mirror of https://gitee.com/openkylin/libvirt.git
kbase: Add document outlining internals of incremental backup in qemu
Outline the basics and how to integrate with externally created overlays. Other topics will continue later. Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Eric Blake <eblake@redhat.com>
This commit is contained in:
Peter Krempa
parent
5f2fdcc867
commit
da5e5a1e15
|
|
@ -42,6 +42,9 @@
|
|||
|
||||
<dt><a href="kbase/kvm-realtime.html">KVM real time</a></dt>
|
||||
<dd>Run real time workloads in guests on a KVM hypervisor</dd>
|
||||
|
||||
<dt><a href="kbase/incrementalbackupinternals.html">Incremental backup internals</a></dt>
|
||||
<dd>Incremental backup implementation details relevant for users</dd>
|
||||
</dl>
|
||||
</div>
|
||||
|
||||
|
|
|
|||
|
|
@ -0,0 +1,217 @@
|
|||
================================================
|
||||
Internals of incremental backup handling in qemu
|
||||
================================================
|
||||
|
||||
.. contents::
|
||||
|
||||
Libvirt's implementation of incremental backups in the ``qemu`` driver uses
|
||||
qemu's ``block-dirty-bitmaps`` under the hood to track the guest visible disk
|
||||
state changes corresponding to the points in time described by a libvirt
|
||||
checkpoint.
|
||||
|
||||
There are some semantica implications with how libvirt creates and manages the
|
||||
bitmaps which de-facto become API as they are written into the disk images, and
|
||||
this document will try to summarize them.
|
||||
|
||||
Glossary
|
||||
========
|
||||
|
||||
See the knowledge base article on
|
||||
`domain state capture <https://libvirt.org/kbase/domainstatecapture.html>`_ for
|
||||
a deeper explanation of some of the concepts.
|
||||
|
||||
Checkpoint
|
||||
|
||||
A libvirt object which represents a named point in time of the life of the
|
||||
vm where libvirt tracks writes the VM has done, thereby allowing a backup of
|
||||
only the blocks which changed. Note that state of the VM memory is _not_
|
||||
captured.
|
||||
|
||||
A checkpoint can be created either explicitly via the corresponding API
|
||||
(although this isn't very useful on its own), or simultaneously with an
|
||||
incremental or full backup of the VM using the ``virDomainBackupBegin`` API
|
||||
which allows a next backup to only copy the differences.
|
||||
|
||||
Backup
|
||||
|
||||
A copy of either all blocks of selected disks (full backup) or blocks changed
|
||||
since a checkpoint (incremental backup) at the time the backup job was
|
||||
started. (Blocks modified while the backup job is running are not part of the
|
||||
backup!)
|
||||
|
||||
Snapshot
|
||||
|
||||
Similarly to a checkpoint it's a point in time in the lifecycle of the VM
|
||||
but the state of the VM including memory is captured at that point allowing
|
||||
returning to the state later.
|
||||
|
||||
Blockjob
|
||||
|
||||
A long running job which modifies the shape and/or location of the disk
|
||||
backing chain (images storing the disk contents). Libvirt supports
|
||||
``block pull`` where data is moved up the chain towards the active layer,
|
||||
``block commit`` where data is moved down the chain towards the base/oldest
|
||||
image. These blockjobs always remove images from the backing chain. Lastly
|
||||
``block copy`` where image is moved to a different location (and possibly
|
||||
collapsed moving all of the data into the new location into the one image).
|
||||
|
||||
block-dirty-bitmap (bitmap)
|
||||
|
||||
A data structure in qemu tracking which blocks were written by the guest
|
||||
OS since the bitmap was created.
|
||||
|
||||
Relationships of bitmaps, checkpoints and VM disks
|
||||
==================================================
|
||||
|
||||
When a checkpoint is created libvirt creates a block-dirty-bitmap for every
|
||||
configured VM disk named the same way as the chcheckpoint. The bitmap is
|
||||
actively recording which blocks were changed by the guest OS from that point on.
|
||||
Other bitmaps are not impacted by any way as they are self-contained:
|
||||
|
||||
::
|
||||
|
||||
+----------------+ +----------------+
|
||||
| disk: vda | | disk: vdb |
|
||||
+--------+-------+ +--------+-------+
|
||||
| |
|
||||
+--------v-------+ +--------v-------+
|
||||
| vda-1.qcow2 | | vdb-1.qcow2 |
|
||||
| | | |
|
||||
| bitmaps: chk-a | | bitmaps: chk-a |
|
||||
| chk-b | | chk-b |
|
||||
| | | |
|
||||
+----------------+ +----------------+
|
||||
|
||||
Bitmaps are created at the same time to track changes to all disks in sync and
|
||||
are active and persisted in the QCOW2 image. Other formats currently don't
|
||||
support this feature.
|
||||
|
||||
Modification of bitmaps outside of libvirt is not recommended, but when adhering
|
||||
to the same semantics which the document will describe it should be safe to do
|
||||
so, even if we obviously can't guarantee that.
|
||||
|
||||
|
||||
Integration with external snapshots
|
||||
===================================
|
||||
|
||||
Handling of bitmaps
|
||||
-------------------
|
||||
|
||||
Creating an external snapshot involves adding a new layer to the backing chain
|
||||
on top of the previous chain. In this step there are no new bitmaps created by
|
||||
default, which would mean that backups become impossible after this step.
|
||||
|
||||
To prevent this from happening we need to re-create the active bitmaps in the
|
||||
new top/active layer of the backing chain which allows us to continue tracking
|
||||
the changes with same granularity as before and also allows libvirt to stitch
|
||||
together all the corresponding bitmaps to do a backup across snapshots.
|
||||
|
||||
After taking a snapshot of the ``vda`` disk from the example above placed into
|
||||
``vda-2.qcow2`` the following topology will be created:
|
||||
|
||||
::
|
||||
|
||||
+----------------+
|
||||
| disk: vda |
|
||||
+-------+--------+
|
||||
|
|
||||
+-------v--------+ +----------------+
|
||||
| vda-2.qcow2 | | vda-1.qcow2 |
|
||||
| | | |
|
||||
| bitmaps: chk-a +----> bitmaps: chk-a |
|
||||
| chk-b | | chk-b |
|
||||
| | | |
|
||||
+----------------+ +----------------+
|
||||
|
||||
Checking bitmap health
|
||||
----------------------
|
||||
|
||||
QEMU optimizes disk writes by only updating the bitmaps in certain cases. This
|
||||
also can cause problems in cases when e.g. QEMU crashes.
|
||||
|
||||
For a chain of corresponding bitmaps in a backing chain to be considered valid
|
||||
and eligible for use with ``virDomainBackupBegin`` it must conform to the
|
||||
following rules:
|
||||
|
||||
1) Top image must contain the bitmap
|
||||
2) If any of the backing images in the chain contain the bitmap too, all
|
||||
contiguous images must have the bitmap (no gaps)
|
||||
3) all of the above bitmaps must be marked as active
|
||||
(``auto`` flag in ``qemu-img`` output, ``recording`` in qemu)
|
||||
4) none of the above bitmaps can be inconsistent
|
||||
(``in-use`` flag in ``qemu-img`` provided that it's not used on image which
|
||||
is currently in use by a qemu instance, or ``inconsistent`` in qemu)
|
||||
|
||||
::
|
||||
|
||||
# check that image has bitmaps
|
||||
$ qemu-img info vda-1.qcow2
|
||||
image: vda-1.qcow2
|
||||
file format: qcow2
|
||||
virtual size: 100 MiB (104857600 bytes)
|
||||
disk size: 220 KiB
|
||||
cluster_size: 65536
|
||||
Format specific information:
|
||||
compat: 1.1
|
||||
compression type: zlib
|
||||
lazy refcounts: false
|
||||
bitmaps:
|
||||
[0]:
|
||||
flags:
|
||||
[0]: in-use
|
||||
[1]: auto
|
||||
name: chk-a
|
||||
granularity: 65536
|
||||
[1]:
|
||||
flags:
|
||||
[0]: auto
|
||||
name: chk-b
|
||||
granularity: 65536
|
||||
refcount bits: 16
|
||||
corrupt: false
|
||||
|
||||
(See also the ``qemuBlockBitmapChainIsValid`` helper method in
|
||||
``src/qemu/qemu_block.c``)
|
||||
|
||||
Creating external snapshots manually
|
||||
--------------------------------------
|
||||
|
||||
To create the same topology outside of libvirt (e.g when doing snapshots offline)
|
||||
a new ``qemu-img`` which supports the ``bitmap`` subcommand is recommended. The
|
||||
following algorithm then ensures that the new image after snapshot will work
|
||||
with backups (note that ``jq`` is a JSON processor):
|
||||
|
||||
::
|
||||
|
||||
#!/bin/bash
|
||||
|
||||
# arguments
|
||||
SNAP_IMG="vda-2.qcow2"
|
||||
BACKING_IMG="vda-1.qcow2"
|
||||
|
||||
# constants - snapshots and bitmaps work only with qcow2
|
||||
SNAP_FMT="qcow2"
|
||||
BACKING_IMG_FMT="qcow2"
|
||||
|
||||
# create snapshot overlay
|
||||
qemu-img create -f "$SNAP_FMT" -F "$BACKING_IMG_FMT" -b "$BACKING_IMG" "$SNAP_IMG"
|
||||
|
||||
BACKING_IMG_INFO=$(qemu-img info --output=json -f "$BACKING_IMG_FMT" "$BACKING_IMG")
|
||||
BACKING_BITMAPS=$(jq '."format-specific".data.bitmaps' <<< "$BACKING_IMG_INFO")
|
||||
|
||||
if [ "x$BACKING_BITMAPS" = "xnull" ]; then
|
||||
exit 0
|
||||
fi
|
||||
|
||||
for BACKING_BITMAP_ in $(jq -c '.[]' <<< "$BACKING_BITMAPS"); do
|
||||
BITMAP_FLAGS=$(jq -c -r '.flags[]' <<< "$BACKING_BITMAP_")
|
||||
BITMAP_NAME=$(jq -r '.name' <<< "$BACKING_BITMAP_")
|
||||
|
||||
if grep 'in-use' <<< "$BITMAP_FLAGS" ||
|
||||
grep -v 'auto' <<< "$BITMAP_FLAGS"; then
|
||||
continue
|
||||
fi
|
||||
|
||||
qemu-img bitmap -f "$SNAP_FMT" "$SNAP_IMG" --add "$BITMAP_NAME"
|
||||
|
||||
done
|
||||
Loading…
Reference in New Issue