Re: [Xen-devel] Blkfront support for get geometry ioctl

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>]

> 
> > Continuing my battle with LCFGng installation onto Xen, I was puzzled by 
> > the 
> > fact that whatever size drive I exported to Xen, the partitioning program 
> > always seemed to think the disk size was approx 2.1 GB. After some probing, 
> > I realised that the program LCFG uses to determine disk size (based on 
> > RedHat's libfdisk I think) does so by performing an ioctl to get the 
> > geometry of the disk. The code in blkfront "implements" the HDIO_GETGEO 
> > ioctl by returning garbage results for cyls, heads, and sectors/track. (the 
> > figures in question: 63*255*262 -- equals approx 2.1 GB!).
> 
> As you've discovered, exporting a device to a whole disk target
> hasn't been widely tested -- most people export devices to
> partitions.

Stephen,

It's worth having a look at the following patch to add whole disk
partition table support to the loop back device:

 
ftp://ftp.hq.nasa.gov/pub/ig/ccd/enhanced_loopback/patches/enhanced_loop-2.4.20.patch

They seem to initialise the geometry information to all zeros,
and simply return all zeros, unless someone uses HDIO_SETGEO to
set it to something else.

I wander if a similar strategy could work for you, rather than
inventing a bogus geometry? Would you mind trying this out? 

It might be worth strace'ing fdisk to see if it uses HDIO_SETGEO
to configure a geometry if it gets all zeros back from GETGEO.


Thanks,
Ian


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