================================================ 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 `_ 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 =================================== External snapshot terminology ----------------------------- External snapshots on a disk level consist of layered chains of disk images. An image in the chain can have a ``backing image`` placed below. Any chunk in the current image which was not written explicitly is transparent and if read the data from the backing image is passed through. An image placed on top of the current image is called ``overlay``. The bottommost backing image at the end of the chain is also usually described as ``base image``. The topmost overlay is the image which is being written to by the VM and is also described as the ``active`` layer or image. Handling of bitmaps during snapshots ------------------------------------ Creating an external snapshot involves adding a overlay on top of the previously active image. Libvirt requires that all ``block-dirty-bitmaps`` which correspond to the checkpoint must be created in the new overlay before any write from the guest reaches the overlay to continue tracking which blocks are dirtied. Since there are no new bitmaps created by ``qemu`` or ``qemu-img`` by default when creating an overlay, we need to re-create the appropriate bitmaps (see below) in the new overlay based on the previously active bitmaps in the active image. The new bitmaps are created with the same granularity. 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