kvm虚拟化小结（八）外部快照External snapshot
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。
1[root@localhost test]# cd /usr/libexec/
2[root@localhost libexec]# ./qemu-kvm -version
3QEMU 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：
1[root@localhost libvirt]#virsh snapshot-create-as ppd_centos snap1-centos "snap1 description" --diskspec vda,file=/file/snap1-centos.qcow2 --disk-only --atomic
2error: 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：
1[root@localhost qemu-1.7.1]# ./configure --prefix=/usr/local/qemu
2ERROR: glib-2.12 required to compile QEMU
3[root@localhost qemu-1.7.1]#yum -y install glib*
4[root@localhost qemu-1.7.1]# ./configure --prefix=/usr/local/qemu
5Install prefix /usr/local/qemu
6BIOS directory /usr/local/qemu/share/qemu
7binary directory /usr/local/qemu/bin
8library directory /usr/local/qemu/lib
9libexec directory /usr/local/qemu/libexec
10include directory /usr/local/qemu/include
11config directory /usr/local/qemu/etc
12local state directory /usr/local/qemu/var
13Manual directory /usr/local/qemu/share/man
14ELF interp prefix /usr/gnemul/qemu-%M
15Source path /usr/local/src/qemu-1.7.1
16C compiler cc
17Host C compiler cc
18C++ compiler c++
19Objective-C compiler cc
20ARFLAGS rv
21CFLAGS -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 -g
22QEMU_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
23LDFLAGS -Wl,--warn-common -Wl,-z,relro -Wl,-z,now -pie -m64 -g
24make make
25install install
26python python -B
27smbd /usr/sbin/smbd
28host CPU x86_64
29host big endian no
30target 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
31tcg debug enabled no
32gprof enabled no
33sparse enabled no
34strip binaries yes
35profiler no
36static build no
37-Werror enabled no
38pixman internal
39SDL support no
40GTK support no
41curses support no
42curl support no
43mingw32 support no
44Audio drivers oss
45Block whitelist (rw)
46Block whitelist (ro)
47VirtFS support no
48VNC support yes
49VNC TLS support no
50VNC SASL support no
51VNC JPEG support no
52VNC PNG support no
53VNC WS support no
54xen support no
55brlapi support no
56bluez support no
57Documentation no
58GUEST_BASE yes
59PIE yes
60vde support no
61Linux AIO support no
62ATTR/XATTR support yes
63Install blobs yes
64KVM support yes
65RDMA support no
66TCG interpreter no
67fdt support yes
68preadv support yes
69fdatasync yes
70madvise yes
71posix_madvise yes
72sigev_thread_id yes
73uuid support no
74libcap-ng support no
75vhost-net support yes
76vhost-scsi support yes
77Trace backend nop
78Trace output file trace-<pid>
79spice support no (/)
80rbd support no
81xfsctl support no
82nss used no
83libusb no
84usb net redir no
85GLX support no
86libiscsi support no
87build guest agent yes
88QGA VSS support no
89seccomp support no
90coroutine backend ucontext
91coroutine pool yes
92GlusterFS support no
93virtio-blk-data-plane no
94gcov gcov
95gcov enabled no
96TPM support no
97libssh2 support no
98TPM passthrough no
99QOM debugging yes
100vhdx no
101[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：
1[root@localhost file]# virsh destroy ppd_centos
2Domain ppd_centos destroyed
3[root@localhost file]# virsh snapshot-create-as ppd_centos snap1-centos "snap1 description" --disk-only --atomic
4Domain snapshot snap1-centos created
5[root@localhost file]# qemu-img info centos.qcow2
6image: centos.qcow2
7file format: qcow2
8virtual size: 30G (32212254720 bytes)
9disk size: 1.6G
10cluster_size: 65536
11Format specific information:
12 compat: 0.10
13[root@localhost file]# qemu-img info centos.snap1-centos
14image: centos.snap1-centos
15file format: qcow2
16virtual size: 30G (32212254720 bytes)
17disk size: 196K
18cluster_size: 65536
19backing file: /file/centos.qcow2
20backing file format: qcow2
21Format specific information:
22 compat: 0.10
23[root@localhost file]# virsh snapshot-list ppd_centos
24 Name Creation Time State
25------------------------------------------------------------
26 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：
1[root@localhost libexec]# virsh domblklist ppd_centos --details
2Type Device Target Source
3------------------------------------------------
4file disk vda /file/centos.snap1-centos
5block 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：
1[root@localhost test]# ll /file/
2total 39673072
3-rwxr-xr-x. 1 root root 32212254720 Apr 14 16:39 centos.img
4-rw-r--r--. 1 root root 1704854016 Apr 15 18:20 centos.qcow2
5-rw-r--r--. 1 root root 197120 Apr 15 18:20 centos.snap1-centos
6drwx------. 2 root root 16384 Apr 9 18:08 lost+found
7drwxr-xr-x. 8 1001 1002 4096 Apr 14 09:58 ntfs-3g_ntfsprogs-2014.2.15
8-rwxr-xr-x. 1 qemu qemu 53687091200 Apr 15 19:29 win2008.img
9-rwxr-xr-x. 1 qemu qemu 32212254720 Apr 15 19:27 win7.img
10[root@localhost test]# ll
11total 0
12[root@localhost test]# ll /file/
13total 39772532
14-rwxr-xr-x. 1 root root 32212254720 Apr 14 16:39 centos.img
15-rw-r--r--. 1 qemu qemu 1704854016 Apr 15 18:20 centos.qcow2
16-rw-r--r--. 1 qemu qemu 102105088 Apr 15 19:33 centos.snap1-centos
17drwx------. 2 root root 16384 Apr 9 18:08 lost+found
18drwxr-xr-x. 8 1001 1002 4096 Apr 14 09:58 ntfs-3g_ntfsprogs-2014.2.15
19-rwxr-xr-x. 1 qemu qemu 53687091200 Apr 15 19:32 win2008.img
20-rwxr-xr-x. 1 qemu qemu 32212254720 Apr 15 19:32 win7.img
21[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：
1# 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 。
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- Author: shisekong
- Link: https://blog.361way.com/external-snapshot/3183.html
- License: This work is under a 知识共享署名-非商业性使用-禁止演绎 4.0 国际许可协议. Kindly fulfill the requirements of the aforementioned License when adapting or creating a derivative of this work.