FreeBSD 9 and Windows XP

Carl Johnson carlj at
Mon Mar 11 23:42:08 UTC 2013

Warren Block <wblock at> writes:

> On Mon, 11 Mar 2013, Carl Johnson wrote:
>> It is my understanding that FreeBSD doesn't allow using part of a disk,
>> but grabs the entire disk.  That means that VirtualBox can't use
>> partitions on a disk that any other partitions are being used by
>> anything else, including FreeBSD itself.  Am I wrong about this?  I use
>> VirtualBox using vdmk for an entire disk, but I have never been able to
>> share with anything else.
> It's very hard to tell what situation is being described here.  If the
> VMDK is a pointer to a whole physical disk, that would probably make
> the disk only usable by one VM.  It should be possible to make the
> VMDK point to just one partition on the disk.  Then other VMs or a
> physical machine could use those other partitions while the FreeBSD VM
> was running.

I was thinking of the case where I tried to allow direct access by a
virtual machine to a slice on the same disk that I was running FreeBSD
off of.  I just looked further into that and discovered that it is
possible, but not allowed by geom by default.  It can be done by setting
'sysctl kern.geom.debugflags=0x10'.  I am sure that you are aware of the
dangers, but for anybody else reading this check out the warning in the
geom(4) manpage.  They refer to this option as 'allow foot shooting' for
a reason.

Carl Johnson		carlj at

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