KVM的外部快照（External snapshot）功能比较实用，可以支持仅对disk进行快照，也支持live snapshot，很多虚拟化云方案中一般也会使用外部快照功能创建快照。不过遗憾的是在centos6的rpm默认源里安装的qemu版本过低不支持–live模式的外部快照 。目前
qemu.org
上的qemu的最新版本是2.0.0rc3，最新稳定版是1.7.1，而目前centos6的源中使用的qemu版本是0.12.1。
[root@localhost test]# cd /usr/libexec/
[root@localhost libexec]# ./qemu-kvm -version
QEMU PC emulator version 0.12.1 (qemu-kvm-0.12.1.2), Copyright (c) 2003-2008 Fabrice Bellard如下所示，使用–live功能时会报错：
[root@localhost libvirt]#virsh snapshot-create-as ppd_centos snap1-centos "snap1 description" --diskspec vda,file=/file/snap1-centos.qcow2 --disk-only --atomic
error: Operation not supported: live disk snapshot not supported with this QEMU binary
在serverfault上有人提到了该问题，链接为：
http://serverfault.com/questions/567234/how-to-replace-qemu-binary-with-newer-version-for-libvirt-live-snapshots
，同时我也尝试了使用升级qemu为最新的1.7.1测试–live，发现问题也依旧 。目前估计还会对内核版本和libvirt的版本有依赖或兼容性问题。编译步骤如下：
[root@localhost qemu-1.7.1]# ./configure --prefix=/usr/local/qemu
ERROR: glib-2.12 required to compile QEMU
[root@localhost qemu-1.7.1]#yum -y install glib*
[root@localhost qemu-1.7.1]# ./configure --prefix=/usr/local/qemu
Install prefix /usr/local/qemu
BIOS directory /usr/local/qemu/share/qemu
binary directory /usr/local/qemu/bin
library directory /usr/local/qemu/lib
libexec directory /usr/local/qemu/libexec
include directory /usr/local/qemu/include
config directory /usr/local/qemu/etc
local state directory /usr/local/qemu/var
Manual directory /usr/local/qemu/share/man
ELF interp prefix /usr/gnemul/qemu-%M
Source path /usr/local/src/qemu-1.7.1
C compiler cc
Host C compiler cc
C++ compiler c++
Objective-C compiler cc
ARFLAGS rv
CFLAGS -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g
QEMU_CFLAGS -fPIE -DPIE -m64 -D_GNU_SOURCE -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -Wstrict-prototypes -Wredundant-decls -Wall -Wundef -Wwrite-strings -Wmissing-prototypes -fno-strict-aliasing -Wendif-labels -Wmissing-include-dirs -Wempty-body -Wnested-externs -Wformat-security -Wformat-y2k -Winit-self -Wignored-qualifiers -Wold-style-declaration -Wold-style-definition -Wtype-limits -fstack-protector-all -I$(SRC_PATH)/pixman/pixman -I$(BUILD_DIR)/pixman/pixman -I$(SRC_PATH)/dtc/libfdt
LDFLAGS -Wl,--warn-common -Wl,-z,relro -Wl,-z,now -pie -m64 -g
make make
install install
python python -B
smbd /usr/sbin/smbd
host CPU x86_64
host big endian no
target list alpha-softmmu arm-softmmu cris-softmmu i386-softmmu lm32-softmmu m68k-softmmu microblazeel-softmmu microblaze-softmmu mips64el-softmmu mips64-softmmu mipsel-softmmu mips-softmmu moxie-softmmu or32-softmmu ppc64-softmmu ppcemb-softmmu ppc-softmmu s390x-softmmu sh4eb-softmmu sh4-softmmu sparc64-softmmu sparc-softmmu unicore32-softmmu x86_64-softmmu xtensaeb-softmmu xtensa-softmmu alpha-linux-user armeb-linux-user arm-linux-user cris-linux-user i386-linux-user m68k-linux-user microblazeel-linux-user microblaze-linux-user mips64el-linux-user mips64-linux-user mipsel-linux-user mips-linux-user mipsn32el-linux-user mipsn32-linux-user or32-linux-user ppc64abi32-linux-user ppc64-linux-user ppc-linux-user s390x-linux-user sh4eb-linux-user sh4-linux-user sparc32plus-linux-user sparc64-linux-user sparc-linux-user unicore32-linux-user x86_64-linux-user
tcg debug enabled no
gprof enabled no
sparse enabled no
strip binaries yes
profiler no
static build no
-Werror enabled no
pixman internal
SDL support no
GTK support no
curses support no
curl support no
mingw32 support no
Audio drivers oss
Block whitelist (rw)
Block whitelist (ro)
VirtFS support no
VNC support yes
VNC TLS support no
VNC SASL support no
VNC JPEG support no
VNC PNG support no
VNC WS support no
xen support no
brlapi support no
bluez support no
Documentation no
GUEST_BASE yes
PIE yes
vde support no
Linux AIO support no
ATTR/XATTR support yes
Install blobs yes
KVM support yes
RDMA support no
TCG interpreter no
fdt support yes
preadv support yes
fdatasync yes
madvise yes
posix_madvise yes
sigev_thread_id yes
uuid support no
libcap-ng support no
vhost-net support yes
vhost-scsi support yes
Trace backend nop
Trace output file trace-<pid>
spice support no (/)
rbd support no
xfsctl support no
nss used no
libusb no
usb net redir no
GLX support no
libiscsi support no
build guest agent yes
QGA VSS support no
seccomp support no
coroutine backend ucontext
coroutine pool yes
GlusterFS support no
virtio-blk-data-plane no
gcov gcov
gcov enabled no
TPM support no
libssh2 support no
TPM passthrough no
QOM debugging yes
vhdx no
[root@localhost qemu-1.7.1]# make && make install 编译完成后，通过软链接替换qemu-kvm和qemu-img两个可执行文件，由于测试结果是失败的，所以这里过程也略过。
在destroy关机情况下，测试的–disk外部快照功能如下：
[root@localhost file]# virsh destroy ppd_centos
Domain ppd_centos destroyed
[root@localhost file]# virsh snapshot-create-as ppd_centos snap1-centos "snap1 description" --disk-only --atomic
Domain snapshot snap1-centos created
[root@localhost file]# qemu-img info centos.qcow2
image: centos.qcow2
file format: qcow2
virtual size: 30G (32212254720 bytes)
disk size: 1.6G
cluster_size: 65536
Format specific information:
compat: 0.10
[root@localhost file]# qemu-img info centos.snap1-centos
image: centos.snap1-centos
file format: qcow2
virtual size: 30G (32212254720 bytes)
disk size: 196K
cluster_size: 65536
backing file: /file/centos.qcow2
backing file format: qcow2
Format specific information:
compat: 0.10
[root@localhost file]# virsh snapshot-list ppd_centos
Name Creation Time State
------------------------------------------------------------
snap1-centos 2014-04-15 18:20:54 +0800 shutoff从上面的查询中不难看出centos.snap1-centos的backing file来自于/file/centos.qcow2 。同样，也可以利用下面的命令进行查看：
[root@localhost libexec]# virsh domblklist ppd_centos --details
Type Device Target Source
------------------------------------------------
file disk vda /file/centos.snap1-centos
block cdrom hdc -创建完外部快照后，原镜像文件会变成只读，新的变更都会写入到新的快照文件中。如下，我做完快照后，上传到一个文件到guest主机中，查看文件状态发现如下：
[root@localhost test]# ll /file/
total 39673072
-rwxr-xr-x. 1 root root 32212254720 Apr 14 16:39 centos.img
-rw-r--r--. 1 root root 1704854016 Apr 15 18:20 centos.qcow2
-rw-r--r--. 1 root root 197120 Apr 15 18:20 centos.snap1-centos
drwx------. 2 root root 16384 Apr 9 18:08 lost+found
drwxr-xr-x. 8 1001 1002 4096 Apr 14 09:58 ntfs-3g_ntfsprogs-2014.2.15
-rwxr-xr-x. 1 qemu qemu 53687091200 Apr 15 19:29 win2008.img
-rwxr-xr-x. 1 qemu qemu 32212254720 Apr 15 19:27 win7.img
[root@localhost test]# ll
total 0
[root@localhost test]# ll /file/
total 39772532
-rwxr-xr-x. 1 root root 32212254720 Apr 14 16:39 centos.img
-rw-r--r--. 1 qemu qemu 1704854016 Apr 15 18:20 centos.qcow2
-rw-r--r--. 1 qemu qemu 102105088 Apr 15 19:33 centos.snap1-centos
drwx------. 2 root root 16384 Apr 9 18:08 lost+found
drwxr-xr-x. 8 1001 1002 4096 Apr 14 09:58 ntfs-3g_ntfsprogs-2014.2.15
-rwxr-xr-x. 1 qemu qemu 53687091200 Apr 15 19:32 win2008.img
-rwxr-xr-x. 1 qemu qemu 32212254720 Apr 15 19:32 win7.img
[root@localhost test]#从上面不难发现，centos.snap1-centos的大小在增大，而原centos.qcow2的大小不变。至于快照的删除、回滚和内部快照操作没有区别。不过外部快照多了一个快照合并功能：
# virsh blockpull --domain ppd_centos --path /file/centos.snap1-centos以上命令就可以将快照的变化写入境像文件中。
关于centos功能支持性的问题，外部快照功能测试的有点草率。回头有时间换到ubuntu、fedora或重新编译安装再做测试 。