Re: virtio implementation?

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Arnd Bergmann wrote:
> On Wednesday 18 July 2007, Gerd Hoffmann wrote:
>   
>> Rusty Russell wrote:
>>     
>>> You mean backend?  For networking it makes a great deal of sense.  For
>>> block it makes far less sense (COW, weird formats, etc).
>>>       
>> For block you probably want both:  userspace driver which can handle all
>> sorts of funny image files, and a kernel driver doing a 1:1 mapping to a
>> block device (lvm volume).
>>
>>     
>
> Interestingly, once you have the kernel driver that maps a block device,
> you can do most of the useful user scenarios by means of /dev/loop
> and/or device mapper.
>   

Not quite.  Using device mapper to implement something like qcow turns 
out to be pretty painful.  qcow, for instance, doesn't guarantee that 
blocks will be aligned to a sector boundary.  It also doesn't guarantee 
that blocks are >= PAGE_SIZE.  The other problem is that device mapper 
doesn't very much like to have very large tables so you end up having to 
use the table as a cache.

See dm-userspace for an actual implementation of all of this.  I'm 
beginning to think that a more proper userspace block device would be 
interesting.

http://wiki.xensource.com/xenwiki/DmUserspace

Regards,

Anthony Liguori

> 	Arnd <><
>
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