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