Xen devel - Jan 2005 - how to see the data changing in a running domain?

hello,

I wonder how I can see the (on-disk) data changing in a running
domainU. I tried to do it, but the data looking from outside remains
intact, no matter what I modified inside the domainU.

I did it this way: I use a file-based rootfs (got from ttylinux), and
created a domainU on that rootfs. Everything was fine. Then from
domain0, I mounted that "rootfs" file in read-only mode, with this
command: "mount -o loop -o ro rootfs /mnt". After that I could see the
data inside the rootfs.

Then I suppose that from outside (domain0), I can watch for the data
of domainU changing in realtime. To test, I just did a comand like
this from inside domainU: "ls /etc > /root/etclist", and created a
"etclist" in "/root". Then I supposed that from domain0, in
a mounted
"/mnt" above, I could see the new file created as
"/mnt/root/etclist",
but I was disappointed not to see anything at all.

So clearly everything made changed inside domainU cannot be seen from
outside (domain0). Anybody can please explain to me why, and if
possible, how could I watch data of domainU (in realtime) from
domain0?

Thank you a lot,
AQ


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> I wonder how I can see the (on-disk) data changing in a running
> domainU. I tried to do it, but the data looking from outside remains
> intact, no matter what I modified inside the domainU.
Use NFS root, otherwise the caching (in both kernels) will stop you
seeing what''s going on.

An alternative is to use GFS or OCFS2.

Ian
 
> I did it this way: I use a file-based rootfs (got from ttylinux), and
> created a domainU on that rootfs. Everything was fine. Then from
> domain0, I mounted that "rootfs" file in read-only mode, with
this
> command: "mount -o loop -o ro rootfs /mnt". After that I could
see the
> data inside the rootfs.
> 
> Then I suppose that from outside (domain0), I can watch for the data
> of domainU changing in realtime. To test, I just did a comand like
> this from inside domainU: "ls /etc > /root/etclist", and
created a
> "etclist" in "/root". Then I supposed that from
domain0, in a mounted
> "/mnt" above, I could see the new file created as
"/mnt/root/etclist",
> but I was disappointed not to see anything at all.
> 
> So clearly everything made changed inside domainU cannot be seen from
> outside (domain0). Anybody can please explain to me why, and if
> possible, how could I watch data of domainU (in realtime) from
> domain0?
> 
> Thank you a lot,
> AQ
> 
> 
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> https://lists.sourceforge.net/lists/listinfo/xen-devel
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> So clearly everything made changed inside domainU cannot be seen from
> outside (domain0).
> Anybody can please explain to me why, and if 
> possible, how could I watch data of domainU (in realtime) from
> domain0?
The DomU will be caching the updates to its disk in memory before writing them 
to the disk.  Likewise, Dom0 will be caching data it''s read and
assuming it
stays the same (most filesystems assume only one writer, so dom0 thinks it 
knows when the disk is modified).

The result of this is that dom0 won''t immediately notice the changes
domU has
made.  When it does notice, it''ll likely take the form of an error
along the
lines of "Something changed on this filesystem that I wasn''t
expecting."

As Ian says, the GFS and OCFSv2 filesystems are explicitly aware of concurrent 
access so they''ll cope with this OK.

The easiest solution for your purposes would be to use NFS root in the domU 
(with the root FS exported from dom0) or to run an NFS server in the domU and 
export the root FS to dom0.

HTH,
Mark


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> OK, so I suppose that there is a (fixed) policy that determines when
> to "flush" the "dirty" data from the cache to the
virtual disk? If I
> dont like to use NFS or other special filesystems, can I expect that
> after certain "timeout", I will see the updated data in domainU
from
> domain0?
Dom0 will see the data but it''ll likely just get confused if
you''re using a
conventional filesystem.  Ext, Reiser, etc, all assume that they have total 
control of what gets written to the disk.  The filesystem driver in dom0 will 
therefore get very confused if it sees stuff changing on the disk that it 
didn''t write itself - it''ll just look like the hard disk
corrupting on its
own.

A network filesystem or a cluster filesystem is really the only way to do this 
sanely.

HTH,
Mark


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On Sun, 23 Jan 2005 14:22:59 -0000, Ian Pratt <m+Ian.Pratt@cl.cam.ac.uk>
wrote:
> > I wonder how I can see the (on-disk) data changing in a running
> > domainU. I tried to do it, but the data looking from outside remains
> > intact, no matter what I modified inside the domainU.
> 
> Use NFS root, otherwise the caching (in both kernels) will stop you
> seeing what''s going on.
> 
> An alternative is to use GFS or OCFS2.
> 
OK, so I suppose that there is a (fixed) policy that determines when
to "flush" the "dirty" data from the cache to the virtual
disk? If I
dont like to use NFS or other special filesystems, can I expect that
after certain "timeout", I will see the updated data in domainU from
domain0?

Best regards,
AQ


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