Re: [PATCH 32/32] 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(a)redhat.com&gt; --- docs/kbase.html.in | 3 + docs/kbase/incrementalbackupinternals.rst | 210 ++++++++++++++++++++++ 2 files changed, 213 insertions(+) create mode 100644 docs/kbase/incrementalbackupinternals.rst diff --git a/docs/kbase.html.in b/docs/kbase.html.in index c586e0f676..4257e52b7e 100644 --- a/docs/kbase.html.in +++ b/docs/kbase.html.in @@ -36,6 +36,9 @@ <dt><a href="kbase/virtiofs.html">Virtio-FS</a></dt> <dd>Share a filesystem between the guest and the host</dd> + + <dt><a href="kbase/incrementalbackupinternals.html">Incremental backup internals</a></dt> + <dd>Incremental backup implementation details relevant for users</dd> </dl> </div> diff --git a/docs/kbase/incrementalbackupinternals.rst b/docs/kbase/incrementalbackupinternals.rst new file mode 100644 index 0000000000..adf12002d2 --- /dev/null +++ b/docs/kbase/incrementalbackupinternals.rst @@ -0,0 +1,210 @@ +================================================ +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 correspoiding to the points in time described by a libvirt +checkpoint. + +There are some semantical implications 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 sumarize them. + +Glossary +======== + +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 and allows then a backup of + block which changed. Note that state of the VM memory is _not_ captured. + + A checkpoint can be created either explicitly via the corresponding API + which isn't very useful or is created as part of creating 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 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. Oter formats currently don't +support this feature. + +Modification of bitmaps outside of libvirt is not recommended, but when adrering +to the same semantics which the document will describe it should be safe to do +so but obviously we 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 acorss 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 bitmaps corresponding 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 checkpoints manually +-------------------------------------- + +To create the same topology outside of libvirt (e.g when doing snapshots offline) +a new ``qemu-img`` which supports the ``bitmap`` subcomand is necessary. The +following algorithm then ensures that the new image after snapshot will work +with backups (note that ``jq`` is a JSON processor): + +:: + + # 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 -- 2.26.2