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

[Anthony Liguori <anthony@codemonkey.ws>]

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<jan.kiszka@web.de>
>>>
>>>        
>> 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