Re: [libvirt PATCH 1/2] docs: kbase: Add a doc on live full disk backup

This is a rewrite of: https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit 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/index.rst | 3 + docs/kbase/live_full_disk_backup.rst | 111 +++++++++++++++++++++++++++ 2 files changed, 114 insertions(+) create mode 100644 docs/kbase/live_full_disk_backup.rst

diff --git a/docs/kbase/index.rst b/docs/kbase/index.rst index 532804fe05..5ee3b5fce8 100644 --- a/docs/kbase/index.rst +++ b/docs/kbase/index.rst @@ -12,6 +12,9 @@ Usage Explanation of how disk backing chain specification impacts libvirt's behaviour and basic troubleshooting steps of disk problems. +`Live full disk backup <live_full_disk_backup.html>`__ + A walkthrough of how to take effective live full disk backups. + `Virtio-FS <virtiofs.html>`__ Share a filesystem between the guest and the host diff --git a/docs/kbase/live_full_disk_backup.rst b/docs/kbase/live_full_disk_backup.rst new file mode 100644 index 0000000000..f4ef7f2cc7 --- /dev/null +++ b/docs/kbase/live_full_disk_backup.rst @@ -0,0 +1,111 @@ +Efficient live full disk backup +===============================

+ +Overview +--------

+ +Live full disk backups are preferred in many scenarios, *despite* their +space requirements. The following outlines an efficient method to do +that using libvirt's APIs. This method involves concepts: the notion of +`backing chains <https://libvirt.org/kbase/backing_chains.html>`_, +`QCOW2 overlays +<https://qemu.readthedocs.io/en/latest/interop/live-block-operations.html#disk-image-backing-chain-notation>`_, +and a special operation called "active block-commit", which allows +live-merging an overlay disk image into its backing file. + +At a high-level, this is the approach to take a live ful disk backup: +Assuming a guest with a single disk image, create a temporary live QCOW2 +overlay (commonly called as "external snapshot") to track the live guest +writes. Then backup the original disk image while the guest (live QEMU) +keeps writing to the temporary overlay. Finally, perform the "active +block-commit" opertion to live-merge the temporary overlay disk contents +into the original image — i.e. the backing file — and "pivot" the live +QEMU process to point to it. + +The below assumes you're using *at least* these versions (preferably +much newer): QEMU 2.1 and libvirt-1.2.9. + +Detailed procedure +------------------ + +1. Start with a guest with a single disk image, ``base.raw``, which is

+ where the live QEMU is pointing at, and recording the guest writes:: + + base.raw (live QEMU) + +2. List the current block device(s) in use:: + + $ virsh domblklist vm1 + Target Source + ------------------------------------------------ + vda /var/lib/libvirt/images/base.raw + +3. Create the live "external disk snapshot" (or more correctly, "an + overlay"):: + + $ virsh snapshot-create-as --domain vm1 overlay1 \ + --diskspec vda,file=/var/lib/libvirt/images/overlay1.qcow2 \ + --disk-only + + The disk image chain looks as follows:: + + base.raw <-- overlay1.qcow2 (live QEMU) + +.. note::

+ Above, if you have QEMU guest agent installed in your virtual + machine, use the ``--quiesce`` option with ``virsh + snapshot-create-as [...]`` to ensure you have a consistent disk + state. + + Optionally, you can also supply the ``--no-metadata`` option to + ``virsh snapshot-create-as`` to tell libvirt not track the snapshot + metadata. Otherwise, when you decide to merge snapshot overlays, + you have to explicitly clean the libvirt metadata using ``virsh + snapshot-delete vm1 --metadata [name|--current]``. + +4. Now, take a backup the orignal image, ``base.raw``, to a different + location using ``cp`` or ``rsync``:: + + $ cp /var/lib/libvirt/images/base.raw + /export/backups/copy1_base.raw + + # Or: + + $ rsync -avhW --progress /var/lib/libvirt/images/base.raw \ + /export/backups/copy1_base.raw + +5. Enumerate the current block device(s) in use, again. Notice that the + current disk image in use is the above-created overlay, + ``overlay1.qcow2``:: + + $ virsh domblklist vm1 + Target Source + ------------------------------------------------ + vda vda,file=/var/lib/libvirt/images/overlay1.qcow2 + +6. Once the backup of the original image completes, now perform the + "active block-commit" to live-merge the contents of + ``overlay1.qcow2`` into ``base.raw`` *and* pivot the live QEMU back + to the original:: + + $ virsh blockcommit vm1 vda --active --verbose --pivot + +7. After the above operation completes, again list the current block + device(s) in use. And notice that the live QEMU is now writing to + the original base image:: + + $ virsh domblklist vm1 + Target Source + ------------------------------------------------ + vda /var/lib/libvirt/images/base.raw + + +The final updated disk image "chain" will be a single consolidated +disk:: + + [base.raw] (live QEMU) + + +Now you can safely **discard the overlay image**, ``overlay1.qcow2`` — +it is no longer valid; and its contents are now fully merged into the +base image.