mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-07-06 09:55:46 +00:00
docs: kbase: Add a doc on merging disk image chains
This is a rewrite of:
https://wiki.libvirt.org/page/Live-merge-an-entire-disk-image-chain-including-current-active-disk
Once this commit merges, the above wiki should point to this kbase
document.
Signed-off-by: Kashyap Chamarthy <kchamart@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
This commit is contained in:
Kashyap Chamarthy
Michal Privoznik
parent
e4f3f9d063
commit
f98759170e
@ -40,6 +40,9 @@ Usage
|
||||
`Live full disk backup <live_full_disk_backup.html>`__
|
||||
A walkthrough of how to take effective live full disk backups.
|
||||
|
||||
`Merging disk image chains <merging_disk_image_chains.html>`__
|
||||
Ways to reduce or consolidate disk image chains.
|
||||
|
||||
`KVM real time <kvm-realtime.html>`__
|
||||
Run real time workloads in guests on a KVM hypervisor
|
||||
|
||||
|
||||
200
docs/kbase/merging_disk_image_chains.rst
Normal file
200
docs/kbase/merging_disk_image_chains.rst
Normal file
@ -0,0 +1,200 @@
|
||||
===============================
|
||||
Merging disk image image chains
|
||||
===============================
|
||||
|
||||
.. contents::
|
||||
|
||||
Context
|
||||
=======
|
||||
|
||||
Sometimes a `disk image chain
|
||||
<https://libvirt.org/kbase/backing_chains.html>`_ can get long and
|
||||
cumbersome. For the purpose of illustration, consider this smaller disk
|
||||
image chain::
|
||||
|
||||
base.raw <-- a.qcow2 <-- b.qcow2 <-- c.qcow2 (live QEMU)
|
||||
|
||||
You may want to reduce the backing chain length, or consolidate *all*
|
||||
the disk images in the chain into a single image. But you want to
|
||||
accomplish this *without* incurring guest down time. Here's how to go
|
||||
about it.
|
||||
|
||||
The same principles used in the `live full disk backup
|
||||
<https://libvirt.org/kbase/live_full_disk_backup.html>` document are
|
||||
used here too.
|
||||
|
||||
Reducing the disk image chain length
|
||||
====================================
|
||||
|
||||
Starting the earlier image chain::
|
||||
|
||||
base.raw <-- a.qcow2 <-- b.qcow2 <-- c.qcow2 (live QEMU)
|
||||
|
||||
Reduce the length of the chain by two images, with the resulting chain
|
||||
being::
|
||||
|
||||
base.raw <-- c.qcow2 (live QEMU)
|
||||
|
||||
Where the ``base.raw`` contains the contents of ``a.qcow2`` and
|
||||
``b.qcow2``.
|
||||
|
||||
#. Start by listing the current active disk image in use::
|
||||
|
||||
$ virsh domblklist vm1
|
||||
Target Source
|
||||
------------------------------------------------
|
||||
vda /var/lib/libvirt/images/base.raw
|
||||
|
||||
#. Create the image chain by creating three QCOW2 overlays (or "external
|
||||
snapshots") on top of each other, while adding some differentiating
|
||||
content in each image::
|
||||
|
||||
$ virsh snapshot-create-as --domain vm1 snap1 \
|
||||
--diskspec vda,file=/var/lib/libvirt/images/a.qcow2 \
|
||||
--disk-only --no-metadata
|
||||
|
||||
# <Add a file in the guest>
|
||||
|
||||
$ virsh snapshot-create-as --domain vm1 snap2 \
|
||||
--diskspec vda,file=/var/lib/libvirt/images/b.qcow2 \
|
||||
--disk-only --no-metadata
|
||||
|
||||
# <Add another file in the guest>
|
||||
|
||||
$ virsh snapshot-create-as --domain vm1 snap3 \
|
||||
--diskspec vda,file=/var/lib/libvirt/images/c.qcow2 \
|
||||
--disk-only --no-metadata
|
||||
|
||||
#. Enumerate the backing file chain (here the ``force-share`` option
|
||||
simply allows ``qemu-img`` to safely query the disk image status
|
||||
while it is active) ::
|
||||
|
||||
$ qemu-img info --force-share --backing-chain /var/lib/libvirt/images/cur.qcow2
|
||||
[...]
|
||||
|
||||
#. Again, list the current active disk image in use::
|
||||
|
||||
$ virsh domblklist vm1
|
||||
Target Source
|
||||
------------------------------------------------
|
||||
vda /var/lib/libvirt/images/c.qcow2
|
||||
|
||||
#. Perform the "block-commit" by specify the "base" and "top" images,
|
||||
i.e. merge the contents of ``b.qcow2`` *and* ``a.qcow2`` into
|
||||
``base.raw``, *and* ::
|
||||
|
||||
$ virsh blockcommit vm1 vda \
|
||||
--base=/var/lib/libvirt/images/base.raw
|
||||
--top=/var/lib/libvirt/images/b.qcow2
|
||||
|
||||
A note on accessing 'base' and 'top' images
|
||||
-------------------------------------------
|
||||
|
||||
Specifying file paths, as above, make sense when your disks are in the
|
||||
local filesystem. However, when using more complicated setups such as
|
||||
network block device (NBD) disks, there are no file paths. Hhere is
|
||||
where accessing the disk by its index number comes into picture.
|
||||
|
||||
As an example, the below is the ``<disk>`` element of the guest XML for
|
||||
with the original disk image chain of four images::
|
||||
|
||||
...
|
||||
<disk type='file' device='disk'>
|
||||
<driver name='qemu' type='qcow2'/>
|
||||
<source file='/var/lib/libvirt/images/c.qcow2' index='4'/>
|
||||
<backingStore type='file' index='3'>
|
||||
<format type='qcow2'/>
|
||||
<source file='/var/lib/libvirt/images/b.qcow2'/>
|
||||
<backingStore type='file' index='2'>
|
||||
<format type='qcow2'/>
|
||||
<source file='/var/lib/libvirt/images/a.qcow2'/>
|
||||
<backingStore type='file' index='1'>
|
||||
<format type='raw'/>
|
||||
<source file='/var/lib/libvirt/images/base.raw'/>
|
||||
<backingStore/>
|
||||
</backingStore>
|
||||
</backingStore>
|
||||
</backingStore>
|
||||
<target dev='vda' bus='virtio'/>
|
||||
<alias name='virtio-disk0'/>
|
||||
<address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
|
||||
</disk>
|
||||
...
|
||||
|
||||
And we can merge the images ``a.qcow2`` and ``b.qcow2`` into
|
||||
``base.qcow2`` using the index numbers 1 (for ``base.qcow2``) and ::
|
||||
|
||||
$> virsh blockcommit vm1 vda --base='vda[1]' --top='vda[3]'
|
||||
|
||||
Note that the shell quoting is necessary here, since unquoted ``[1]``
|
||||
would do an unexpected shell "globbing" (i.e. file name expansion) if
|
||||
you have a file '1' in the current directory
|
||||
|
||||
Accessing the disk images via their index numbers is more useful when
|
||||
you're using blockCommit() API programmatically.
|
||||
|
||||
|
||||
Consolidating an entire disk image chain into a single image
|
||||
============================================================
|
||||
|
||||
Again, starting the original image chain::
|
||||
|
||||
base.raw <-- a.qcow2 <-- b.qcow2 <-- c.qcow2 (live QEMU)
|
||||
|
||||
Reduce the length of the chain by two images, with the resulting chain
|
||||
being::
|
||||
|
||||
base.raw (live QEMU)
|
||||
|
||||
Where the ``base.raw`` contains the contents of ``a.qcow2``, ``b.qcow2``
|
||||
and ``c.qcow2``; *and* the live QEMU is piovoted to point to the
|
||||
``base.raw``.
|
||||
|
||||
|
||||
#. Use the same procedure discussed earlier to create the disk image
|
||||
chain.
|
||||
|
||||
|
||||
#. Now perform the "active block-commit" operation::
|
||||
|
||||
$ virsh blockcommit vm1 vda --verbose --pivot --active
|
||||
Block Commit: [100 %]
|
||||
Successfully pivoted
|
||||
|
||||
Notes:
|
||||
|
||||
- ``--active``: It performs a two-stage operation: first, the contents
|
||||
from top images (``a.qcow2``, ``b.qcow2``, and ``c.qcow2``) are
|
||||
committed into the base image; and in the second stage, the
|
||||
"block-commit" operation remains awake to synchronize any further
|
||||
changes from top images into base. Here the user can take two
|
||||
actions: cancel the job, or pivot the job, i.e. adjust the base
|
||||
image as the current active image.
|
||||
|
||||
- ``--pivot``: Once data is committed from sn1, sn2 and current into
|
||||
base, it pivots the live QEMU to use base as the active image.
|
||||
|
||||
- ``--verbose``: It shows the progress of block operation.
|
||||
|
||||
|
||||
#. Again, check the current active block device in use::
|
||||
|
||||
$ virsh domblklist vm1
|
||||
Target Source
|
||||
------------------------------------------------
|
||||
vda /var/lib/libvirt/images/base.raw
|
||||
|
||||
|
||||
#. Enumerate the backing file chain::
|
||||
|
||||
$ qemu-img info --backing-chain /var/lib/libvirt/images/base.raw
|
||||
[...]
|
||||
|
||||
And the final resulting disk image "chain" will be a single,
|
||||
consolidated disk image::
|
||||
|
||||
[base] (live QEMU)
|
||||
|
||||
It is worth bearing in mind that once the above pivot completes, *all*
|
||||
three overlay files — ``a.qcow2``, ``b.qcow2``, and ``c.qcow2`` — are no
|
||||
longer valid, and can be safely discarded.
|
||||
@ -10,6 +10,7 @@ docs_kbase_files = [
|
||||
'locking-lockd',
|
||||
'locking',
|
||||
'locking-sanlock',
|
||||
'merging_disk_image_chains',
|
||||
'migrationinternals',
|
||||
'qemu-passthrough-security',
|
||||
'rpm-deployment',
|
||||
|
||||
Loading…
Reference in New Issue
Block a user