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Re: [Qemu-discuss] Multiple snapshots with one guest user

From: Jakob Bohm
Subject: Re: [Qemu-discuss] Multiple snapshots with one guest user
Date: Mon, 16 Jan 2017 16:36:20 +0100
User-agent: Mozilla/5.0 (Windows NT 6.3; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.6.0

On 16/01/2017 16:06, Mahmood Naderan wrote:
This is what I want to do via the Qemu: I have installed a windows guest with two drives (c and d). I will create an admin user and install some basic softwares. Then I will create a snapshot for each user. Each user is then able to boot his/her snapshot (saved in his home folder) and install/run what ever he wants.

So is that possible?

I want to be sure that if linux_user_1 install a software on his snapshot, then linux_user_2 is not able to see that. Note that both access their windows session via one windows_user which I have created before snapshots.

Any comment on that?

You would need to create the "common" hard disk image as a file
that the (host) users can read but not write.

Next create each users "snapshot" as a new .qcow2 disk image file
referencing the common disk image as its "base file".

Once any user makes changes in their snapshot .qcow2 file, you
cannot make new changes in the "common" hard disk image without
causing those users to see a corrupted windows disk, where disk
sectors changed by the "common" image but not the user image
contain the change made in "common", sectors changed by the user
image but not the "common" image contain the change made by the
user and sectors changed by both user and "common" image contain
the user change but not the "common" change.

So if you later want to change something in the "common" image,
you should change the filename of the "common" image and require
users to recreate their changes in a new snapshot image.

Thus except that you will be using less real host disk space, it
will be as if each user is given a copy of the "common" image.

To make things easier for users, you might create a virtual
machine with two virtual hard disks (not two partitions), where
the second hard disk is blank in its disk file in the "common"
image, and not changed when changing the "common" image, thus
allowing each user to keep their changes that are on the second
disk, but still having to recreate their changes (such as
software installations) on the first disk.


Jakob Bohm, CIO, Partner, WiseMo A/S.  https://www.wisemo.com
Transformervej 29, 2860 Søborg, Denmark.  Direct +45 31 13 16 10
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