Efficient live disk backup with active blockcommit

列出当前正在使用的block device

$ virsh domblklist vm1
Target     Source
------------------------------------------------
vda        /export/images/base.qcow2 

创建一个external disk snapshot

$ virsh snapshot-create-as --domain vm1 sn1 
    --diskspec vda,file=/export/images/sn1.qcow2 
    --disk-only --atomic 

现在disk image chain为：

[base] <-- [sn1] (live QEMU)

NOTE-1:如果guest中安装有QEMU guest agent，可以'--quiesce' option来得到consistent disk state

备份origin disk：

$ cp /export/images/base.img /export/images/copy.img

[or]

$ rsync -avh --progress /export/images/base.img 
        /export/images/copy.img

再看当前正在使用的block device为sn1：

$ virsh domblklist vm1
Target     Source
------------------------------------------------
vda        /export/images/sn1.qcow2

执行blockcommit:

$ virsh blockcommit vm1 vda --active --verbose --pivot

再看当前正在使用的block device为origin disk：

$ virsh domblklist vm1
Target     Source
------------------------------------------------
vda        /export/images/base.qcow2

disk image chain:

[base] (live QEMU)

http://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit