[libvirt PATCH v2 2/2] 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-inclu... Once this commit merges, the above wiki should point to this kbase document. Signed-off-by: Kashyap Chamarthy <kchamart(a)redhat.com&gt; --- docs/kbase/merging_disk_image_chains.rst | 200 +++++++++++++++++++++++ docs/kbase/meson.build | 1 + 2 files changed, 201 insertions(+) create mode 100644 docs/kbase/merging_disk_image_chains.rst diff --git a/docs/kbase/merging_disk_image_chains.rst b/docs/kbase/merging_disk_image_chains.rst new file mode 100644 index 0000000000..7635bd3eec --- /dev/null +++ b/docs/kbase/merging_disk_image_chains.rst @@ -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 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. diff --git a/docs/kbase/meson.build b/docs/kbase/meson.build index 51d4bc7b90..7631b47018 100644 --- a/docs/kbase/meson.build +++ b/docs/kbase/meson.build @@ -10,6 +10,7 @@ docs_kbase_files = [ 'locking-lockd', 'locking', 'locking-sanlock', + 'merging_disk_image_chains', 'migrationinternals', 'qemu-passthrough-security', 'rpm-deployment', -- 2.30.2