一、Btrfs文件系統簡介
Btrfs透明壓縮文件系統 (B-tree, Butter FS, Better FS)是一種COW(copy-on-write式)文件系統，有着傳統文件系統(ext3/4)所沒有的一些特性，如支持可寫的磁盤快照(snapshots)，以及支持遞歸的快照(snapshots of snapshots)，支持內建磁盤陣列（RAID），支持子卷(Subvolumes)的概念，允許在線調整文件系統大小等。
二、核心特性及命令
核心特性：
1、多物理卷支持：btrfs可由多個底層物理卷組成；支持RAID，以聯機“添加”、“移除”，“修改”；
2、寫時複製更新機制(CoW)：複製、更新及替換指針，而非“就地”更新；
3、數據及元數據校驗碼：checksum
4、子卷：sub_volume，子卷實質上是一個保存文件和目錄的命名的B樹。它們的inode保存在樹根之樹中，可以爲非根用戶和組所有。子卷可選設定塊配額。子卷內的所有塊和文件區段都有引用計數以便做快照。和虛擬機存儲的動態擴展相似，其只按需使用設備空間，消除了許多半滿的分區。用戶也可用不同的掛載選項掛載子卷，得到更靈活的安全性。
5、快照：支持快照的快照； 命令：btrfs subvolume snapshot
6、透明壓縮：分區裏的每個文件都被自動壓縮，不僅減小了文件的大小，還提高了性能。
7、Btrfs支持在線碎片整理。命令： btrfs filesystem defragment
相關命令：
1、文件系統創建：
命令：mkfs.btrfs
選項：-L 'LABEL'
-d <type>: raid0, raid1, raid5, raid6, raid10, single
-m <profile>: raid0, raid1, raid5, raid6, raid10, single, dup
-O <feature>
-O list-all: 列出支持的所有feature；
2、文件系統命令管理，包括卷標管理，文件系統大小調整，及碎片管理
命令：btrfs filesystem <subcommand> <args>
屬性查看：btrfs filesystem show
3、掛載文件系統：
命令：mount -t btrfs /dev/sdb MOUNT_POINT
4、透明壓縮機制：
命令：mount -o compress={lzo|zlib} DEVICE MOUNT_POINT
5、 管理子卷和快照命令
命令：btrfs subvolume <subcommand> [<args>]
6、設備管理命令
命令：btrfs device <subcommand> <args>
7、塊組均衡管理
命令：btrfs [filesystem] balance <subcommand>|<args>
三、實驗
1、創建btrfs文件系統
在vmware中新增兩塊20G磁盤，/dev/sdb與/dev/sdc
[root@localhost ~]# mkfs.btrfs -L 'B-tree-fs' -f /dev/sd{b,c}
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 2 FS bytes used 112.00KiB
devid 1 size 20.00GiB used 2.03GiB path /dev/sdb
devid 2 size 20.00GiB used 2.01GiB path /dev/sdc
2、掛載文件系統
[root@localhost ~]# mkdir /btrdata
[root@localhost ~]# mount /dev/sdb /btrdata
[root@localhost ~]# df -h | grep btrdata
/dev/sdb 40G 1.0M 38G 1% /btrdata
3、建立子卷
[root@localhost ~]# btrfs subvolume create /btrdata/mydata
Create subvolume '/btrdata/mydata'
[root@localhost ~]# cp -r /root/* /btrdata/mydata # 複製數據
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 2 FS bytes used 735.67MiB
devid 1 size 20.00GiB used 2.03GiB path /dev/sdb
devid 2 size 20.00GiB used 2.01GiB path /dev/sdc
4、擴展文件系統
[root@localhost ~]# btrfs device add /dev/sdd /btrdata
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 3 FS bytes used 735.67MiB
devid 1 size 20.00GiB used 2.03GiB path /dev/sdb
devid 2 size 20.00GiB used 2.01GiB path /dev/sdc
devid 3 size 20.00GiB used 0.00 path /dev/sdd
[root@localhost ~]# df -h | grep btrdata
/dev/sdb 60G 772M 56G 2% /btrdata
5、重新均衡文件系統
[root@localhost ~]# btrfs balance start /btrdata
Done, had to relocate 6 out of 6 chunks
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 3 FS bytes used 736.20MiB
devid 1 size 20.00GiB used 1.03GiB path /dev/sdb
devid 2 size 20.00GiB used 2.00GiB path /dev/sdc
devid 3 size 20.00GiB used 2.03GiB path /dev/sdd
6、移除硬盤
移除硬盤的時候會先把數據移動到其他盤，不需要像LVM那樣需要手動移動數據
[root@localhost ~]# btrfs device delete /dev/sdb /btrdata
[root@localhost ~]# df -h | grep btrdata
/dev/sdc 40G 772M 38G 2% /btrdata
[root@localhost ~]# ls /btrdata/
commdir mydata
[root@localhost ~]# head /btrdata/mydata/anaconda-ks.cfg
#version=RHEL7
# System authorization information
auth --enableshadow --passalgo=sha512
# Use CDROM installation media
cdrom
# Use graphical install
graphical
# Run the Setup Agent on first boot
firstboot --enable
7、修改數據或元數據的RAID級別
[root@localhost ~]# btrfs device add /dev/sdb /btrdata
[root@localhost ~]# btrfs filesystem df /btrdata
Data, RAID0: total=2.00GiB, used=699.96MiB
System, RAID1: total=32.00MiB, used=16.00KiB
Metadata, RAID1: total=1.00GiB, used=35.98MiB
GlobalReserve, single: total=16.00MiB, used=0.00
[root@localhost ~]# btrfs balance start -mconvert=raid5 /btrdata # 此處修改元數據RAID級別
Done, had to relocate 2 out of 3 chunks
[root@localhost ~]# btrfs filesystem df /btrdata
Data, RAID0: total=2.00GiB, used=700.21MiB
System, RAID5: total=64.00MiB, used=16.00KiB
Metadata, RAID5: total=1.00GiB, used=35.91MiB
GlobalReserve, single: total=16.00MiB, used=0.00
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 3 FS bytes used 735.89MiB
devid 2 size 20.00GiB used 1.53GiB path /dev/sdc
devid 3 size 20.00GiB used 1.53GiB path /dev/sdd
devid 4 size 20.00GiB used 544.00MiB path /dev/sdb
8、創建快照
[root@localhost ~]# btrfs subvolume snapshot /btrdata/mydata /btrdata/mydata_snapshot
Create a snapshot of '/btrdata/mydata' in '/btrdata/mydata_snapshot'
[root@localhost ~]# cat /btrdata/mydata/1.txt
1
[root@localhost ~]# echo 'just a test' >> /btrdata/mydata/1.txt
[root@localhost ~]# cat /btrdata/mydata/1.txt
1
just a test
[root@localhost ~]# cat /btrdata/mydata_snapshot/1.txt
1
9、轉換
[root@localhost ~]# btrfs balance start -dconvert=single -mconvert=raid1 /btrdata
Done, had to relocate 3 out of 3 chunks
[root@localhost ~]# btrfs device delete /dev/sdd /btrdata
[root@localhost ~]# fdisk /dev/sdd
Welcome to fdisk (util-linux 2.23.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Device does not contain a recognized partition table
Building a new DOS disklabel with disk identifier 0xadd41340.
Command (m for help): n
Partition type:
p primary (0 primary, 0 extended, 4 free)
e extended
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-41943039, default 2048):
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-41943039, default 41943039): +5G
Partition 1 of type Linux and of size 5 GiB is set
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
[root@localhost ~]# mke2fs -t ext4 /dev/sdd1
mke2fs 1.42.9 (28-Dec-2013)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
327680 inodes, 1310720 blocks
65536 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=1342177280
40 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
[root@localhost ~]# fsck -f /dev/sdd1
fsck from util-linux 2.23.2
e2fsck 1.42.9 (28-Dec-2013)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/sdd1: 12/327680 files (0.0% non-contiguous), 58463/1310720 blocks
[root@localhost ~]# btrfs-convert /dev/sdd1 # 將ext4文件系統轉換爲btrfs
creating btrfs metadata.
creating ext2fs image file.
cleaning up system chunk.
conversion complete.
[root@localhost ~]# btrfs filesystem show
Label: 'B-tree-fs' uuid: b254b565-0418-4601-8317-f9a5306c6f19
Total devices 2 FS bytes used 735.81MiB
devid 2 size 20.00GiB used 2.03GiB path /dev/sdc
devid 4 size 20.00GiB used 1.03GiB path /dev/sdb
Label: none uuid: 12170ec7-9847-4a5a-85ee-13215a478a3c
Total devices 1 FS bytes used 228.42MiB
devid 1 size 5.00GiB used 5.00GiB path /dev/sdd1
Btrfs v3.16.2
[root@localhost ~]# btrfs-convert -r /dev/sdd1 # 將btrfs文件系統回滾原來的文件系統
rollback complete.
[root@localhost ~]# blkid /dev/sdd1
/dev/sdd1: UUID="fbbd653f-fb3f-4715-8c42-8c942564c8f4" TYPE="ext4"