Re: [kvm-devel] [Qemu-devel] Re: [RFC][PATCH] Modify loop device to be able to manage partitions of the image disk

[Laurent Vivier]

Le mercredi 16 janvier 2008 à 08:57 -0600, Anthony Liguori a écrit :
> > Le mardi 15 janvier 2008 à 23:54 +0000, Daniel P. Berrange a écrit :
[...]
> >>> 2- I'd like to mount qcow2 or others disk image formats, so perhaps it's
> >>> easier to modify loop device driver (but perhaps you know another magic
> >>> tool ?)
> >>>       
> >> There has been some work in this area wrt to Xen - the DM-Userspace project
> >> had some working code providing a device mapper target calling out to a 
> >> userspace daemon to handle non-raw file formats like qcow. I don't
> >> know what the state of it is now wrt to upstream kernel / device-mapper,
> >> or even whether it is more than just 'proof of concept', but the project
> >> page is here with some info:
> >>
> >>   http://wiki.xensource.com/xenwiki/DmUserspace
> 
> FWIW, I still think a userspace block device is the Right Way to support 

I agree with you, it was my first idea too, but it introduces complexity
to manage communications between the kernel part of the driver and the
userspace daemon: I don't like complexity.

> these sort of things.  dm-userspace turned out to be difficult as device 
> mapper has some rather strict requirements about alignment that some 
> formats (like qcow) cannot satisfy.
> 
> The loop driver is a terrible base to start from as it does not preserve 
> data integrity.
[...]

But everyone already uses loop as it is currently, so why not to add
more supported formats for the disk image ?
Why do you say it doesn't preserve data integrity ?

Regards,
Laurent
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