New tool: virt-resize

Virt-resize lets you resize existing virtual machines (not live however).

$ rm -f /tmp/centos.img
$ truncate -s 12G /tmp/centos.img
$ virt-resize --resize sda1=200% --resize sda2=11.2G \
    /dev/vg_trick/CentOS5x32 /tmp/centos.img
Summary of changes:
/dev/sda1: partition will be resized from 101.9M to 203.9M
/dev/sda2: partition will be resized from 9.9G to 11.2G
There is a surplus of 644971316 bytes (615.1M).
An extra partition will be created for the surplus.
Copying /dev/sda1 ... done
Copying /dev/sda2 ... done
$ rm -f /tmp/centos.img
$ truncate -s 12G /tmp/centos.img
$ virt-resize --resize sda1=200% --expand sda2 \
    /dev/vg_trick/CentOS5x32 /tmp/centos.img 
Summary of changes:
/dev/sda1: partition will be resized from 101.9M to 203.9M
/dev/sda2: partition will be resized from 9.9G to 11.8G
Copying /dev/sda1 ... done
Copying /dev/sda2 ... done

After some discussion on the list we decided to start with a simple / primitive tool and work upwards. So virt-resize as implemented now will not resize filesystems and PVs. You have to do that as a separate step after running the tool (either running pvresize/resize2fs in the guest or using guestfish for offline changes.

The new tool’s man page after the cut.

    virt-resize - Resize a virtual machine disk

     virt-resize [--options] indisk outdisk

    Virt-resize is a tool which can resize a virtual machine disk, making it
    larger or smaller overall, and resizing or deleting any partitions
    contained within.

    Virt-resize cannot resize disk images in-place. Virt-resize should not
    be used on live virtual machines - for consistent results, shut the
    virtual machine down before resizing it.

    If you are not familiar with the associated tools:
    virt-list-partitions(1), virt-list-filesystems(1) and virt-df(1), we
    recommend you go and read those manual pages first.

    This describes the common case where you want to expand an image to give
    your guest more space. Shrinking images is considerably more complicated

    1. Locate disk image
        Locate the disk image that you want to resize. It could be in a
        local file or device. If the guest is managed by libvirt, you can
        use "virsh dumpxml" like this to find the disk image name:

         # virsh dumpxml guestname | xpath /domain/devices/disk/source
         Found 1 nodes:
         -- NODE --
         <source dev="/dev/vg/lv_guest" />

    2. Look at current sizing
        Use virt-list-partitions(1) to display the current partitions and

         # virt-list-partitions -lh /dev/vg/lv_guest
         /dev/sda1 ext3 101.9M
         /dev/sda2 pv 7.9G

        (This example is a virtual machine with an 8 GB disk which we would
        like to expand up to 10 GB).

    3. Create destination disk
        Virt-resize cannot do in-place disk modifications. You have to have
        space to store the resized destination disk.

        To store the resized disk image in a file, create a file of a
        suitable size:

         # rm -f outdisk
         # truncate -s 10G outdisk

        Use lvcreate(1) to create a logical volume:

         # lvcreate -L 10G -n lv_name vg_name

        Or use virsh(1) vol-create-as to create a libvirt storage volume:

         # virsh pool-list
         # virsh vol-create-as poolname newvol 10G

    4. Resize
         virt-resize indisk outdisk

        This command just copies disk image "indisk" to disk image "outdisk"
        *without* resizing or changing any existing partitions. If "outdisk"
        is larger, then an extra, empty partition is created at the end of
        the disk covering the extra space. If "outdisk" is smaller, then it
        will give an error.

        To resize, you need to pass extra options (for the full list see the
        "OPTIONS" section below).

        "--expand" is the most useful option. It expands the named partition
        within the disk to fill any extra space:

         virt-resize --expand /dev/sda2 indisk outdisk

        (In this case, an extra partition is *not* created at the end of the
        disk, because there will be no unused space).

        If /dev/sda2 in the image contains a filesystem or LVM PV, then this
        content is not automatically resized. You can resize it afterwards
        either using guestfish(1) (offline) or using commands inside the
        guest (online resizing).

        "--resize" is the other commonly used option. The following would
        increase the size of /dev/sda1 by 200M, and expand /dev/sda2 to fill
        the rest of the available space:

         virt-resize --resize /dev/sda1=+200M --expand /dev/sda2 \
           indisk outdisk

        Other options are covered below.

    5. Test
        Thoroughly test the new disk image *before* discarding the old one.

        If you are using libvirt, edit the XML to point at the new disk:

         # virsh edit guestname

        Change <source ...>, see
        libvirt documentation

        Then start up the domain with the new, resized disk:

         # virsh start guestname

        and check that it still works.

        Note that to see the extra space in the guest, you may need to use
        guest commands to resize PVs, LVs and/or filesystems to fit the
        extra space available. Three common guest commands for doing this
        for Linux guests are pvresize(8), lvresize(8) and resize2fs(8). It
        is also possible to do this offline (eg. for scripting changes)
        using guestfish(1).

        Display help.

        Display version number and exit.

    --resize part=size
        Resize the named partition (expanding or shrinking it) so that it
        has the given size.

        "size" can be expressed as an absolute number followed by
        b/K/M/G/T/P/E to mean bytes, Kilobytes, Megabytes, Gigabytes,
        Terabytes, Petabytes or Exabytes; or as a percentage of the current
        size; or as a relative number or percentage. For example:

         --resize /dev/sda2=10G

         --resize /dev/sda4=90%

         --resize /dev/sda2=+1G

         --resize /dev/sda2=-200M

         --resize /dev/sda1=+128K

         --resize /dev/sda1=+10%

         --resize /dev/sda1=-10%

        You can increase the size of any partition.

        You can *only* decrease the size of partitions that contain
        filesystems or PVs which have already been shrunk. Virt-resize will
        check this has been done before proceeding, or else will print an
        error (see also "--resize-force").

        You can give this option multiple times.

    --resize-force part=size
        This is the same as "--resize" except that it will let you decrease
        the size of any partition. Generally this means you will lose any
        data which was at the end of the partition you shrink, but you may
        not care about that (eg. if shrinking an unused partition, or if you
        can easily recreate it such as a swap partition).

        See also the "--ignore" option.

    --expand part
        Expand the named partition so it uses up all extra space (space left
        over after any other resize changes that you request have been

        Any filesystem inside the partition is *not* expanded. You will need
        to expand the filesystem (or PV) to fit the extra space either using
        guestfish(1) (offline) or online guest tools.

        Note that you cannot use "--expand" and "--shrink" together.

    --shrink part
        Shrink the named partition until the overall disk image fits in the
        destination. The named partition must contain a filesystem or PV
        which has already been shrunk using another tool (eg. guestfish(1)
        or other online tools). Virt-resize will check this and give an
        error if it has not been done.

        The amount by which the overall disk must be shrunk (after carrying
        out all other operations requested by the user) is called the
        "deficit". For example, a straight copy (assume no other operations)
        from a 5GB disk image to a 4GB disk image results in a 1GB deficit.
        In this case, virt-resize would give an error unless the user
        specified a partition to shrink and that partition had more than a
        gigabyte of free space.

        Note that you cannot use "--expand" and "--shrink" together.

    --ignore part
        Ignore the named partition. Effectively this means the partition is
        allocated on the destination disk, but the content is not copied
        across from the source disk. The content of the partition will be
        blank (all zero bytes).

        You can give this option multiple times.

    --delete part
        Delete the named partition. It would be more accurate to describe
        this as "don't copy it over", since virt-resize doesn't do in-place
        changes and the original disk image is left intact.

        Note that when you delete a partition, then anything contained in
        the partition is also deleted. Furthermore, this causes any
        partitions that come after to be *renumbered*, which can easily make
        your guest unbootable.

        You can give this option multiple times.

        By default, virt-resize copies over some sectors at the start of the
        disk (up to the beginning of the first partition). Commonly these
        sectors contain the Master Boot Record (MBR) and the boot loader,
        and are required in order for the guest to boot correctly.

        If you specify this flag, then this initial copy is not done. You
        may need to reinstall the boot loader in this case.

        By default, virt-resize creates an extra partition if there is any
        extra, unused space after all resizing has happened. Use this option
        to prevent the extra partition from being created. If you do this
        then the extra space will be inaccessible until you run fdisk,
        parted, or some other partitioning tool in the guest.

        Note that if the surplus space is smaller than 10 MB, no extra
        partition will be created.

    -d | --debug
        Enable debugging messages.

    -n | --dryrun
        Print a summary of what would be done, but don't do anything.

    -q | --quiet
        Don't print the summary.

    virt-list-partitions(1), virt-list-filesystems(1), virt-df(1),
    guestfs(3), guestfish(1), lvm(8), pvresize(8), lvresize(8),
    resize2fs(8), virsh(1), Sys::Guestfs(3).

    Richard W.M. Jones

    Copyright (C) 2010 Red Hat Inc.

    This program is free software; you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by the
    Free Software Foundation; either version 2 of the License, or (at your
    option) any later version.

    This program is distributed in the hope that it will be useful, but
    WITHOUT ANY WARRANTY; without even the implied warranty of
    Public License for more details.

    You should have received a copy of the GNU General Public License along
    with this program; if not, write to the Free Software Foundation, Inc.,
    675 Mass Ave, Cambridge, MA 02139, USA.


1 Comment

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One response to “New tool: virt-resize

  1. Pingback: Quick quiz: Resizing VMs « Richard WM Jones

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