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: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

docs/kbase.html.in

@@ -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>
 

docs/kbase/incrementalbackupinternals.rst

+================================================
+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