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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.
NB: I've intentionally left out the example for pull-based full backups.
I'll tackle it once QMP `x-blockdev-reopen` comes out of experimental
mode in upstream QEMU. Then pull-based can be described for both full
and and differntial backups.
Overall, future documents should cover:
- full backups using both push- and pull-mode
- differential backups using both push- and pull-mode
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
39954c76a6
commit
e4f3f9d063
@ -37,6 +37,9 @@ Usage
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`Launch security <launch_security_sev.html>`__
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Securely launching VMs with AMD SEV
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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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`KVM real time <kvm-realtime.html>`__
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Run real time workloads in guests on a KVM hypervisor
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186
docs/kbase/live_full_disk_backup.rst
Normal file
186
docs/kbase/live_full_disk_backup.rst
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@ -0,0 +1,186 @@
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===============================
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Efficient live full disk backup
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===============================
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.. contents::
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Overview
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========
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Live full disk backups are preferred in many scenarios, *despite* their
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space requirements. The following outlines an efficient method to do
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that using libvirt's APIs. This method involves concepts: the notion of
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`backing chains <https://libvirt.org/kbase/backing_chains.html>`_,
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`QCOW2 overlays
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<https://qemu.readthedocs.io/en/latest/interop/live-block-operations.html#disk-image-backing-chain-notation>`_,
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and a special operation called "active block-commit", which allows
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live-merging an overlay disk image into its backing file.
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Two kinds of backup: "push" and "pull"
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======================================
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QEMU and libvirt combine the concept of `bitmaps
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<https://qemu-project.gitlab.io/qemu/interop/bitmaps.html>`_ and network
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block device (NBD) to allow copying out modified data blocks. There are
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two approaches to it: In the first, "push mode", when a user requests
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for it, libvirt creates a full backup in an external location (i.e.
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libvirt "pushes" the data to the target).
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In the other, "pull mode", libvirt (in coordination with QEMU) exposes
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the data that needs to be written out and allows a third-party tool to
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copy them out reliably (i.e. the data is being "pulled" from libvirt).
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The pull-based backup provides more flexibility by letting an external
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tool fetch the modified bits as it sees fit, rather than waiting on
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libvirt to push out a full backup to a target location.
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The push- and pull-mode techniques also apply for differential backups
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(it also includes incremental backups), which track what has changed
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since *any* given backup.
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This document covers only the full backups using the "push" mode.
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Full disk backup using "push" mode
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==================================
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The below approach uses the modern backup API, virDomainBackupBegin().
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This requires libvirt-7.2.0 and QEMU-4.2, or higher versions.
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#. Start the guest::
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$> virsh start vm1
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Domain 'vm1' started
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#. Enumerate the disk(s) in use::
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$> virsh domblklist vm1
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Target Source
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--------------------------------------
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vda /var/lib/libvirt/images/vm1.qcow2
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#. Begin the backup::
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$> virsh backup-begin vm1
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Backup started
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#. Check the job status ("None" means the job has likely completed)::
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$> virsh domjobinfo vm1
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Job type: None
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#. Check the completed job status::
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$> virsh domjobinfo vm1 --completed
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Job type: Completed
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Operation: Backup
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Time elapsed: 183 ms
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File processed: 39.250 MiB
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File remaining: 0.000 B
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File total: 39.250 MiB
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#. Now we see the copy of the backup::
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$> ls -lash /var/lib/libvirt/images/vm1.qcow2*
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15M -rw-r--r--. 1 qemu qemu 15M May 10 12:22 vm1.qcow2
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21M -rw-------. 1 root root 21M May 10 12:23 vm1.qcow2.1620642185
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Full backup with older libvirt versions
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=======================================
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This is the alternative in case you cannot use libvirt-7.2.0 and
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QEMU-4.2 for some reason. But this assumes you're using *at least* QEMU
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2.1 and libvirt-1.2.9.
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This backup approach is slightly more involved, and predates the
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virDomainBackupBegin() API: Assuming a guest with a single disk image,
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create a temporary live QCOW2 overlay (commonly called as "external
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snapshot") to track the live guest writes. Then backup the original
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disk image while the guest (live QEMU) keeps writing to the temporary
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overlay. Finally, perform the "active block-commit" opertion to
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live-merge the temporary overlay disk contents into the original image —
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i.e. the backing file — and "pivot" the live QEMU process to point to
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it.
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#. Start with a guest with a single disk image, ``base.raw``, which is
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where the live QEMU is pointing at, and recording the guest writes::
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base.raw (live QEMU)
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#. List the current block device(s) 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 live "external disk snapshot" (or more correctly, "an
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overlay")::
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$ virsh snapshot-create-as --domain vm1 overlay1 \
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--diskspec vda,file=/var/lib/libvirt/images/overlay1.qcow2 \
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--disk-only
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The disk image chain looks as follows::
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base.raw <-- overlay1.qcow2 (live QEMU)
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.. note::
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Above, if you have QEMU guest agent installed in your virtual
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machine, use the ``--quiesce`` option with ``virsh
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snapshot-create-as [...]`` to ensure you have a consistent disk
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state.
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Optionally, you can also supply the ``--no-metadata`` option to
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``virsh snapshot-create-as`` to tell libvirt not track the snapshot
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metadata. Otherwise, when you decide to merge snapshot overlays,
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you have to explicitly clean the libvirt metadata using ``virsh
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snapshot-delete vm1 --metadata [name|--current]``.
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#. Now, take a backup the orignal image, ``base.raw``, to a different
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location using ``cp`` or ``rsync``::
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$ cp /var/lib/libvirt/images/base.raw
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/export/backups/copy1_base.raw
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# Or:
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$ rsync -avhW --progress /var/lib/libvirt/images/base.raw \
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/export/backups/copy1_base.raw
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#. Enumerate the current block device(s) in use, again. Notice that the
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current disk image in use is the above-created overlay,
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``overlay1.qcow2``::
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$ virsh domblklist vm1
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Target Source
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------------------------------------------------
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vda vda,file=/var/lib/libvirt/images/overlay1.qcow2
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#. Once the backup of the original image completes, now perform the
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"active block-commit" to live-merge the contents of
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``overlay1.qcow2`` into ``base.raw`` *and* pivot the live QEMU back
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to the original::
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$ virsh blockcommit vm1 vda --active --verbose --pivot
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#. After the above operation completes, again list the current block
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device(s) in use. And notice that the live QEMU is now writing to
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the original base image::
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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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The final updated disk image "chain" will be a single consolidated
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disk::
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[base.raw] (live QEMU)
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Now you can safely **discard the overlay image**, ``overlay1.qcow2`` —
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it is no longer valid; and its contents are now fully merged into the
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base image.
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@ -6,6 +6,7 @@ docs_kbase_files = [
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'index',
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'kvm-realtime',
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'launch_security_sev',
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'live_full_disk_backup',
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'locking-lockd',
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'locking',
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'locking-sanlock',
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