qmpbackup 0.46 - add image fleecing
I’ve released qmpbackup 0.46 which now utilizes the image fleecing technique for backup.
Usually, during backup, Qemu will use a so called copy-before-write filter so
that data for new guest writes is sent to the backup target first, the guest
write blocks until this operation is finished.
If the backup target is flaky, or becomes unavailable during backup operation, this could lead to high I/O wait times or even complete VM lockups.
To fix this, a so called “fleecing” image is introduced during backup being used as temporary cache for write operations by the guest. This image can be placed on the same storage as the virtual machine disks, so is independent from the backup target performance.
The documentation on which steps are required to get this going, using the Qemu QMP protocol is, lets say.. lacking..
The following examples show the general functionality, but should be enhanced
to use transactions where possible. All commands are in qmp-shell command
format.
Lets start with a full backup:
# create a new bitmap
block-dirty-bitmap-add node=disk1 name=bitmap persistent=true
# add the fleece image to the virtual machine (same size as original disk required)
blockdev-add driver=qcow2 node-name=fleecie file={"driver":"file","filename":"/tmp/fleece.qcow2"}
# add the backup target file to the virtual machine
blockdev-add driver=qcow2 node-name=backup-target-file file={"driver":"file","filename":"/tmp/backup.qcow2"}
# enable the copy-before-writer for the first disk attached, utilizing the fleece image
blockdev-add driver=copy-before-write node-name=cbw file=disk1 target=fleecie
# "blockdev-replace": make the copy-before-writer filter the major device (use "query-block" to get path parameter value, qdev node)
qom-set path=/machine/unattached/device[20] property=drive value=cbw
# add the snapshot-access filter backing the copy-before-writer
blockdev-add driver=snapshot-access file=cbw node-name=snapshot-backup-source
# create a full backup
blockdev-backup device=snapshot-backup-source target=backup-target-file sync=full job-id=test
[ wait until block job finishes]
# remove the snapshot access filter from the virtual machine
blockdev-del node-name=snapshot-backup-source
# switch back to the regular disk
qom-set path=/machine/unattached/device[20] property=drive value=node-disk1
# remove the copy-before-writer
blockdev-del node-name=cbw
# remove the backup-target-file
blockdev-del node-name=backup-target-file
# detach the fleecing image
blockdev-del node-name=fleecieAfter this process, the temporary fleecing image can be deleted/recreated. Now lets go for a incremental backup:
# add the fleecing and backup target image, like before
blockdev-add driver=qcow2 node-name=fleecie file={"driver":"file","filename":"/tmp/fleece.qcow2"}
blockdev-add driver=qcow2 node-name=backup-target-file file={"driver":"file","filename":"/tmp/backup-incremental.qcow2"}
# add the copy-before-write filter, but utilize the bitmap created during full backup
blockdev-add driver=copy-before-write node-name=cbw file=disk1 target=fleecie bitmap={"node":"disk1","name":"bitmap"}
# switch device to the copy-before-write filter
qom-set path=/machine/unattached/device[20] property=drive value=cbw
# add the snapshot-access filter
blockdev-add driver=snapshot-access file=cbw node-name=snapshot-backup-source
# merge the bitmap created during full backup to the snapshot-access device so
# the backup operation can access it. (you better use an transaction here)
block-dirty-bitmap-add node=snapshot-backup-source name=bitmap
block-dirty-bitmap-merge node=snapshot-backup-source target=bitmap bitmaps=[{"node":"disk1","name":"bitmap"}]
# create incremental backup (you better use an transaction here)
blockdev-backup device=snapshot-backup-source target=backup-target-file job-id=test sync=incremental bitmap=bitmap
[ wait until backup has finished ]
[ cleanup like before ]
# clear the dirty bitmap (you better use an transaction here)
block-dirty-bitmap-clear node=disk1 name=bitmapOr, use a simple reproducer by directly passing qmp commands via stdio:
#!/usr/bin/bash
qemu-img create -f raw disk 1M
qemu-img create -f raw fleece 1M
qemu-img create -f raw backup 1M
qemu-system-x86_64 -drive node-name=disk,file=disk,format=file -qmp stdio -nographic -nodefaults <<EOF
{"execute": "qmp_capabilities"}
{"execute": "block-dirty-bitmap-add", "arguments": {"node": "disk", "name": "bitmap"}}
{"execute": "blockdev-add", "arguments": {"node-name": "fleece", "driver": "file", "filename": "fleece"}}
{"execute": "blockdev-add", "arguments": {"node-name": "backup", "driver": "file", "filename": "backup"}}
{"execute": "blockdev-add", "arguments": {"node-name": "cbw", "driver": "copy-before-write", "file": "disk", "target": "fleece", "bitmap": {"node": "disk", "name": "bitmap"}}}
{"execute": "query-block"}
{"execute": "qom-set", "arguments": {"path": "/machine/unattached/device[4]", "property": "drive", "value": "cbw"}}
{"execute": "blockdev-add", "arguments": {"node-name": "snapshot", "driver": "snapshot-access", "file": "cbw"}}
{"execute": "block-dirty-bitmap-add", "arguments": {"node": "snapshot", "name": "tbitmap"}}
{"execute": "block-dirty-bitmap-merge", "arguments": {"node": "snapshot", "target": "tbitmap", "bitmaps": [{"node": "disk", "name": "bitmap"}]}}
[..]
{"execute": "quit"}
EOF