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btrfs文件系统简介及CentOS7下实例由Oracle于2007年宣布并进行中的COW(copy-on-write式)文件系统。目标是取代Linux目前的ext3文件系统，改善ext3的限制，特别是单一文件大小的限制，总文件系统大小限制以及加入文件校验和特性。加入目前ext3/4未支持的一些功能，例如可写的磁盘快照(snapshots)，以及支持递归的快照(snapshots of snapshots)，内建磁盘阵列（RAID）支持，支持子卷(Subvolumes)的概念，允许在线调整文件系统大小。（自百度百科）以下自马哥课程笔记和CentOS7帮助文档：核心特性：多物理卷支持：btrfs可由多个底层物理卷组成；支持RAID，以联机“添加”、“移除”，“修改”；支持写时复制更新机制(CoW)：复制、更新及替换指针，而非“就地”更新；支持数据及元数据校验码：checksum支持子卷：sub_volume支持快照：支持快照的快照；支持透明压缩；命令介绍：文件系统创建：mkfs.btrfs-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；属性查看：btrfs filesystem show [--mounted|--all-devices|<path>|<uuid>|<device>|<label>]orblkid [DEVICE]管理命令介绍：btrfs文件系统管理命令使用子命令的方式进行管理，具体了命令的功能可通过man SUBCOMMEND来查看。btrfs命令的常用子命令：filesystem, device, balance, subvolumebtrfs:NAME btrfs - control a btrfs filesystemSYNOPSIS btrfs <command> [<args>]COMMANDSbalance Balance btrfs filesystem chunks across single or several devices. See btrfs-balance(8) for details.check Do off-line check on a btrfs filesystem. See btrfs-check(8) for details.device Manage devices managed by btrfs, including add/delete/scan and so on. See btrfs-device(8) for details.filesystem Manage a btrfs filesystem, including label setting/sync and so on. See btrfs-filesystem(8) for details. receive Receive subvolume data from stdin/file for restore and etc. See btrfs-receive(8) for details.replace Replace btrfs devices. See btrfs-replace(8) for details. subvolume Create/delete/list/manage btrfs subvolume. See btrfs-subvolume(8) for details. btrfs filesystem:NAME btrfs-filesystem - control btrfs filesystemSYNOPSIS btrfs filesystem <subcommand> <args>SUBCOMMANDdf [options] <path> Show space usage information for a mount point.show [--mounted|--all-devices|<path>|<uuid>|<device>|<label>]Show the btrfs filesystem with some additional info.resize [<devid>:][+/-]<size>[kKmMgGtTpPeE]|[<devid>:]max <path>Resize a filesystem identified by <path> for the underlying device devid online.不常用，一般调整子卷。label [<dev>|<mountpoint>] [<newlabel>]Show or update the label of a filesystem.btrfs device:NAME btrfs-device - control btrfs devicesSYNOPSIS btrfs device <subcommand> <args> DEVICE MANAGEMENT RAID level Btrfs filesystem supports most of the standard RAID level: 0/1/5/6/10.Balance btrfs-balance(8) subcommand can be used to balance or rebuild chunks to the desired profile.Device add/remove/replace Device can be added/removed using btrfs-replace(8) subcommand and replaced using btrfs-replace(8). When device is removed or replaced, btrfs will do the chunk rebuild if needed. SUBCOMMANDadd [-Kf] <dev> [<dev>...] <path> Add device(s) to the filesystem identified by <path>.delete <dev> [<dev>...] <path> Remove device(s) from a filesystem identified by <path>. 拆除物理卷时会自动迁移要移除的物理卷中的数据。ready <device> Check device to see if it has all of it’s devices in cache for mounting.scan [(--all-devices|-d)|<device> [<device>...]] Scan devices for a btrfs filesystem.stats [-z] <path>|<device> Read and print the device IO stats for all devices of the filesystem identified by <path> or for a single <device>. btrfs balance:NAME btrfs-balance - balance btrfs filesystemSYNOPSIS btrfs balance <subcommand> <args> SUBCOMMANDcancel <path> Cancel running or paused balance.pause <path> Pause running balance.resume <path> Resume interrupted balance.start [options] <path> Balance chunks across the devices online. Options: -d[<filters>] act on data chunks. See FILTERS section for details about <filters>. 修改数据的组织机制。 -m[<filters>] act on metadata chunks. See FILTERS section for details about <filters>. 修改元数据的组织机制。 -s[<filters>] act on system chunks (only under -f). See FILTERS section for details about <filters>. -v be verbose -f force reducing of metadata integritystatus [-v] <path> Show status of running or paused balance. If -v option is given, output will be verbose.FILTERSconvertConvert each selected block group to the given profile name identified by parameters.btrfs subvolume:NAME btrfs-subvolume - control btrfs subvolume(s)SYNOPSIS btrfs subvolume <subcommand> [<args>]DESCRIPTION btrfs subvolume is used to control the filesystem to create/delete/list/show subvolumes and snapshots.SUBCOMMANDcreate [-i <qgroupid>] [<dest>]<name> Create a subvolume <name> in <dest>. If <dest> is not given, subvolume <name> will be created in the currently directory.Options -i <qgroupid> Add the newly created subvolume to a qgroup. This option can be given multiple times.delete [options] <subvolume> [<subvolume>...] -c|--commit-after wait for transaction commit at the end of the operation -C|--commit-each wait for transaction commit after delet each subvolumelist <path> List the subvolumes present in the filesystem <path>.snapshot [-r] <source> <dest>|[<dest>/]<name> 挂载文件系统：mount -t btrfs /dev/sdb MOUNT_POINT启用透明压缩机制：压缩和解压缩自动透明运行，启用后会增加CPU负载mount -o compress={lzo|zlib} DEVICE MOUNT_POINTbtrfs文件系统管理实例：（1）、CentOS7中加载磁盘设备（多块）；如/dev/sd{b,c,d}；（2）、将硬盘格式化为btrfs文件系统；命令：# mkfs.btrfs -L MYDATA /dev/sd{b,c}查看：# btrfs filesystem show（3）、挂载btrfs文件系统；命令：# mount -t btrfs /dev/sdc /mydata/设备可为任意一个btrfs设备名，如/dev/sdb或/dev/sdc启用透明压缩机制（可选）：# mount -t btrfs -o compress=lzo /dev/sdc /mydata/（4）、在线调整btrfs文件系统空间大小；命令：# btrfs filesystem resize -20G /mydata/（5）、btrfs文件系统中添加/删除磁盘设备；添加设备命令：# btrfs device add /dev/sdb /mydata/删除设备命令：# btrfs device delete /dev/sdd /mydata/（6）、调整数据分部，使数据均衡的分部到各磁盘设备；启动balance：# btrfs balance start /mydata/查看balance运行时的状态：# btrfs balance status /mydata/（7）、动态修改磁盘设备卷的RAID级别；修改数据的RIAD级别为RAID5：# btrfs balance start -dconvert=raid5 /mydata/修改元数据的RIAD级别为RAID5：# btrfs balance start -mconvert=raid5 /mydata/检查：# btrfs filesystem df /mydata/（8）、btrfs文件系统上创建和管理子卷；查看子卷信息：# btrfs subvolume list /mydata/创建子卷，结果类似于在父卷下创建子目录：#btrfssubvolumecreate/mydata/binID269gen133toplevel5pathbin//ID269为该子卷的ID号单独挂载子卷，如果挂载父卷，则父卷下的所有子卷都会自动挂载：1) # umount /mydata///首先卸载父卷2) # mount -o subvol=logs /dev/sdb /mnt///使用子卷名挂载or# mount -o subvolid=269 /dev/sdb /mnt///使用子卷ID挂载查看子卷的详细信息：#btrfssubvolumeshow/mnt删除子卷：#btrfssubvolumedelete/mydata/bin/（9）、btrfs文件系统中子卷和文件快照的管理和使用；创建子卷快照：# btrfs subvolume snapshot /mydata/logs/ /mydata/logs_snapshot//子卷的快照必须和该子卷在同一父卷下。查看子卷是否生成：# btrfs subvolume list /mydata/挂载/删除btrfs文件系统子卷快照的方法和挂载/删除子卷一致：挂载子卷：#mount-osubvolid=269/dev/sdb/mnt/删除子卷快照：#btrfssubvolumedelete/mydata/logs_snapshot/创建文件快照：#cp--reflink/mydata/fstab/mydata/fstab_snap（10）、ext4和btrfs文件系统转换；ext4 to btrfs:1） 首先卸载原ext4文件系统的磁盘，并强制检测：#umount/mnt/;fsck-f/dev/sdd12） 使用btrfs-convert转换文件系统：#btrfs-convert/dev/sdd1creatingbtrfsmetadata.copyinodes[o][0/12]creatingext2fsimagefile.cleaningupsystemchunk.conversioncomplete.3） 检查btrfs文件系统是否完成转换：#btrfsfilesystemshow4） 挂载使用：#mount/dev/sdd1/mnt/btrfs to ext4:1） 卸载文件系统：# umount /mnt/2）btrfs降级为ext4：#btrfs-convert-r/dev/sdd1rollbackcomplete.3） 检查：#blkid/dev/sdd1/dev/sdd1:UUID="af904001-4dac-4b10-87b5-8ff9c06f9e36"TYPE="ext4"（11）、拆除btrfs文件系统；1） 拆除前首先需要确认当前数据和无数据的RAID是否满足RAID的最少设备限制，若不满足修改之。若全部拆除可修改为single。查看当前设备卷的RAID状态：#btrfsfilesystemdf/mydata/Data,RAID5:total=2.00GiB,used=788.00KiBSystem,RAID5:total=64.00MiB,used=16.00KiBMetadata,RAID5:total=1.00GiB,used=144.00KiBGlobalReserve,single:total=16.00MiB,used=0.00B修改数据和元数据的RAID级别：#btrfsbalancestart-mconvert=single/mydata/#btrfsbalancestart-f-mconvert=single/mydata/检查：#btrfsfilesystemdf/mydata/Data,single:total=1.00GiB,used=532.00KiBSystem,single:total=32.00MiB,used=16.00KiBMetadata,single:total=1.00GiB,used=144.00KiBGlobalReserve,single:total=16.00MiB,used=0.00B2） 删除btrfs卷中的磁盘卷：#btrfsdevicedelete/dev/sdd/mydata/
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