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

To: Michel Gaudet <Michel.Gaudet@xxxxxxxx>
Subject: Re: [Xen-users] Share of disks in read only between dom0 and domU.
From: Goswin von Brederlow <brederlo@xxxxxxxxxxxxxxxxxxxxxxxxxxx>
Date: Thu, 27 Sep 2007 11:20:05 +0200
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In-reply-to: <46FB61FD.5010709@xxxxxxxx> (Michel Gaudet's message of "Thu, 27 Sep 2007 09:55:41 +0200")
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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.


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