[ubuntu-cloud] Greater than 1TiB boot disks for the Cloud Image

[Patrick Devine]

Hey peeps,

I ran into an interesting problem yesterday with regards to the cloud
image.  The image contains a single ext4 partition across an MSDOS
partition table, which I believe is done to make it expedient to
resize the partition; it's simple to add space to the image and extend
the the single partition to different instance sizes.

There ends up being two problems, however.  Since it's partitioned
with MSDOS, the total size of the disk is limited to 2 TiB.  This is
annoying, but if you're OK with the limitation it will work out of the
box with OpenStack and KVM.  That is, it will work until a new grub
package is released.  Grub has a limitation that it can only read
partitions which are less than 1TiB in size, so if you've resized your
instance flavour to larger than 1 TiB, you're going to be in trouble.
Everything will work fine initially because Grub gets installed
initially as part of image creation when the partition size is small.
When it gets _re-installed_ however (as was the case yesterday when a
new grub-pc package was released), things will blow up leaving your
image in a broken state.

I haven't tried the uefi image as I'm not certain whether or not it
will have issues with KVM.  My guess is that it would at least have a
GPT partition table, but I don't know if it still suffers from the
single partition problem which chokes up Grub.

Is there any documentation on the cloud images?  The Community Help
Page does not mention any limitations, and doesn't even explain how to
import the image into OpenStack.  It would be nice to know what the
limitations are up front so that when you are using them in a
production environment they don't end up biting you.

Thanks!
--Patrick.