LVM逻辑卷实现磁盘镜像
大家都知道用lvm作分区扩展很方便，其实用lvs还可以用作磁盘镜像，类似于raid1，即多块磁盘互相同步备份，可以有效避免数据丢失的尴尬。
1.新建磁盘分区
新建4个分区/dev/sdb1,/dev/sdb2,/dev/sdb3,/dev/sdb4，如：
Disk /dev/sdb: 1999.8 GB, 1999844147200 bytes
255 heads, 63 sectors/track, 243133 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 1 374 3004123+ 83 Linux
/dev/sdb2 375 748 3004155 83 Linux
/dev/sdb3 749 1122 3004155 83 Linux
/dev/sdb4 1123 1496 3004155 83 Linux
其中/dev/sdb1,/dev/sdb2用作数据磁盘，/dev/sdb2用作/dev/sdb1的镜像；
/dev/sdb3用作日志；/dev/sdb4作为备用，当sdb1或sdb2损害时备用，类似于raid5中的热备盘。
2.创建pv和vg
pvcreate /dev/sdb1
pvcreate /dev/sdb2
pvcreate /dev/sdb3
pvcreate /dev/sdb4
vgcreate vg_test /dev/sdb1 /dev/sdb2 /dev/sdb3
注：/dev/sdb4暂未使用，待备用
3.创建包含镜像功能的逻辑卷(mirror_test)
[root@localhost ~]# lvcreate -L 2.8G -m1 -n mirror_test vg_test /dev/sdb1 /dev/sdb2 /dev/sdb3
Rounding up size to full physical extent 2.80 GB
Logical volume "mirror_test" created
[root@localhost ~]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Convert Devices
mirror_test vg_test mwi-a- 2.80G mirror_test_mlog 24.27 mirror_test_mimage_0(0),mirror_test_mimage_1(0)
[mirror_test_mimage_0] vg_test Iwi-ao 2.80G /dev/sdb1(0)
[mirror_test_mimage_1] vg_test Iwi-ao 2.80G /dev/sdb2(0)
[mirror_test_mlog] vg_test lwi-ao 4.00M /dev/sdb3(0)
其中：-m1参数为创建镜像；-L参数为设定镜像卷的大小为2.8G；-n参数为指定镜像名称。这个镜像包括三部分：sdb1和sdb2位数据卷和镜像卷，sdb3为日志卷。
4.格式化挂载
[root@localhost ~]# mkfs.ext3 /dev/vg_test/mirror_test
[root@localhost ~]# mkdir -p /test
[root@localhost ~]# mount /dev/vg_test/mirror_test /test
5.测试
(1)创建测试文件
for x in {1..100};do echo "$x test test test " >> /test/test.txt ;done
(2)模拟对/dev/sdb2进行破坏
[root@localhost ~]# dd if=/dev/zero of=/dev/sdb2 count=10 bs=1M
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.008574 seconds, 1.2 GB/s
[root@localhost ~]# lvs -a -o +devices
Couldn't find device with uuid z74R0l-ZLUV-X6TS-QrpF-nXDZ-gc74-UvwGC0.
LV VG Attr LSize Origin Snap% Move Log Copy% Convert Devices
mirror_test vg_test mwi-ao 2.80G mirror_test_mlog 100.00 mirror_test_mimage_0(0),mirror_test_mimage_1(0)
[mirror_test_mimage_0] vg_test iwi-ao 2.80G /dev/sdb1(0)
[mirror_test_mimage_1] vg_test iwi-ao 2.80G unknown device(0)
[mirror_test_mlog] vg_test lwi-ao 4.00M /dev/sdb3(0)
查看状态发现/dev/sdb2处于“unknown device(0)”状态。
[root@localhost ~]# lvscan
Couldn't find device with uuid z74R0l-ZLUV-X6TS-QrpF-nXDZ-gc74-UvwGC0.
ACTIVE '/dev/vg_test/mirror_test' [2.80 GB] inherit
(3)重新挂载确认数据可读取
[root@localhost ~]# umount /test
[root@localhost test]# cd
[root@localhost ~]# mount /dev/vg_test/mirror_test /test
[root@localhost ~]# cat /test/test.txt
lvs mirror test
(4)将坏掉的设备删除
[root@localhost ~]# vgreduce --removemissing --force vg_test
Couldn't find device with uuid z74R0l-ZLUV-X6TS-QrpF-nXDZ-gc74-UvwGC0.
WARNING: Bad device removed from mirror volume, vg_test/mirror_test
WARNING: Mirror volume, vg_test/mirror_test converted to linear due to device failure.
Wrote out consistent volume group vg_test
移除后vgdisplay /dev/vg_test发现mirrored volumes选项没有了；并且由于sdb2的实效，
镜像mirror_test也已经有mirror模式转变成线性模式，如果恢复我们需要用到lvconvert命令来恢复。
(5)镜像数据恢复
[root@localhost ~]# vgextend vg_test /dev/sdb4
Volume group "vg_test" successfully extended
[root@localhost ~]# lvconvert -m1 /dev/vg_test/mirror_test /dev/sdb1 /dev/sdb4 /dev/sdb3
vg_test/mirror_test: Converted: 0.0%
vg_test/mirror_test: Converted: 20.8%
vg_test/mirror_test: Converted: 40.4%
vg_test/mirror_test: Converted: 60.1%
vg_test/mirror_test: Converted: 79.4%
vg_test/mirror_test: Converted: 99.2%
vg_test/mirror_test: Converted: 100.0%
ok，恢复完成，我们来查看下状态：
[root@localhost ~]# lvdisplay /dev/vg_test
--- Logical volume ---
LV Name /dev/vg_test/mirror_test
VG Name vg_test
LV UUID RUamnD-nYCt-D9yp-cNdX-ix1y-r9P9-wenSjb
LV Write Access read/write
LV Status available
# open 1
LV Size 2.80 GB
Current LE 717
Mirrored volumes 2
Segments 1
Allocation inherit
Read ahead sectors auto
- currently set to 256
Block device 253:8
[root@localhost ~]# cat /test/test.txt
lvs mirror test
[root@localhost ~]# lvs -a -o +devices
LV VG Attr LSize Origin Snap% Move Log Copy% Convert Devices
mirror_test vg_test mwi-ao 2.80G mirror_test_mlog 100.00 mirror_test_mimage_0(0),mirror_test_mimage_1(0)
[mirror_test_mimage_0] vg_test iwi-ao 2.80G /dev/sdb1(0)
[mirror_test_mimage_1] vg_test iwi-ao 2.80G /dev/sdb4(0)
[mirror_test_mlog] vg_test lwi-ao 4.00M /dev/sdb3(0)
镜像卷由原来的sdb2转变为sdb4
总结：在某些不支持raid的服务器上，我们可以使用lvm的磁盘镜像来解决防止数据丢失的问题了。
