Re: [Xen-users] Where does xen chache domUs grub.conf ?

[Alain BARTHE <alain.barthe@xxxxxxxx>]

Thanks Al, that's exactly the explanation
I waited for.

Alain.

xen-users-bounces@xxxxxxxxxxxxxxxxxxx a écrit sur
29/01/2008 03:29:10 :

> On Dec 11, 2007 8:08 AM, John Madden <jmadden@xxxxxxxxxxx> wrote:
> > > By "reboot", I meant "shutdown/restart".
Among others things, I tried
> > > to run pygrub on the dom0 on the LV that support the domU's
virtual
> > > disk, while the domU is off. Without seeing any change.
> > > I also verified the grub.conf to be effectively updated
by mounting
> > > the domU boot partition on the dom0, while the domU is off.
The
> > > grub.conf was actually changed. That's why I suppose something
to be
> > > cached somewhere on the dom0.
> >
> > dom0 can/will certainly cache the contents of the disk you're
sharing to
> > the domU: I had another case where "grub wasn't getting
installed inside
> > domU" when it certainly was; less -f /dev/disk from dom0
didn't show the
> > grub sectors on the disk when they were definitely there, a domU
> > shutdown "cleared the cache."  So again, my experience
has been that
> > shutting it down completely fixed the problem, but perhaps it
doesn't
> > for you.
> 
> A domU shutdown will not necessarily clear the cache that is causing
> the pygrub problem.   In fact, the problem has nothing to do
with Xen
> or pygrub at all.   It's the linux kernel that's caching the
blocks in
> RAM and not noticing that they've changed on the disk.    
I ran into
> this when editing the grub configuration by mounting a kpartx device
> then trying to boot the VM off the normal LVM device.    The
kernel
> had no way of knowing that the pages containing grub.conf were dirty,
> so it just returned what it had cached in RAM.    I'll let
a kernel
> hacker can explain this more thoroughly.
> 
> Either of the following commands should clear the cache and let pygrub
> get the right information:
> 
> echo 1 > /proc/sys/vm/drop_caches
> blockdev --flushbufs <device>
> 
> See also:
> http://linux-mm.org/Drop_Caches
> blockdev(8)
> linux-2.6.xx/Documentation/filesystems/proc.txt (from the linux kernel
source)
> 
> -Al Tobey
> 
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