Re: [Xen-users] RFH: loopback & blktap(2) and CDROM

[Philipp Hahn <hahn@xxxxxxxxxxxxx>]

Hello,

On Friday 16 November 2012 17:54:57 Stefano Stabellini wrote:
> On Wed, 14 Nov 2012, Ian Campbell wrote:
> > On Wed, 2012-11-14 at 11:13 +0000, Philipp Hahn wrote:
...
> > > For testing I created 60 partitions (3 primary+57 extended) on hdb, but
> > > as most SCSI, SATA and XEN majors only support 16 minors per device, I
> > > only see the first 15 partitions on /dev/xvde{,1..15}. With
> > > "..unplug=never" I see them all, but 16..60 are provided by block-major
> > > 259 (blkext).
> >
> > Hrm. I wonder of blkfront needs to do some magic to enable this blkext
> > thing then?
>
> I thought that it is not possible to have more than 16 partitions on an
> IDE disk, that would be the reason why you also can't have more than 16
> partitions on a PV disk corresponding to an emulated IDE disk (xvda
> corresponding to hda).

That's the difference between the old PATA 'hdX' and the newer 
SATA/SCSI/SAS 'sdX' block devices: the former support up to 63 partitions.
linux/Documentation/devices.txt:
>   3 block       First MFM, RLL and IDE hard disk/CD-ROM interface
>                   0 = /dev/hda          Master: whole disk (or CD-ROM)
>                  64 = /dev/hdb          Slave: whole disk (or CD-ROM)
>
>                 For partitions, add to the whole disk device number:
>                   0 = /dev/hd?          Whole disk
>                   1 = /dev/hd?1         First partition
>                   2 = /dev/hd?2         Second partition
>                     ...
>                  63 = /dev/hd?63        63rd partition
>
>                 For Linux/i386, partitions 1-4 are the primary
>                 partitions, and 5 and above are logical partitions.
>                 Other versions of Linux use partitioning schemes
>                 appropriate to their respective architectures.
...
>   8 block       SCSI disk devices (0-15)
>                   0 = /dev/sda          First SCSI disk whole disk
>                  16 = /dev/sdb          Second SCSI disk whole disk
>                  32 = /dev/sdc          Third SCSI disk whole disk
>                     ...
>                 240 = /dev/sdp          Sixteenth SCSI disk whole disk
>
>                 Partitions are handled in the same way as for IDE
>                 disks (see major number 3) except that the limit on
>                 partitions is 15.
But with LVM that's mostly irrelevant (I hope).

BYtE
Philipp
-- 
Philipp Hahn           Open Source Software Engineer      hahn@xxxxxxxxxxxxx
Univention GmbH        be open.                       fon: +49 421 22 232- 0
Mary-Somerville-Str.1  D-28359 Bremen                 fax: +49 421 22 232-99
                                                   http://www.univention.de/

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