Re: [Xen-users] read only disk option in domUs

[Steve Dobbelstein <steved@xxxxxxxxxx>]

list user <xktnniuymlla@xxxxxxxxxxxxxx> wrote on 04/25/2006 03:18:45 PM:

> Hi all,
>
> Successfully running a built from source xen 3.0.2-2 & linux 2.6.16 on
> fedora 4 with lvm.
>
> I'm trying to share a read only /usr across multiple domU's.
>
> Logical volume "/dev/vg0/usr" is read only.  Within the domU's fstab
> /usr is "ro".  Here is the disk option from the domU definition.
>
> disk = [ 'phy:vg0/fc4,hda1,w' ,
>           'phy:vg0/usr,hda2,r' ,
>           'phy:vg0/swap,hda3,w' ]
>
> This fails with an unable to write block error.
>
> Now if the vg0/usr volume is specified with 'w' it works without
> complaint until I try to use it in a different domU.  Then the complaint
> is that it is already in use in another guest domain.
>
> Q 1: is inability to specify a phy: as read only a bug?
> Q 2: what is the proper way to share/use a read only volume among domU's?
>
> Thanks for any help,
> Mike Wright

We use a shared read-only device for /usr in our test setups quite
successfully.  The disk line in the domU config file looks like:
disk =
['phy:/dev/virt-blkdev-backend/dom1,sda1,w','phy:/dev/virt-blkdev-backend/usr,sda2,r','phy:/dev/virt-blkdev-disk_io_test/dom1,sda3,w']
First device is /, second device is /usr, third device is shows up as a raw
disk for testing.  We are running FC4 in the domUs.

What is it that fails?  xm create? the boot up in the domU?

In our setup I have seen an error or two during bootup in the domU because
something in the initialization wants to write to /usr.  It's usually some
function we don't care about so we ignore it.  Is something in your init
scripts trying to write to /usr?

Steve D.


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