Re: [Qemu-devel] [PATCH 1/4] hd-geometry.c: Integrate HDIO_GETGEO in guessing for target-s390x

[Christian Borntraeger]

On 20/08/14 10:19, Paolo Bonzini wrote:
> Il 29/07/2014 14:27, Ekaterina Tumanova ha scritto:
>> The new HDIO_GETGEO logic is required for two use cases:
>> a) Support for geometries of Direct Attached Storage Disks (DASD)
>> on s390x configured as backing of virtio block devices.
> 
> Is this still relevant now that QEMU can emulate 512-byte sectors on top
> of host devices with 4k sectors?

Yes, the geometry and the block size define the layout of the DASD disk format. 
This defines how the ibm disk layout partition table looks like. So partition 
detection of the IBM layout only works if values are correct. (see linux 
block/partitions/ibm.c. it needs these values to correctly locate the data 
structures).

Furthermore, if you do an mkfs in the host and the guest sees a different block 
size all kind of strange things will happen when mounting, no?

> 
>> b) Support for FCP attached SCSI disks that do not yet have a
>> partition table. Without this patch, fdisk -l on the host would
>> return different results then fdisk -l in the guest.
> 
> Can you provide an example?

scsi disk in the host:
address@hidden ~]#  sfdisk -g 
/dev/disk/by-id/wwn-0x6005076305ffc1ae0000000000002593
/dev/disk/by-id/wwn-0x6005076305ffc1ae0000000000002593: 1011 cylinders, 34 
heads, 61 sectors/track

same scsi disk in the guest as virtio-blk:
scsi disk in the guest:
address@hidden ~]# sfdisk -g /dev/disk/by-id/virtio-d20
/dev/disk/by-id/virtio-d20: 2080 cylinders, 16 heads, 63 sectors/track

16/63 is currently hardcoded by QEMU, no matter what the host thinks. This gets 
overridden as soon as there is a partition table.

command line was:
-drive 
file=/dev/disk/by-id/scsi-36005076305ffc1ae0000000000002593,if=none,id=drive-virtio-disk20,format=raw,serial=d20,cache=none,aio=native

> 
> Paolo
>