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

[Jan Kiszka <jan.kiszka@web.de>]

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Anthony Liguori wrote:
> 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?

Nope. It's a bit like libguestfs as it uses Linux to access the
filesystems, but that Linux runs in UML mode, thus does not require any
qemu/kvm underneath. It simply maps the FUSE requests on corresponding
VFS services in the UML kernel.

> 
> Does it preserve ownership?

Yep.

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

At least my default config does not prevent running qemu-img map as root
and then performing a classic "mount -o loop" on the partitions it
provides. Or what do you mean?


What mountlo is lacking (at least so far) are things like LVM or
soft-RAID. There were some posts on the fuse lists announcing work on
it, but that dates 2 years back without any code traces. But if this
path turns out to be useful for us (or libguestfs), I guess that should
be easy to add.

Jan


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