Red Hat Software Raid
This is a quick and dirty document on software raid there are many more documents on the web that go into greater detail, the following is covered in this document:
- Creating a raid array with a hot spare
- Checking the array
- Simulating a drive failure
- Removing a disk from an array
- Adding a disk to the array
- Extend the array
- Starting stopping array
The following was tested using a centos 4 installation on Dell hardware, three partitions have already been created /dev/sdb10, /dev/sdb11 and /dev/sdb12 all are 1Gb in size.
Set the partition type
| Set partition type |
[root]# fdisk /dev/sdb
Command (m for help): t Paratition number (1-12): 10
Partition ID (L to list options): fd
Command (m for help): w Command (m for help): q
Note: repeat above for /dev/sdb11 and /dev/sdb12
fd = raid autodetect
|
| create partition table of new disk |
sfdisk -d /dev/sda | sfdisk /dev/sdb
Note: this is like the prtvtoc ... | fmthard ... command in Solaris
|
Create the array
| Using mkraid | 1. Update the configuration file /etc/raidtab with these lines of code [root]# vi /etc/raidtab raiddev /dev/md0 raid-level 1 nr-raid-disks 2 nr-spare-disks 1 persistent-superblock chunk-size 4 device /dev/sdb10 raid-disk 0 device /dev/sdb11 raid-disk 1 device /dev/sdb12 spare-disk 0
2. Now make the RAID device md0 and create a filesystem on it
[root]# mkraid /dev/md0 [root]# mke2fs -j –b 4096 –R stride=8 /dev/md0
3. Now add entry to /etc/fstab and mount it:
Add line to fstab:
/dev/md0 /mirror1 ext3 defaults 1 2
Create the mount point:
[root]# mkdir /mirror1
Mount the mirror:
[root]# mount /mirror1 |
| Using mdadm |
mdadm -C /dev/md0 -l1 -n2 /dev/sdb10 /dev/sdb11 -x1 /dev/sdb12
-C create an array -l the raid level (raid 1 in this case) -n Number of devices in raid (2 devices in this case) -x Number of spare disks in the raid (1 in this case) |
| detail, examine and assemble | detail = applies to the whole array which is currently active examine = applies to the devices which are a component of the array assemble = assemble the array using all devices (will use config file first then scan devices) |
Commands
| |
| Saving the configuration |
echo "DEVICE partitions" > /etc/mdadm.conf
mdadm --detail --scan >> /etc/mdadm.conf
Note: by default the system will look in /etc/mdadm.conf then /etc/mdadm/mdadm.conf
|
| Lost the configuration file | echo "DEVICE partitions" > /etc/mdadm.conf mdadm --examine --scan >> /etc/mdadm.conf mdadm --assemble --scan |
| /proc/mdstat | cat /proc/mdstat |
| lsraid | lsraid –a /dev/md0 |
| mdadm | mdadm --detail /dev/md0 mdadm -D /dev/md0 |
| Display device configuration | mdadm --misc -D /dev/md0 mdadm --misc -E /dev/sdb11 |
Simulating a drive failure
To test the raid integrity you might want to simulate a disk failure again there are a number of ways to do this.
| raidtools | raidsetfaulty /dev/md0 /dev/sdb11 |
| mdadm | mdadm -–manage /dev/md0 –f /dev/sdb11 |
Remove a disk from the array
To remove a disk from the raid array use the following commands, the disk has to be faulty not faulted (see above) to allow this option to work, at this point the disk can be physically removed.
| raidtools | raidhotremove /dev/md0 /dev/sdb11 |
| mdadm | mdadm --manage /dev/md0 –r /dev/sdb11 |
Add a disk to the array
Adding a disk to the array could result in two outcomes, if the array is already degraded the new will be used to fix the fault (if a hot spare has not already been used), if not then the disk will be used as a hot spare.
| raidtools | raidhotadd /dev/md0 /dev/sdb11 |
| mdadm | mdadm --manage /dev/md0 –a /dev/sdb11 |
Extend/Shrink the array
| grow | ## First you need to change the configuration mdadm --grow /dev/md0 -n3 ## Then you can add the disks mdadm --manage /dev/md0 -a /dev/sdb12 ## Now you can resize the filesystem resize2fs /dev/md0 |
| shrink | ## MAKE SURE EVERYTHING IS BACKED UP ## First fail the disk you want to remove, mdadm --manage /dev/md0 -f /dev/sdb12 # Then alter the configuration, yes I know the grow actually shrinks the array???? mdadm --grow /dev/md0 -n2 ## Now you can remove the disk mdadm --manage /dev/md0 -r /dev/sdb12 |
It is possible to change the type of the array from a raid 1 to a raid 5, I would advise to backup first but it worked without any problems for me
| change from raid 1 to raid 5 | ## First you need to unmount any filesystem from the array umount /array1 ## Then you need to stop the array mdadm -S /dev/md0 ## Then recreate the array mdadm -C /dev/md0 -l5 -n3 /dev/sdb10 /dev/sdb11 /dev/sdb12 -x1 /dev/sdb13 ## you will get errors stating the below, select Y to carry on creating the array mdadm: /dev/sdb1 appears to contain an ext2fs file system size=51072K mtime=Tue Feb 8 10:30:06 2011 mdadm: /dev/sdb1 appears to be part of a raid array: level=raid1 devices=2 ctime=Tue Feb 8 10:26:56 2011 mdadm: /dev/sde1 appears to contain an ext2fs file system size=51072K mtime=Tue Feb 8 10:30:06 2011 mdadm: /dev/sde1 appears to be part of a raid array: level=raid1 devices=2 ctime=Tue Feb 8 10:26:56 2011 Continue creating array? y mdadm: array /dev/md0 started. ## Once the array has started then mount it again and check everything mount /dev/md0 /array1 ## check the array mdadm -D /dev/md0 ## Don't forget to update the configuration file cp /etc/mdadm.conf /etc/mdadm.conf_old echo "DEVICE partitions" > /etc/mdadm.conf mdadm --detail --scan >> /etc/mdadm.conf |
Starting and stopping array
| raidtools | raidstop /dev/md0 raidstart /dev/md0 |
| mdadm | ## Make sure you have the configuration /etc/mdadm.conf file created mdadm -S /dev/md0 mdadm -A -R /dev/md0 -S stopping -A assemble -R starting |
| Remove the array | mdadm --stop /dev/md0 mdadm --remove /dev/md0 ## Once you run the below command there is no going back mdadm --zero-superblock /dev/sdb11 mdadm --zero-superblock /dev/sdb12 |
| Restore the array | ## standard header for mdadm.conf file echo "DEVICE partitions" > /etc/mdadm.conf ## get the information from the disks and append that information to mdadm.conf file mdadm --examine --scan /dev/sdb1 /dev/sdb2 /dev/sdb3 >> /etc/mdadm.conf ## use the mdadm.conf file to start the array mdadm -A -s |
Persistant-superblockWhen an array is initialised with the persistent-superblock option in the /etc/raidtab file, a special superblock is written in the beginning of all disks participating in the array. This allows the kernel to read the configuration of RAID devices directly from the disks involved, instead of reading from some configuration file that may not be available at all times.
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