FAQ
I am investigating the procedure to follow when moving a
KVM guest instance from one host to another where the
guest uses LVM as its storage. As a preliminary cut I
have cobbled the following together from various sources
located through Google searches:

1. Log in to vmhost_old

2. Shutdown guest

3. Create an LVM snapshot of the guest volume
3.a. lvcreate -s -L 300m -n LVM_guest_snapshot guest_lv

4. Convert snapshot to a file image
4.a dd if=/dev/vhost/LVM_guest_snapshot
of=/tmp/fs/LVM_guest_snapshot.img bs@96

5. Remove the original snapshot
5.a lvremove /dev/vhost/LVM_guest_snapshot

6. Move the snapshot image file to the new KVM host. Note
that for large vm guest images tape transport is probably
better than network transfers.

7. Log in to vhost_new

8. Create a new lv on the new vhost
8.a lvcreate -n guest_name_lv -L 60G vhost_new_vg

9. Copy transferred image to new LV
9.a dd if=tmp/LVM_guest_snapshot.img
of=/dev/vhost_new/guest_name_lv bs@96

10. Copy guest xml file from vhost_old via sftp
10.a. get /etc/libvirt/qemu/guest_name.xml \
/etc/libvirt/qemu/guest_name.xml

11. Start new guest?


Is there anything obviously wrong or omitted from this?


--
*** E-Mail is NOT a SECURE channel ***
James B. Byrne mailto:ByrneJB at Harte-Lyne.ca
Harte & Lyne Limited http://www.harte-lyne.ca
9 Brockley Drive vox: +1 905 561 1241
Hamilton, Ontario fax: +1 905 561 0757
Canada L8E 3C3

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  • Ed Heron at Nov 29, 2011 at 5:11 pm

    On Tue, 2011-11-29 at 15:51 -0500, James B. Byrne wrote:
    I am investigating the procedure to follow when moving a
    KVM guest instance from one host to another where the
    guest uses LVM as its storage. As a preliminary cut I
    have cobbled the following together from various sources
    located through Google searches:
    ...
    If you are moving the virtual disk, you can't run the virtual machine while you are moving it. Therefore, taking a snapshot (and removing it later) is superfluous.

    I use md5sum to give myself some extra re-assurance that a copy that big is accurate.

    Are the virtualization servers at the same location?

    How often do you move virtual machines?

    If your virtualization servers are in the same location, you might want to look into setting up a LAN for transferring virtual machine images. It would only take an extra network card in each virtualization server and a Gb Ethernet switch (or better).

    If you are interested in virtual disk replication, you might want to look at DRBD. It adds an extra layer to your storage, but allows for hot replication of disk space which would allow you to start a virtual machine on any virtualization server.
  • James B. Byrne at Nov 30, 2011 at 11:23 am

    On Tue, November 29, 2011 17:11:01, Ed Heron Ed at Heron-ent.com wrote:

    If you are moving the virtual disk, you can't run
    the virtual machine while you are moving it.
    Therefore, taking a snapshot (and removing it later)
    is superfluous.
    Point taken.
    I use md5sum to give myself some extra re-assurance
    that a copy that big is accurate.
    Good idea.
    Are the virtualization servers at the same location?
    At the moment. It is planned that there will be two vhosts
    at each of two different locations, one backing up the
    other.
    How often do you move virtual machines?
    I have not moved any as of yet. I am investigating how to
    do this as part of setting up our first virtual server
    host. I am discovering what is possible and the
    techniques used. Moving vm guests is not being considered
    for load balancing but is under consideration for
    providing warm spares for use in a DR situation.
    If your virtualization servers are in the same
    location, you might want to look into setting up a
    LAN for transferring virtual machine images. It
    would only take an extra network card in each
    virtualization server and a Gb Ethernet switch
    (or better).
    Our vhosts all (will) have two nics, one for public IPs
    and one for private IPs. The private IPs are used for
    vhost management. The experimental vhost is so equipped.
    If you are interested in virtual disk replication,
    you might want to look at DRBD. It adds an extra layer
    to your storage, but allows for hot replication of
    disk space which would allow you to start a virtual
    machine on any virtualization server.
    This is a useful suggestion. However, I find that the
    availability of a DRDB package on CentOS-6 is presently an
    issue. And, our vm project has already expanded into
    multiple areas with commensurate delays. I think that I
    will leave DRDB for a later time.

    So, what I now have for a proposed procedure is:

    1. Log in to vmhost_old

    2. Shutdown guest

    3. Convert guest lvs to file images for each lv
    3.a dd if=/dev/vhost/lv_vhost_guest
    of=/tmp/fs/lv_vhost_guest.img bs@96

    4. Create MD5 hash for each image file
    4.a md5sum /tmp/fs/lv_vhost_guest.img > \
    /tmp/fs/lv_vhost_guest.md5

    5. Move the guest image file(s) and the checksum file to
    the new vhost; preserving both the directory structure and
    file names or subsequently modifying the file names in the
    md5 checksum file. Note that for large vm guest images
    tape transport is probably better than network transfers.

    6. Log in to vhost_new

    7. Verify md5 checksum(s) on transferred file(s).
    7.a md5sum -c /tmp/fs/lv_vhost_guest.md5

    8. Create a new lv(s) on the new vhost. Ensure sufficient
    size.
    8.a lvcreate -n lv_guest_name -L 60G vhost_new_vg

    9. Copy transferred image to new LV
    9.a dd if=/tmp/fs/lv_vhost_guest.img
    of=/dev/vhost_new/lv_guest_name bs@96

    10. Copy guest xml file from vhost_old via sftp
    10.a. get /etc/libvirt/qemu/guest_name.xml \
    /etc/libvirt/qemu/guest_name.xml

    11. Start new guest?


    Thanks,


    --
    *** E-Mail is NOT a SECURE channel ***
    James B. Byrne mailto:ByrneJB at Harte-Lyne.ca
    Harte & Lyne Limited http://www.harte-lyne.ca
    9 Brockley Drive vox: +1 905 561 1241
    Hamilton, Ontario fax: +1 905 561 0757
    Canada L8E 3C3

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postedNov 29, '11 at 3:51p
activeNov 30, '11 at 11:23a
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James B. Byrne: 2 posts Ed Heron: 1 post

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