Re: [Qemu-devel] [PATCH] qemu-img: add FUSE-based image access

[Anthony Liguori]

On 03/25/2010 04:46 PM, Jan Kiszka wrote:
> Anthony Liguori wrote:
>    
> > On 03/25/2010 12:52 PM, Jan Kiszka wrote:
> >      
> > > This adds the "map" subcommand to qemu-img. It is able to expose the raw
> > > content of a disk image via a FUSE filesystem. Both the whole disk can
> > > be accessed, e.g. to run partitioning tools against it, as well as
> > > individual partitions. This allows to create new filesystems in the
> > > image or loop-back mount exiting ones. Using the great mountlo tool
> > > from the FUSE collection [1][2], the latter can even be done by non-root
> > > users (the former anyway).
> > >
> > > There are some dependency to fulfill to gain all features: Partition
> > > scanning is done via recent libblkid (I used version 2.17.1). If this
> > > library is not available, only the disk file is provide. Fortunately,
> > > mountlo can do partition scanning as well ("-p n") to work around this.
> > >
> > > Moreover, libfuse>= 2.8 and a host kernel>= 2.6.29 is required for
> > > seamless disk access via fdisk. Otherwise, the BLKGETSIZE64 IOCTL cannot
> > > be provided, and the number of cylinders has to set explicitly (e.g. via
> > > "-C n").
> > >
> > > This work was inspired by Ashley Saulsbury's qemu-diskp [3].
> > >
> > > [1]
> > > http://sourceforge.net/apps/mediawiki/fuse/index.php?title=FileSystems#Mountlo
> > >
> > > [2] http://sourceforge.net/projects/fuse/files/mountlo/
> > > [3] http://www.saulsbury.org/software/virtualization.html
> > >
> > > Signed-off-by: Jan Kiszka<address@hidden>
> > >
> > >        
> > This has been proposed quite a few times.
> >
> > In fact, I wrote something like this prior to implementing qemu-nbd.
> >
> > The problem with fuse is that as default configured, you can't actually
> > enter into a fuse filesystem as root and since you need to be root to
> > loopback mount it, it pretty nasty from a usability perspective.
> >      
> You don't, see mountlo.
>    

That definitely changes things.  I assume it just uses libe2fs et al to 
display filesystem contents?

Does it preserve ownership?

You still can't do things as root I take it which is problematic.

> > So why did you go the fuse route instead of using qemu-nbd?
> >      
> Mostly usability. It's really straightforward to stack mountlo on top of
> the mapped image. And you can run (almost) all the filesystem and
> partitioning tools.
>
> Moreover, blkid_partlist provides a more complete partition parser than
> rolling your own version

We do in qemu-nbd and in qemu (to guess disk geometry).

Regards,

Anthony Liguori