[Qemu-devel] Re: file system sharing

["Eric S. Johansson" <esj@harvee.org>]

Mark Williamson wrote:
> If only one machine (host or guest) has mounted the device then it should 
> always be safe to do this.  You may get away with read only mounting in one 
> and writing in the other but it's not a reliable solution.  Never allow more 
> than one writer to the filesystem - this does bad things to your filesystem!

I am well aware of these issues having worked on filesystems in the 
1980s and having designed and built one of the first RPC based networked 
filesystems in the late 80s for a CAD/CAM company.  To this day the nova 
architecture gives me the willies.

I suspect based on a comment that someone else made about caching that I 
would need some sort of event to trigger a flush either by an explicit 
flush call or a close.  Would unmounting the disk image create such an 
event?

> 
> If you're using a file-based disk and it's partitioned you'll need to use 
> lomount http://www.dad-answers.com/qemu/utilities/QEMU-HD-Mounter/lomount/ to 
> mount the right partition in the host.

so what I can do is create the partition image on the host, start up 
qemu with that multi-partition disk image, do what I need to in qemu, 
shut down qemu and then I have a modified disk image.  if I need to 
modify it from the host, then I can use lomount to make it accessible as 
the local filesystem.

Very cool.

one more question on this theme.  How do I know when the guest OS has 
finished booting?  The reason I ask is I am planning on using ssh to 
perform various operations on the guest OS once it's up.


>>ideally, I would like to "import" my flash memory device into the guest
>>OS side (USB based) but if I can create a "virtual" flash disk and when
>>I'm done modifying it, physically copy the file based image to the
>>physical flash, I would be happy.
> 
> 
> I imagine just giving the guest access to the device file would work.

I'm not entirely sure how to do that.  would I just do something like:

-hdc /dev/sda1

as part of the command line?
> 
> HTH,
> Mark