Re: Different geoms for an rbd block device

[Josh Durgin <josh.durgin@inktank.com>]

On 10/28/2012 03:02 AM, Andrey Korolyov wrote:
> Hi,
>
> Should following behavior considered to be normal?
>
> $ rbd map test-rack0/debiantest --user qemukvm --secret qemukvm.key
> $ fdisk /dev/rbd1
>
> Command (m for help): p
>
> Disk /dev/rbd1: 671 MB, 671088640 bytes
> 255 heads, 63 sectors/track, 81 cylinders, total 1310720 sectors
> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 4194304 bytes / 4194304 bytes
> Disk identifier: 0x00056f14
>
>       Device Boot      Start         End      Blocks   Id  System
> /dev/rbd1p1            2048       63487       30720   82  Linux swap / Solaris
> Partition 1 does not start on physical sector boundary.
> /dev/rbd1p2           63488     1292287      614400   83  Linux
> Partition 2 does not start on physical sector boundary.
>
> Meanwhile, in the guest vm over same image:
>
> fdisk /dev/vda
>
> Command (m for help): p
>
> Disk /dev/vda: 671 MB, 671088640 bytes
> 16 heads, 63 sectors/track, 1300 cylinders, total 1310720 sectors

I'm guessing the reported number of cylinders is the issue?
You can control that with a qemu option. I think

-drive ...cyls=81

will do it. You can also set the min/opt i/o sizes via
qemu device properties min_io_size and opt_io_size in
the same way you can adjust discard granularity:

http://ceph.com/docs/master/rbd/qemu-rbd/#enabling-discard-trim

Unfortunately min_io_size is a uint16 in qemu, so it won't
be able to store 4194304.

> Units = sectors of 1 * 512 = 512 bytes
> Sector size (logical/physical): 512 bytes / 512 bytes
> I/O size (minimum/optimal): 512 bytes / 512 bytes
> Disk identifier: 0x00056f14
>
>     Device Boot      Start         End      Blocks   Id  System
> /dev/vda1            2048       63487       30720   82  Linux swap / Solaris
> /dev/vda2           63488     1292287      614400   83  Linux
>
> The real pain starts when I try to repartition disk from after 'rbd
> map' using its geometry - it simply broke partition layout, for
> example, first block offset moves from 2048b to 8192. Of course I can
> specify geometry by hand, but before that I may need to start vm at
> least once or do something else which will print me out actual layout.
>
> Thanks!

Setting the geometry at qemu boot time should work, and is a bit easier.
qemu actually has code to try to guess disk geometry from a partition
table, but perhaps it doesn't support the format you're using.

Josh