Linux下使用mdadm创建和管理软raid 注：本次操作以RHEL4为例，但应该可以应用到其它大部分的distro上（guess）。
mdadm的几个常用参数
- -C 创建Raid，后面跟参数，代表raid设备的名称。比如：/dev/md0，/dev/md1。
- -n 用于创建磁盘阵列的磁盘个数。
- -l Raid的级别。
- -x 指定用于hotspare（热备盘）的磁盘个数。如果阵列中有一块硬盘坏了，它会立刻顶上，并rebuild；
- -D 显示软raid的详细信息；
- -s 扫描配置文件（/etc/mdadm.conf）或'/proc/mdstat'来查看遗漏的信息f；
创建软raid的大体流程
- 使用fdisk工具为新磁盘创建分区；
- 使用mkfs.XXXX工具将刚才划分好的分区格式化成某种格式的文件系统。比如：ext3，reiserfs等；
- 使用mdadm来创建软raid；
- 创建/etc/mdadm.conf文件（注意文件的格式，包括是否有逗号等等。该文件是为了系统在重启后能够自动启用软raid。可以查看/etc/rc.sysinit脚本，搜索'mdadm'字符串就明白了）；
示例：创建软raid5（+hotspare）以下是我的一次实际操作的完整过程：
- 这是用'fdisk -l'命令查看到的我当前的磁盘和分区情况（只有/dev/sda在使用，其它四个都是新磁盘，没有分区，没有格式化）：
# fdisk -l
Disk /dev/sda: 6442 MB, 6442450944 bytes
255 heads, 63 sectors/track, 783 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 720 5783368+ 83 Linux
/dev/sda2 721 783 506047+ 82 Linux swap
Disk /dev/sdb: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk /dev/sdb doesn't contain a valid partition table
Disk /dev/sdc: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk /dev/sdc doesn't contain a valid partition table
Disk /dev/sdd: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk /dev/sdd doesn't contain a valid partition table
Disk /dev/sde: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Disk /dev/sde doesn't contain a valid partition table
- 使用fdisk创建分区（本例中将整块磁盘划分为一个主分区。其余几块磁盘也做相同的操作。）：
#
fdisk /dev/sdbDevice contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)
Command (m for help):
nCommand action
e extended
p primary partition (1-4)
pPartition number (1-4):
1First cylinder (1-204, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-204, default 204):
Using default value 204
Command (m for help):
wThe partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
- 为刚才新建的分区建立文件系统（其余几个分区依次做相同的操作）：
# mkfs.ext3 /dev/sdb1
mke2fs 1.35 (28-Feb-2004)
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
52416 inodes, 208880 blocks
10444 blocks (5.00%) reserved for the super user
First data block=1
Maximum filesystem blocks=67371008
26 block groups
8192 blocks per group, 8192 fragments per group
2016 inodes per group
Superblock backups stored on blocks:
8193, 24577, 40961, 57345, 73729, 204801
Writing inode tables: done
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
所有磁盘都操作完后，再次用'fdisk -l'查看磁盘及分区状态：
# fdisk -l
Disk /dev/sda: 6442 MB, 6442450944 bytes
255 heads, 63 sectors/track, 783 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Device Boot Start End Blocks Id System
/dev/sda1 * 1 720 5783368+ 83 Linux
/dev/sda2 721 783 506047+ 82 Linux swap
Disk /dev/sdb: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot Start End Blocks Id System
/dev/sdb1 1 204 208880 83 Linux
Disk /dev/sdc: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot Start End Blocks Id System
/dev/sdc1 1 204 208880 83 Linux
Disk /dev/sdd: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot Start End Blocks Id System
/dev/sdd1 1 204 208880 83 Linux
Disk /dev/sde: 214 MB, 214748160 bytes
64 heads, 32 sectors/track, 204 cylinders
Units = cylinders of 2048 * 512 = 1048576 bytes
Device Boot Start End Blocks Id System
/dev/sde1 1 204 208880 83 Linux
- 使用mdadm创建一个软raid，raid级别：5；并有一个hotspare盘：
# mdadm -C /dev/md0 -l5 -n3 -x1 /dev/sd[b-e]1
mdadm: array /dev/md0 started.
输出信息显示软raid(/dev/md0)已经启用了。使用mdadm的-D参数（--detail）可以查看软raid状态：
# mdadm -D /dev/md0
/dev/md0:
Version : 00.90.01
Creation Time : Wed Aug 23 15:10:19 2006
Raid Level : raid5
Array Size : 417536 (407.75 MiB 427.56 MB)
Device Size : 208768 (203.88 MiB 213.78 MB)
Raid Devices : 3
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Wed Aug 23 15:10:21 2006
State : clean
Active Devices : 3
Working Devices : 4
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 64K
Number Major Minor RaidDevice State
0 8 17 0 active sync /dev/sdb1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
3 8 65 -1 spare /dev/sde1
UUID : f8283de5:39c73d89:b9fbc266:fdceb416
Events : 0.2
# mdadm -D -s >/etc/mdadm.conf
查看一下：
# cat /etc/mdadm.conf
ARRAY /dev/md0 level=raid5 num-devices=3 UUID=f8283de5:39c73d89:b9fbc266:fdceb416
devices=/dev/sdb1,/dev/sdc1,/dev/sdd1,/dev/sde1
修改（把上面devices=后面的磁盘，都放到DEVICE后面，并且不要逗号。而/dev/md0之后的内容，都要用逗号来分隔）：
DEVICE /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1
ARRAY /dev/md0 level=raid5,num-devices=3,UUID=f8283de5:39c73d89:b9fbc266:fdceb416
重启一下，检测配置好的软raid是否能够在系统重启后自动启用。
重启后，查看'/proc/mdstat'文件就可以看到软raid的状态：
# cat /proc/mdstat
Personalities : [raid5]
md0 : active raid5 sdb1[0] sde1[3] sdd1[2] sdc1[1]
417536 blocks level 5, 64k chunk, algorithm 2 [3/3] [UUU]
unused devices:
That's all.
出现故障后的恢复这里指的出现故障，是指raid中的一块磁盘出现了故障，无法使用。这时候需要使用额外的磁盘来代替它。这里以强制将某块磁盘标记为已损坏，来模拟实际出现故障（注：新的磁盘的容量最好和已损坏的磁盘一致）：
将/dev/sdb1标记为已损坏：
# mdadm /dev/md0 -f /dev/sdb1
mdadm: set /dev/sdb1 faulty in /dev/md0
这时候使用mdadm的-D参数来查看状态，可以看到/dev/sdb1已经被认为是faulty，而hotspare（热备）盘'/dev/sde1'已经顶替了它的位置（这就是hotspare的作用）：
# mdadm -D /dev/md0
/dev/md0:
Version : 00.90.01
Creation Time : Wed Aug 23 15:10:19 2006
Raid Level : raid5
Array Size : 417536 (407.75 MiB 427.56 MB)
Device Size : 208768 (203.88 MiB 213.78 MB)
Raid Devices : 3
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Wed Aug 23 15:42:24 2006
State : clean
Active Devices : 3
Working Devices : 3
Failed Devices : 1
Spare Devices : 0
Layout : left-symmetric
Chunk Size : 64K
Number Major Minor RaidDevice State
0 8 65 0 active sync /dev/sde1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
3 8 17 -1 faulty /dev/sdb1
UUID : f8283de5:39c73d89:b9fbc266:fdceb416
Events : 0.4
既然'/dev/sdb1'出现了故障，当然就要将它移除：
# mdadm /dev/md0 -r /dev/sdb1
mdadm: hot removed /dev/sdb1
现在可以关机了。关机之后拔下这块已损坏的磁盘了，换上你的新磁盘。换好之后，分区，mkfs.XXXX。然后将它加入到软raid中：
# mdadm /dev/md0 -a /dev/sdb1
mdadm: hot added /dev/sdb1
这时候再使用mdadm的'-D'参数，可以看到sdb1已经作为hotspare盘了：
# mdadm -D /dev/md0
/dev/md0:
Version : 00.90.01
Creation Time : Wed Aug 23 15:10:19 2006
Raid Level : raid5
Array Size : 417536 (407.75 MiB 427.56 MB)
Device Size : 208768 (203.88 MiB 213.78 MB)
Raid Devices : 3
Total Devices : 4
Preferred Minor : 0
Persistence : Superblock is persistent
Update Time : Wed Aug 23 16:19:36 2006
State : clean
Active Devices : 3
Working Devices : 4
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 64K
Number Major Minor RaidDevice State
0 8 65 0 active sync /dev/sde1
1 8 33 1 active sync /dev/sdc1
2 8 49 2 active sync /dev/sdd1
3 8 17 -1 spare /dev/sdb1
UUID : f8283de5:39c73d89:b9fbc266:fdceb416
Events : 0.6
misc
- 假如创建了RAID，但是没有生成 /etc/mdadm.conf 文件，那么系统重启后是不会启用RAID的，这时候需要这样做：
# mdadm -A /dev/md0 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1