Re: [Xen-users] xen-create-image puts "swap" before "root"

["Fajar A. Nugraha" <list@xxxxxxxxx>]

On Sat, May 17, 2014 at 4:04 AM, Alexandre Kouznetsov <alk@xxxxxxxxxx> wrote:
> Hello.
>
> I'm setting up a Xen hypervisor on a Debian Wheezy, and facing a issue I
> wanted to deal with since a long time ago.
>
> It's about the order in which xen-create-image handles DomU's disks.  By
> design of /usr/bin/xen-create-image perl script, the "swap" partition always
> goes in the first place on the list, followed by "/" and then the rest of
> them.
>
> Between other things, this setup makes it tricky for PyGrub, there has to be
> specific code in xm.tmpl to make the "/" to appear in the first place.
>
> I'm wondering why is it so. I would expect the "/" to go first, followed by
> swap and then the rest of them. Even better, if a partition schema from
> /etc/xen-tools/partitions.d is used, why not to use the disks in the same
> order as they appear in the partitions file, for sake of predictability.
>
> This is not feature request. There must be a reason to handle the partitions
> in this order. I'm very curious to know which is it, before proposing any
> change.

IIRC in the old days, the earliest most-common implementation for xen
backend storage was a file formatted as loopback device, without any
paritition. This makes it easy if you want to do stuff like extend the
storage, but makes it very different from "normal", non-virtualized
system. Among others, most OS installer won't work on it, you need to
setup the OS using a special way (e.g. using tar.gz-ed template).

After that, comes the age of a file / LVM , with paritiion. Since
you're more likely to extend storage for fileystem (e.g. "/") compared
to swap, the swap space is placed first, and "/" last. This makes
extending the storage somewhat easy enough for experts: you need to
create a new partition table, but you don't need to move any data
around.

Back to the present, LVM seems to be the norm nowadays, so it
shouldn't matter where "/" or swap is located on the disk image. You
simply add another disk/file/block device to the VM, and have LVM
(inside the VM) handle the rest. This setup also makes it similar to
non-virtualized system.

-- 
Fajar

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