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.
 
> Do you think it would be possible to return geometry values that are at 
> least consistent with the size of the device? Something like this (still has 
> hard-coded heads and sectors, but calculates cyls based on size of disk):

It certainly seems sensible to return geometry that approximates
the "disk's" total size. Your current patch can 'waste' up to 8MB
of disk space. Do you think it's worth doing something iterative
that plays around with different numbers of heads and sectors to
find a total <= disk size?

I'll bet there's some stock code we could use to do this...

Ian


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