Re: [Xen-users] Share of disks in read only between dom0 and domU.

[Goswin von Brederlow <brederlo@xxxxxxxxxxxxxxxxxxxxxxxxxxx>]

Michel Gaudet <Michel.Gaudet@xxxxxxxx> writes:

> Hello !
>
> Because of a special config of my grub boot system, i try to export in
> a domU, in read only mode, one of my disk partition mounted in the
> dom0 (in RW)  and i don't succeed because xen refuse to export this
> disk partition in a domU.
>
> extract of the conf :
>
> disk = [ 'phy:/dev/sda,ioemu:hda,r' , 'phy:/dev/sda6,ioemu:sda6,r]' ,
> 'phy:/dev/sda1,ioemu:sda1' ]
>
> The first disk for grub to access the MBR , the second for grub to obtain its 
> 1.5 and
> second stage and the third to reach the guest OS.
>
> I block at the export of the sda6 which is the / of my dom0 with the message :
>
> Error: Device 2054 (vbd) could not be connected.
> Device /dev/sda6 is mounted read-write in the privileged domain,
> and so cannot be mounted read-only by a guest.

And it has good reason to refuse this.

> If i don't mount the /dev/sda6 the domU begin the boot but it freeze with the 
> error :
>
> Booting from Hard Disk
> GRUB Loading stage1.5.
>
> GRUB loading, please wait...
> Error 5
>
> Error 5 is : Partition table invalid or corrupt
>
> But we can boot normally our 2 OS without xen.

You need sda there, not sda6. There is no partition table on sda6.

> I think it is because it don't see the partition onto which is the grub 
> install : /dev/sda6
>
> I understand why it is forbidden to mount in RW the same disk but in R ?

Because the filesystem structures will be in some inconsistent state
and change underneath the caches the domU maintains without
notice. You quickly get a mix of old and new data.

For grub this might not matter as it is so low level but xen doesn't
know you only want to start grub there.

MfG
        Goswin

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