persistent or "steady-state" style disks

persistent or "steady-state" style disks

[Matthew Hook]

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Hi,

I have a requirement (and a limited timeframe) to make a Windows 7 VM
domain roll back to a fixed state on restart, shutdown etc.
Something like a Kiosk Mode.  I think this is a very needed part of the
core of xen and this functionality does
not appear to be part of xen already.  So I've been trying various ways to
make this work.

Has anyone already implemented something like this successfully before?

What I've been experimenting unsuccessfully with currently is the following:

1) Add hooks into the blktap2 code to roll back a VHD (currently based on
the name ending in -persist.vhd).
However, it's not working out as it should.

2) Second attempt was to write an external script and call it periodically.
The idea was to call vhd-snapshot on the parent disk to a new disk (temp
name).  Then do an atomic rename on existing vhd
and unlink the old vhd.  When it restarts you would think it would open up
the new snapshot.  Unfortunately, a reboot seems to
detect that the file is different and so it's not re-started as desired but
instead the domain is shutdown.

I don't like either of the above methods.  They are hacks in my opinion but
I'm not currently familiar enough with xen internals.
Can someone propose a method and/or suggest where I could hook into the xen
tools to implement it?

Regards,
Matthew

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Re: persistent or "steady-state" style disks

[Dave Scott]

Hi,

Matthew Hook wrote:
> I have a requirement (and a limited timeframe) to make a Windows 7 VM domain roll back to a fixed state on restart, shutdown etc.
> Something like a Kiosk Mode.  I think this is a very needed part of the core of xen and this functionality does 
> not appear to be part of xen already.  So I've been trying various ways to make this work.

XCP has a feature which might do what you need.

If you are using .vhd file based storage (SR types 'ext' or 'nfs') then you can do the following: (for a VM called "myvm")

# 1. Ensure the .vhd chain has length 2 (a current restriction)
xe vm-copy vm=myvm new-name-label=nonpersistent.parent
xe vm-clone vm=nonpersistent.parent new-name-label=nonpersistent

# 2. Set the VDI on-boot=reset
xe vm-disk-list vm=nonpersistent
 ... note the VDI uuid of the disk you want to reset ...
xe vdi-param-set uuid=... on-boot=reset

# 3. Start the VM
xe vm-start vm=nonpersistent

HTH,
Dave

Re: persistent or "steady-state" style disks

[Matthew Hook]

Thanks for that information.

It isn't obvious in the XCP code how this feature is being done and
I'm not wanting to use XCP at this point as it's behind without the
latest hypervisor included.

I actually created a patch where you can can set the temporary flag on
the VHD features footer through vhd-util.
Traditionally in the MS world is used to omdocated that the VHD can be
removed on close.
I'm using it for reset-on-boot where if the flag is set during
_vhd_open it tests the flag and if set will regenerate the snapshot
from the parent VHD.

If anyone is interested in this patch right now I can send it through
as is.  However, I'll submit it properly once I've done a bit
more testing.

Comments are welcome.

Matt

On 17 February 2012 13:21, Dave Scott <Dave.Scott@eu.citrix.com> wrote:
>
> Hi,
>
> Matthew Hook wrote:
> > I have a requirement (and a limited timeframe) to make a Windows 7 VM domain roll back to a fixed state on restart, shutdown etc.
> > Something like a Kiosk Mode.  I think this is a very needed part of the core of xen and this functionality does
> > not appear to be part of xen already.  So I've been trying various ways to make this work.
>
> XCP has a feature which might do what you need.
>
> If you are using .vhd file based storage (SR types 'ext' or 'nfs') then you can do the following: (for a VM called "myvm")
>
> # 1. Ensure the .vhd chain has length 2 (a current restriction)
> xe vm-copy vm=myvm new-name-label=nonpersistent.parent
> xe vm-clone vm=nonpersistent.parent new-name-label=nonpersistent
>
> # 2. Set the VDI on-boot=reset
> xe vm-disk-list vm=nonpersistent
>  ... note the VDI uuid of the disk you want to reset ...
> xe vdi-param-set uuid=... on-boot=reset
>
> # 3. Start the VM
> xe vm-start vm=nonpersistent
>
> HTH,
> Dave