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         xfs提供了xfsdump和xfsrestore工具，协助备份xfs文件系统中的数据。xfsdump按inode顺序备份一个xfs文件系统

         在CentOS7中，xfs格式作为默认文件系统，不再使用以前的ext，但仍然支持ext4。xfs专为大数据产生，每个文件系统最大可以支持8eb，单个文件可以支持16tb，不仅数据量大，而且扩展性高。还可以通过xfsdump和xfsrestore来备份和恢复。原始备份数据方法：cp

         与传统的UNIX文件系统不同，xfs不需要备份前卸载；对使用中的xfs文件系统做备份就可以保证镜像的一致性。xfs的备份和恢复的过程是可以被中断然后继续的，无需冻结文件系统。xfsdump甚至特功了高性能的多线程备份操作——它把一次dump拆分成多个数据流，每个数据流可以被发往不同的目的地

         首先了解一下xfsdump的备份级别有以下两种：0，代表完整备份；1~9，代表增量备份。默认为0（即完全备份）。

(1).环境

VMware12       系统CentOS7.6

添加一块虚拟硬盘（准备一个测试分区），分区，格式化，挂载

1）添加完一块虚拟硬盘后如何查看系统是否识别

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[root@xuexi xf]# fdisk -l

磁盘 /dev/sda：21.5 GB, 21474836480 字节，41943040 个扇区

Units = 扇区 of 1 * 512 = 512 bytes

扇区大小(逻辑/物理)：512 字节 / 512 字节

I/O 大小(最小/最佳)：512 字节 / 512 字节

磁盘标签类型：dos

磁盘标识符：0x000c83f7

 

   设备 Boot      Start         End      Blocks   Id  System

/dev/sda1   *        2048      411647      204800   83  Linux

/dev/sda2          411648    21383167    10485760   83  Linux

/dev/sda3        21383168    25577471     2097152   82  Linux swap / Solaris

 

磁盘 /dev/sdb：21.5 GB, 21474836480 字节，41943040 个扇区

Units = 扇区 of 1 * 512 = 512 bytes

扇区大小(逻辑/物理)：512 字节 / 512 字节

I/O 大小(最小/最佳)：512 字节 / 512 字节

[root@xuexi xf]# ls /dev/sd*

/dev/sda  /dev/sda1  /dev/sda2  /dev/sda3  /dev/sdb

2）分区，格式化，挂载可以参考以下文档：

https://wwwblogs/diantong/p/8855683.html

这里创建一个1G的sdb1分区，xfs格式化，并挂载到新建的文件夹/sdb1

3）准备一份备份的测试文件

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[root@xuexi xf]# cd /sdb1/

[root@xuexi sdb1]# ls

[root@xuexi sdb1]# cp /etc/passwd ./

[root@xuexi sdb1]# mkdir test

[root@xuexi sdb1]# touch test/a

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── passwd

└── test

    └── a

1 directory, 2 files

(2).完整备份

1）备份整个分区

xfsdump –f [备份存放路径] [要备份的设备文件或挂载点]

注意：备份的路径这里不能写/sdb1/。可以写成/dev/sdb1或/sdb1

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[root@xuexi sdb1]# mkdir /dump

[root@xuexi sdb1]# xfsdump -f /dump/sdb1 /dev/sdb1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

 ============================= dump label dialog ==============================

please enter label for this dump session (timeout in 300 sec)

 -> dump_sdb1_20181228       //指定备份会话标签，就是对此文件系统的简短描述

session label entered: "dump_sdb1_20181228"

 --------------------------------- end dialog ---------------------------------

xfsdump: level 0 dump of xuexi:/sdb1

xfsdump: dump date: Fri Dec 28 14:11:55 2018

xfsdump: session id: a1afa104-8f0c-42fa-be6e-594b674f13c7

xfsdump: session label: "dump_sdb1_20181228"

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: skipping (no pruning necessary)

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 25856 bytes

xfsdump: /var/lib/xfsdump/inventory created

 ============================= media label dialog =============================

please enter label for media in drive 0 (timeout in 300 sec)

 -> sdb1 //指定设备标签，就是对要备份的设备做一个简短描述

media label entered: "sdb1"

 --------------------------------- end dialog ---------------------------------

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 25552 bytes

xfsdump: dump size (non-dir files) : 2592 bytes

xfsdump: dump complete: 39 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/sdb1 OK (success)

xfsdump: Dump Status: SUCCESS

2）指定备份时免交互操作，方便后期做定时备份

-L 指定备份会话标签

-M 指定设备标签

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[root@xuexi sdb1]# xfsdump -f /dump/sdb1_v2 /sdb1 -L dump_sdb1_20181228_v2 -M sdb1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsdump: level 0 dump of xuexi:/sdb1

xfsdump: dump date: Fri Dec 28 14:26:05 2018

xfsdump: session id: 5595618f-a3c3-4f4d-a5fc-0f6b86513400

xfsdump: session label: "dump_sdb1_20181228_v2"     //备份会话标签

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: skipping (no pruning necessary)

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 25856 bytes

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 25552 bytes

xfsdump: dump size (non-dir files) : 2592 bytes

xfsdump: dump complete: 0 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/sdb1_v2 OK (success)

xfsdump: Dump Status: SUCCESS

3）指定只备份分区中某个目录或文件

-s [文件路径] 只对指定的文件或目录进行备份，路径是相对路径

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[root@xuexi sdb1]# ls /boot/grub2/

device.map  fonts  grub.cfg  grub.cfg.1544939406.rpmsave  grubenv  i386-pc  locale

[root@xuexi sdb1]# xfsdump -f /dump/grub2_grub.cfg -s grub2/grub.cfg /boot -L dump_grub2_grub.cfg_20181228 -M boot-sda1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsdump: level 0 dump of xuexi:/boot

xfsdump: dump date: Fri Dec 28 15:43:25 2018

xfsdump: session id: 6c2c5768-7389-46be-ab7b-11ceaf19c2b5

xfsdump: session label: "dump_grub2_grub.cfg_20181228"

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: skipping (no pruning necessary)

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 29632 bytes

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 28848 bytes

xfsdump: dump size (non-dir files) : 5152 bytes

xfsdump: dump complete: 0 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/grub2_grub.cfg OK (success)

xfsdump: Dump Status: SUCCESS

4）查看备份信息和内容

备份成功后，我们可以在/var/lib/xfsdump/inventory目录下看到生成的档案信息

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[root@xuexi sdb1]# ls /var/lib/xfsdump/inventory/  

02c76feb-497b-4167-8e72-f8b81a79dbf9.InvIndex  43384fc8-7ba5-4f18-b9f9-246e4992962b.StObj  fstab

1eccfe79-a9eb-4f09-83c1-f28aa64db1ce.InvIndex  afe37eaa-9e10-4243-ad45-36aadc180d58.StObj

[root@xuexi sdb1]# xfsdump –I（大写的i） //查看所有备份信息

注意：使用xfsdump时有以下几个限制：

1.xfsdump不支持没有挂载的文件系统备份！

2.xfsdump必须使用root权限才能操作（涉及文件系统的关系）

3.xfsdump只能备份xfs文件系统

4.xfsdump备份下来的数据（档案或存储媒体）只能让xfsrestore解析

5.xfsdump是通过文件系统的UUID来分辨系统各个备份档的，英雌不能备份两个具有相同UUID的文件系统

(3).文件系统恢复

xfsrestore –f [指定需要恢复的备份文件路径] [指定恢复后文件的存放路径]

1）测试恢复：先删除之前创建的内容

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[root@xuexi sdb1]# ls

passwd  test

[root@xuexi sdb1]# pwd

/sdb1

[root@xuexi sdb1]# rm -rf ./*

2）恢复整个分区

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[root@xuexi sdb1]# xfsrestore -f /dump/sdb1 /sdb1/

xfsrestore: using file dump (drive_simple) strategy

xfsrestore: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsrestore: searching media for dump

xfsrestore: examining media file 0

xfsrestore: dump description:

xfsrestore: hostname: xuexi

xfsrestore: mount point: /sdb1

xfsrestore: volume: /dev/sdb1

xfsrestore: session time: Fri Dec 28 14:11:55 2018

xfsrestore: level: 0

xfsrestore: session label: "dump_sdb1_20181228"

xfsrestore: media label: "sdb1"

xfsrestore: file system id: 1eccfe79-a9eb-4f09-83c1-f28aa64db1ce

xfsrestore: session id: a1afa104-8f0c-42fa-be6e-594b674f13c7

xfsrestore: media id: 8db0ba63-465f-42ba-8581-7cc22a24b902

xfsrestore: using online session inventory

xfsrestore: searching media for directory dump

xfsrestore: reading directories

xfsrestore: 2 directories and 3 entries processed

xfsrestore: directory post-processing

xfsrestore: restoring non-directory files

xfsrestore: restore complete: 0 seconds elapsed

xfsrestore: Restore Summary:

xfsrestore:   stream 0 /dump/sdb1 OK (success)

xfsrestore: Restore Status: SUCCESS

[root@xuexi sdb1]# ls /sdb1/

passwd  test

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── passwd

└── test

    └── a

1 directory, 2 files

3）恢复单个文件

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[root@xuexi sdb1]# ls

passwd  test

[root@xuexi sdb1]# xfsrestore -f /dump/grub2_grub.cfg -s grub2/grub.cfg /sdb1/

xfsrestore: using file dump (drive_simple) strategy

xfsrestore: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsrestore: searching media for dump

xfsrestore: examining media file 0

xfsrestore: dump description:

xfsrestore: hostname: xuexi

xfsrestore: mount point: /boot

xfsrestore: volume: /dev/sda1

xfsrestore: session time: Fri Dec 28 15:43:25 2018

xfsrestore: level: 0

xfsrestore: session label: "dump_grub2_grub.cfg_20181228"

xfsrestore: media label: "boot-sda1"

xfsrestore: file system id: 02c76feb-497b-4167-8e72-f8b81a79dbf9

xfsrestore: session id: 6c2c5768-7389-46be-ab7b-11ceaf19c2b5

xfsrestore: media id: dc813998-6f28-4686-96c2-d844525cecff

xfsrestore: using online session inventory

xfsrestore: searching media for directory dump

xfsrestore: reading directories

xfsrestore: 2 directories and 24 entries processed

xfsrestore: directory post-processing

xfsrestore: restoring non-directory files

xfsrestore: restore complete: 0 seconds elapsed

xfsrestore: Restore Summary:

xfsrestore:   stream 0 /dump/grub2_grub.cfg OK (success)

xfsrestore: Restore Status: SUCCESS

[root@xuexi sdb1]# ls

grub2  passwd  test

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── grub2

│   └── grub.cfg

├── passwd

└── test

    └── a

2 directories, 3 files

注意：

[root@xuexi sdb1]# xfsrestore -f /dump/grub2_grub.cfg  -s grub2/grub.cfg /sdb1/

[root@xuexi sdb1]# xfsrestore -f /dump/grub2_grub.cfg  -s grub2 /sdb1/

这两个命令相同效果

(4).增量备份

开始之前看一下备份目录

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[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── grub2

│   └── grub.cfg

├── passwd

└── test

    └── a

2 directories, 3 files

1）  进行第一次完整备份

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[root@xuexi sdb1]# xfsdump -f /dump/full_backup /sdb1 -L full_backup_sdb1 -M sdb1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsdump: level 0 dump of xuexi:/sdb1

xfsdump: dump date: Fri Dec 28 16:54:44 2018

xfsdump: session id: d3809f53-bc01-4138-afd1-98e436fbc48a

xfsdump: session label: "full_backup_sdb1"

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: skipping (no pruning necessary)

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 34688 bytes

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 32312 bytes

xfsdump: dump size (non-dir files) : 7744 bytes

xfsdump: dump complete: 0 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/full_backup OK (success)

xfsdump: Dump Status: SUCCESS

2）  进行第一次增量备份

注意：-l [备份级别]，设置备份级别

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[root@xuexi sdb1]# touch /sdb1/{1.txt,2.txt}    //新增两个文件

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── 1.txt

├── 2.txt

├── grub2

│   └── grub.cfg

├── passwd

└── test

    └── a

2 directories, 5 files

[root@xuexi sdb1]# xfsdump -l 1 -f /dump/incremental_backup1 /sdb1 -L incremental_backup1_sdb1 -M sdb1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsdump: level 1 incremental dump of xuexi:/sdb1 based on level 0 dump begun Fri Dec 28 16:54:44 2018

xfsdump: dump date: Fri Dec 28 16:59:54 2018

xfsdump: session id: 0dd47814-2901-4301-99dc-5203e9a3dac0

xfsdump: session label: "incremental_backup1_sdb1"

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: pruning unneeded subtrees

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 21440 bytes

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 22704 bytes

xfsdump: dump size (non-dir files) : 0 bytes

xfsdump: dump complete: 0 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/incremental_backup1 OK (success)

xfsdump: Dump Status: SUCCESS

3）  进行第二次增量备份

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[root@xuexi sdb1]# touch /sdb1/test/a.txt /sdb1/b.txt

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── 1.txt

├── 2.txt

├── b.txt

├── grub2

│   └── grub.cfg

├── passwd

└── test

    ├── a

    └── a.txt

2 directories, 7 files

[root@xuexi sdb1]# xfsdump -l 2 -f /dump/incremental_backup2 /sdb1 -L incremental_backup2_sdb1 -M sdb1

xfsdump: using file dump (drive_simple) strategy

xfsdump: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsdump: level 2 incremental dump of xuexi:/sdb1 based on level 1 dump begun Fri Dec 28 16:59:54 2018

xfsdump: dump date: Fri Dec 28 17:05:21 2018

xfsdump: session id: 6d59a61c-5e5b-476d-ac8d-07b7610355ae

xfsdump: session label: "incremental_backup2_sdb1"

xfsdump: ino map phase 1: constructing initial dump list

xfsdump: ino map phase 2: pruning unneeded subtrees

xfsdump: ino map phase 3: skipping (only one dump stream)

xfsdump: ino map construction complete

xfsdump: estimated dump size: 21760 bytes

xfsdump: creating dump session media file 0 (media 0, file 0)

xfsdump: dumping ino map

xfsdump: dumping directories

xfsdump: dumping non-directory files

xfsdump: ending media file

xfsdump: media file size 23400 bytes

xfsdump: dump size (non-dir files) : 0 bytes

xfsdump: dump complete: 0 seconds elapsed

xfsdump: Dump Summary:

xfsdump:   stream 0 /dump/incremental_backup2 OK (success)

xfsdump: Dump Status: SUCCESS

4）  恢复

步骤：

1.先恢复完整备份

2.如果两次备份都是1级的，那么只需要恢复最后一次增量备份；但如果第一次是1级备份，第二次是2级备份，那么需要先恢复完整备份，然后恢复1级备份，最后2级备份

实际操作：

删除，测试恢复

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[root@xuexi sdb1]# rm -rf /sdb1/*

先恢复完整备份

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[root@xuexi sdb1]# xfsrestore -f /dump/full_backup  /sdb1/

xfsrestore: using file dump (drive_simple) strategy

xfsrestore: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsrestore: searching media for dump

xfsrestore: examining media file 0

xfsrestore: dump description:

xfsrestore: hostname: xuexi

xfsrestore: mount point: /sdb1

xfsrestore: volume: /dev/sdb1

xfsrestore: session time: Fri Dec 28 16:54:44 2018

xfsrestore: level: 0

xfsrestore: session label: "full_backup_sdb1"

xfsrestore: media label: "sdb1"

xfsrestore: file system id: 1eccfe79-a9eb-4f09-83c1-f28aa64db1ce

xfsrestore: session id: d3809f53-bc01-4138-afd1-98e436fbc48a

xfsrestore: media id: 3f4eeb44-b277-4df8-a7ba-de162e8a5c3e

xfsrestore: using online session inventory

xfsrestore: searching media for directory dump

xfsrestore: reading directories

xfsrestore: 3 directories and 5 entries processed

xfsrestore: directory post-processing

xfsrestore: restoring non-directory files

xfsrestore: restore complete: 0 seconds elapsed

xfsrestore: Restore Summary:

xfsrestore:   stream 0 /dump/full_backup OK (success)

xfsrestore: Restore Status: SUCCESS

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── grub2

│   └── grub.cfg

├── passwd

└── test

    └── a

2 directories, 3 files

恢复1级备份

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[root@xuexi sdb1]# xfsrestore -f /dump/incremental_backup1 /sdb1/

xfsrestore: using file dump (drive_simple) strategy

xfsrestore: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsrestore: searching media for dump

xfsrestore: examining media file 0

xfsrestore: dump description:

xfsrestore: hostname: xuexi

xfsrestore: mount point: /sdb1

xfsrestore: volume: /dev/sdb1

xfsrestore: session time: Fri Dec 28 16:59:54 2018

xfsrestore: level: 1

xfsrestore: session label: "incremental_backup1_sdb1"

xfsrestore: media label: "sdb1"

xfsrestore: file system id: 1eccfe79-a9eb-4f09-83c1-f28aa64db1ce

xfsrestore: session id: 0dd47814-2901-4301-99dc-5203e9a3dac0

xfsrestore: media id: dc9c4bed-671c-4199-9660-31b6a3c560b7

xfsrestore: using online session inventory

xfsrestore: searching media for directory dump

xfsrestore: reading directories

xfsrestore: 1 directories and 5 entries processed

xfsrestore: directory post-processing

xfsrestore: restoring non-directory files

xfsrestore: restore complete: 0 seconds elapsed

xfsrestore: Restore Summary:

xfsrestore:   stream 0 /dump/incremental_backup1 OK (success)

xfsrestore: Restore Status: SUCCESS

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── 1.txt

├── 2.txt

├── grub2

│   └── grub.cfg

├── passwd

└── test

    └── a

2 directories, 5 files

恢复2级备份

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[root@xuexi sdb1]# xfsrestore -f /dump/incremental_backup2 /sdb1/

xfsrestore: using file dump (drive_simple) strategy

xfsrestore: version 3.1.7 (dump format 3.0) - type ^C for status and control

xfsrestore: searching media for dump

xfsrestore: examining media file 0

xfsrestore: dump description:

xfsrestore: hostname: xuexi

xfsrestore: mount point: /sdb1

xfsrestore: volume: /dev/sdb1

xfsrestore: session time: Fri Dec 28 17:05:21 2018

xfsrestore: level: 2

xfsrestore: session label: "incremental_backup2_sdb1"

xfsrestore: media label: "sdb1"

xfsrestore: file system id: 1eccfe79-a9eb-4f09-83c1-f28aa64db1ce

xfsrestore: session id: 6d59a61c-5e5b-476d-ac8d-07b7610355ae

xfsrestore: media id: daf6acdb-921f-437e-a33b-538e47c0ca1a

xfsrestore: using online session inventory

xfsrestore: searching media for directory dump

xfsrestore: reading directories

xfsrestore: 2 directories and 8 entries processed

xfsrestore: directory post-processing

xfsrestore: restoring non-directory files

xfsrestore: restore complete: 0 seconds elapsed

xfsrestore: Restore Summary:

xfsrestore:   stream 0 /dump/incremental_backup2 OK (success)

xfsrestore: Restore Status: SUCCESS

[root@xuexi sdb1]# tree /sdb1/

/sdb1/

├── 1.txt

├── 2.txt

├── b.txt

├── grub2

│   └── grub.cfg

├── passwd

└── test

    ├── a

    └── a.txt

2 directories, 7 files

(5).扩展：

　　完整备份：每次都将所有数据（不管自第一次备份以来有没有修改过），进行一次完整的复制，备份后会清除文件的存档属性，方便日后增量备份或者差异备份进行版本比较。

　　特点：占用空间大，备份速度慢，但恢复时一次恢复到位，恢复速度快。

 

　　增量备份：在第一次完整备份之后，第二次开始每次都将添加了存档属性的文件进行备份，并且在备份之后再把这些存档属性清除。为什么要清除存档属性呢？这就是为了下一次备份的时候判断是否有文件变化，因为用户在每次备份以后修改这些被清除存档属性的文件，存档属性就会自动加上，相当于用户告诉系统，这些文件有变化，你下一次就备份这些文件，其他没有存档属性的就不需要备份，这就是增量备份的工作机制。

　　类比：相当于机器人把地板打扫干净了，你踩过，就会有脚印（增加标记），下次机器人就把脚印记录下来，并且把脚印打扫干净（清除标记），始终保持地板干净。机器人每次记录并打扫的脚印就相当于每次增量备份的内容

　　特点：因每次仅备份自上一次备份（注意是上一次，不是第一次）以来有变化的文件，所  以备份体积小，备份速度快，但是恢复的时候，需要按备份时间顺序，逐个备份版本进行恢复，恢复时间长。

 

　　差异备份：在第一次完整备份之后，第二次开始每次都将所有文件与第一次完整备份的文件做比较，把自第一次完整备份以来所有修改过的文件进行备份，且以后每次备份都是和第一次完整备份进行比较（注意是第一次，不是上一次），备份自第一次完整备份以来所有的修改过的文件。因此，差异备份在备份完毕之后不需要清除文件的存档属性，因为这些文件和下一次备份没有什么关系，它仅仅和第一次完整备份的数据进行比较（第一次完整备份之后是清除存档属性的）。

　　类比：相当于第一次机器人把地板打扫干净了，你踩过，就会有脚印，机器人就把脚印记录下来，但不打扫，下次你又有踩脏的，机器人就把你这几次所有踩脏的地方都记录下来，始终不打扫，每次都这样。机器人每次记录的内容就相当于差异备份的内容

       特点：占用空间比增量备份大，比完整备份小，恢复时仅需要恢复第一个完整版本和最后一次的差异版本，恢复速度介于完整备份和增量备份之间。

 

　　简单的讲，完整备份就是不管三七二十一，每次都把指定的备份目录完整的复制一遍，不管目录下的文件有没有变化；增量备份就是每次将之前（第一次、第二次、直到前一次）做过备份之后有变化的文件进行备份；差异备份就是每次都将第一次完整备份以来有变化的文件进行备份。

 

举例：

　　假设指定备份目录周一包含A、B、C三个文件。周一的时候，做了完整备份。周二新增了D文件，同时A文件发生变化，变成A1文件；周三新增了E文件，同时A文件变成了A2，B文件变成B1；周四新增了F文件，A2变成了A3，D变成了D1，同时删掉了C文件。

　　不同备份方式下的备份情况（假设每天做一次备份）：

       完整备份：周一，备份ABC三个文件；周二，备份A1、B、C、D四个文件；周三，备份A2、B1、C、D、E五个文件；周四，备份A3、B1、D1、E、F五个文件。

　　增量备份：周一，备份ABC三个文件，完整备份；周二，备份与周一相比有变化的文件，即备份A1和D两个文件；周三，备份与之前所有版本相比有变化的文件，即备份A2，B1，E三个文件；周四，备份与之前所有版本相比有变化的文件，即备份A3，D1，F三个文件并删除C。

　　差异备份：周一，备份ABC三个文件，完整备份；周二，备份与周一相比有变化的文件，即备份A1和D两个文件；周三，备份与周一相比有变化的文件，即备份A2、B1、D、E四个文件；周四，备份与周一相比有变化的文件，即备份A3、B1、D1、E、F五个文件并删除C。

本文标签： 文件系统 备份 xfs

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