Comment 3 for bug 623609

Colin, I'm not particularly sure where that code is. My guess is in the xen code, and possibly partially in the xen pv block device driver in the kernel.

Xen has always had this strange but useful functionality of assigning a file or block device in the dom0 to a partition in the guest:

Ie, you can do:
 Host : Guest
 /dev/sdb : /dev/xvda
 /dev/sdb : /dev/xvda1
 /home/smoser/part.img : /dev/xvda1
 /home/smoser/swap.img : /dev/xvda2
 /home/smoser/ephemeral.img : /dev/xvda3

We dont get to control the xen setup of these devices. In EC2, you provide EC2 with a partition image, and they make that data available as the first disk's first partition.

I think in the past, guests used to see a /dev/sda entry (along with the sda1 entries), but it was mostly fictitious. Ie, you couldn't re-write the partition table.

Note, that xen block devices would normally be named xvdX, but our kernel renames them to sdaX. I suppose its possible that there is a bug in our kernel renaming that code that ends up dropping sda.