linux系统中逻辑卷快照

LVM还具备有“快照劵”功能，该功能类似于虚拟机软件的还原时间点功能。例如，可以对某一个逻辑卷设备做一次快照，如果日后发现数据被改错了，就可以

利用之前做好的快照劵进行覆盖还原。LVM的快照劵功能有两个特点：

快照劵的容量必须等同于逻辑卷的容量。

快照劵仅一次有效，一旦执行还原操作后则会被立即自动删除。（linux就该这么学p150）

1、首先查看卷组信息

[root@linuxprobe dev]# vgdisplay  ## 卷组中已经使用了100MB的容量，空闲容量还有39.89G
  --- Volume group ---
  VG Name               vgtest1
  System ID
  Format                lvm2
  Metadata Areas        2
  Metadata Sequence No  4
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                1
  Open LV               1
  Max PV                0
  Cur PV                2
  Act PV                2
  VG Size               39.99 GiB
  PE Size               4.00 MiB
  Total PE              10238
  Alloc PE / Size       25 / 100.00 MiB
  Free  PE / Size       10213 / 39.89 GiB
  VG UUID               iPlgk3-b3h6-K3XB-X4vS-ep0B-TL5x-CpjClo
………………

2、向逻辑卷挂载点的目录中写入一个文件

[root@linuxprobe dev]# ls /linuxprobe/
lost+found
[root@linuxprobe dev]# echo "welcome to linuxprobe" > /linuxprobe/readme.txt
[root@linuxprobe dev]# ls /linuxprobe/
lost+found  readme.txt

3、使用-s参数生成一个快照劵

[root@linuxprobe dev]# lvcreate -L 100M -s -n SNAPTEST /dev/vgtest1/lvtest1  ## -L 指定快照大小，-n制动快照名称
  Logical volume "SNAPTEST" created
[root@linuxprobe dev]# lvdisplay  ## 显示详细信息
……………………
  --- Logical volume ---
  LV Path                /dev/vgtest1/SNAPTEST
  LV Name                SNAPTEST
  VG Name                vgtest1
  LV UUID                yfjyWC-uAbi-NBxh-fqCA-XiXq-9Paf-mvzofR
  LV Write Access        read/write
  LV Creation host, time linuxprobe.com, 2020-10-29 17:47:49 +0800
  LV snapshot status     active destination for lvtest1
  LV Status              available
  # open                 0
  LV Size                100.00 MiB
  Current LE             25
  COW-table size         100.00 MiB
  COW-table LE           25
  Allocated to snapshot  0.01%
  Snapshot chunk size    4.00 KiB
  Segments               1
  Allocation             inherit
………………………………

4、在逻辑卷所挂载的目录中创建一个50MB的垃圾文件，然后在查看快照劵的状态。可以发现存储空间占用量上升了？？。

[root@linuxprobe dev]# dd if=/dev/zero bs=50M count=1 of=/linuxprobe/rubbish.txt  ## 创建垃圾文件
1+0 records in
1+0 records out
52428800 bytes (52 MB) copied, 1.44275 s, 36.3 MB/s
[root@linuxprobe dev]# lvdisplay  ## 查看详细信息
………………
 --- Logical volume ---
  LV Path                /dev/vgtest1/SNAPTEST
  LV Name                SNAPTEST
  VG Name                vgtest1
  LV UUID                yfjyWC-uAbi-NBxh-fqCA-XiXq-9Paf-mvzofR
  LV Write Access        read/write
  LV Creation host, time linuxprobe.com, 2020-10-29 17:47:49 +0800
  LV snapshot status     active destination for lvtest1
  LV Status              available
  # open                 0
  LV Size                100.00 MiB
  Current LE             25
  COW-table size         100.00 MiB
  COW-table LE           25
  Allocated to snapshot  45.32%
  Snapshot chunk size    4.00 KiB
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
……………………

5、先卸载逻辑卷设备的挂载

[root@linuxprobe dev]# df -h
Filesystem                   Size  Used Avail Use% Mounted on
/dev/mapper/rhel-root         18G  2.9G   15G  17% /
devtmpfs                     985M     0  985M   0% /dev
tmpfs                        994M   80K  994M   1% /dev/shm
tmpfs                        994M  8.9M  986M   1% /run
tmpfs                        994M     0  994M   0% /sys/fs/cgroup
/dev/sda1                    497M  119M  379M  24% /boot
/dev/sr0                     3.5G  3.5G     0 100% /run/media/root/RHEL-7.0 Server.x86_64
/dev/mapper/vgtest1-lvtest1   93M   52M   35M  61% /linuxprobe
[root@linuxprobe dev]# umount /linuxprobe/  ## 卸载
[root@linuxprobe dev]# df -h
Filesystem             Size  Used Avail Use% Mounted on
/dev/mapper/rhel-root   18G  2.9G   15G  17% /
devtmpfs               985M     0  985M   0% /dev
tmpfs                  994M   80K  994M   1% /dev/shm
tmpfs                  994M  8.9M  986M   1% /run
tmpfs                  994M     0  994M   0% /sys/fs/cgroup
/dev/sda1              497M  119M  379M  24% /boot
/dev/sr0               3.5G  3.5G     0 100% /run/media/root/RHEL-7.0 Server.x86_64

6、为了校验SNAPTEST快照劵的效果，需要对逻辑卷进行快照还原操作。

[root@linuxprobe dev]# lvconvert --merge /dev/vgtest1/SNAPTEST  ## 还原
  Merging of volume SNAPTEST started.
  lvtest1: Merged: 65.9%
  lvtest1: Merged: 100.0%
  Merge of snapshot into logical volume lvtest1 has finished.
  Logical volume "SNAPTEST" successfully removed

6、快照被自动删除了，rubbish.txt也被删除了

[root@linuxprobe dev]# mount -a  ## 挂载
[root@linuxprobe dev]# ls /dev/vgtest1/
lvtest1
[root@linuxprobe dev]# ls /linuxprobe/
lost+found  readme.txt