration file
this distribution has no user group called
<computeroutput>vboxusers</computeroutput> group which was created by
the VirtualBox installer. Since group numbers are allocated dynamically,
the following example uses 85 as a placeholder. Modify the line
containing (a linebreak has been inserted to improve
necessary options (make sure that everything is in a single
<para>Debian Etch has the mount command in
distribution has no group <computeroutput>usb</computeroutput>, it is
also the easiest solution to allow all members of the group
<computeroutput>vboxusers</computeroutput> to access the USB subsystem.
so that it contains <screen>domount usbfs usbdevfs /proc/bus/usb -onoexec,nosuid,nodev,devgid=85,devmode=664</screen>
As usual, replace the 85 with the actual group number which should get
access to USB devices.</para>
<para>Other distributions do similar operations in scripts stored in the
</sect2>
<sect2>
<para>Linux kernels including the grsec patch (see <literal><ulink
and derivates have to disable PAX_MPROTECT for the VBox binaries to be
able to start a VM. The reason is that VBox has to create executable
code on anonymous memory.</para>
</sect2>
<sect2>
<title>Linux kernel vmalloc pool exhausted</title>
<para>When running a large number of VMs with a lot of RAM on a Linux
system (say 20 VMs with 1GB of RAM each), additional VMs might fail to
start with a kernel error saying that the vmalloc pool is exhausted and
should be extended. The error message also tells you to specify
<computeroutput>vmalloc=256MB</computeroutput> in your kernel parameter
list. If adding this parameter to your GRUB or LILO configuration makes
the kernel fail to boot (with a weird error message such as "failed to
mount the root partition"), then you have probably run into a memory
conflict of your kernel and initial RAM disk. This can be solved by
adding the following parameter to your GRUB configuration:</para>
<screen>uppermem 524288</screen>
</sect2>
</sect1>
<sect1>
<title>Solaris hosts</title>
<sect2>
<title>Cannot start VM, not enough contiguous memory</title>
<para>The ZFS file system is known to use all available RAM as cache if
the default system settings are not changed. This may lead to a heavy
fragmentation of the host memory preventing VirtualBox VMs from being
started. We recommend to limit the ZFS cache by adding a line<screen>set zfs:zfs_arc_max = xxxx</screen>
amount of memory usable for the ZFS cache.</para>
</sect2>
<sect2>
<title>VM aborts with out of memory errors on Solaris 10 hosts</title>
<para>Solaris 10 hosts (bug 1225025) requires swap space equal to, or
greater than the host's physical memory size. For example, 8 GB physical
memory would require at least 8 GB swap. This can be configured during a
Solaris 10 install by choosing a 'custom install' and changing the
default partitions.</para>
<para>For existing Solaris 10 installs, an additional swap image needs
to be mounted and used as swap. Hence if you have 1 GB swap and 8 GB of
physical memory, you require to add 7 GB more swap. This can be done as
follows:</para>
<para>For ZFS (as root user):</para>
<para>To mount if after reboot, add the following line to
<para>Alternatively, you could grow the existing swap using:</para>
<para>And reboot the system for the changes to take effect.</para>
<para>For UFS (as root user):</para>
<para>To mount it after reboot, add the following line to
</sect2>
</sect1>
</chapter>