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.

Every time VirtualBox starts up a VM, a log file is created containing some information about the VM configuration and runtime events. The log file is called VBox.log and resides in the VM log file folder. Typically this will be a directory like this:

$HOME/.VirtualBox/Machines/{machinename}/Logs

When starting a VM, the configuration file of the last run will be renamed to .1, up to .3. 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.

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.

Occasionally, some host file systems provide very poor writing performance and as a consequence cause the guest to time out IDE 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 a VDI 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.

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.

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 VDI 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 using the following command:

VBoxManage setextradata VMNAME 
           "VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/FlushInterval" [b]

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. Only disks support this configuration option. It must not be set for CD-ROM drives.

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.

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.

If desired, the virtual disk images (VDI) can be flushed when the guest issues the IDE FLUSH CACHE command. Normally these requests are ignored for improved performance. To enable flushing, issue the following command:

VBoxManage setextradata VMNAME 
           "VBoxInternal/Devices/piix3ide/0/LUN#[x]/Config/IgnoreFlush" 0

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. Only disks support this configuration option. It must not be set for CD-ROM drives.

Note that this doesn't affect the flushes performed according to the configuration described in 11.1.2. Restoring the default of ignoring flush commands is possible by setting the value to 1 or by removing the key.