Re: [Qemu-devel] [PATCH 1/3] Fix geometry sector calculation

[Christian Borntraeger <borntraeger@de.ibm.com>]

On 02/05/12 12:25, Paolo Bonzini wrote:
> Il 02/05/2012 12:18, Christian Borntraeger ha scritto:
>> Maybe that really points to the problem that we are trying to solve here.
>> For a dasd device, there is usually a 4096 byte block size and on the host
>> these 4096 arereported via getss and getpbsz. 
>> The geometry reported by the device driver is usually 15 head and 12 sectors
>> per track, but actually means 12 sectors of 4096 bytes size (a track ~ 48k).
>>
>> What I want to achieve is that the guest view is identical to the host view
>> for cyls, heads, secs, and all block sizes.
> 
> I think what you want is _not_ to have the same view as the host.  What
> you want is simply to have a default that is consistent with what is
> common on actual s390 disks.

Let me put it in another way:

I want to have these values to match the _device_ that we are passing to the guest
because several tools and the partition detection code for a compatible disk format
(those that can be accessed by z/OS) needs those values to work properly.

That of course means that the guest view is identical to the host view because both
views describe a real property of the hardware.

IOW the geometry for dasd devices is not an artifical number, it has some real meaning
that has a influence on the data structures on the disk.

Thing is, the easiest way of getting the hardware property is to query the host.

Does that make the situation a bit clearer?

Christian