Using RAID in Linux - page 6
The Mysteries of RAID
The easiest way to create a RAID array is to do it during the installation of any new Linux distribution from the graphical installer. In Red Hat the utility named Disk Druid suits our needs. You can create RAID partitions as easy as simple partitions; then you can combine them into one array and set its level. That's all!
However, sometimes Disk Druid is too "clever" and it suggests partitions placement on disks, which goes absolutely against what any system administrator would want.
If this has happened to you, you can easily divide partitions with the
fdisk (don't forget about assigning for
Linux partitions RAID value with partition type 0xfd). In the future, you can combined them
into larger arrays with Disk Druid.
If you don't want to reinstall a distro, this may be the best way to start working with RAID anyway. Though, as with everything else in Linux -- the best of RAID can be achieved by editing its configuration file.
So, with your favorite text editor, create file /etc/raidtab and typing something like this for RAID Linear-mode:
raiddev /dev/md0 # raid device name raid-level linear # linear mode nr-raid-disks 2 # number of used disks chunk-size 32 # in this case it doesn't affect at all persistent-superblock 1 # list of partitions below and their placement device /dev/sdb6 # partition name raid-disk 0 # disk number in array device /dev/sdc5 # ... and so on raid-disk 1 # ...For creating such array we just need to execute:
mkraid /dev/md0After that, while viewing /proc/mdstat, we can make sure of the workability of our array. This device can be run with next command:
raidstart /dev/md0and stopped with this command:
Easy, isn't it?
Once the array is created, the device /dev/md0 can be used for placement of system files as usual--like with any other disk of the system. After reboot, this device will be auto-connected, without any raidstart (or raidstop) needed. You don't need to fix initialization scripts, you don't need to touch absolutely anything!
For RAID 0, the file /etc/raidtab can looks like:
raiddev /dev/md0 # as above raid-level 0 nr-raid-disks 2 persistent-superblock 1 chunk-size 4 # here, size makes sense. look commons below. # everything is the same, like in example above device /dev/sdb6 raid-disk 0 device /dev/sdc5 raid-disk 1The chunk-size argument in this case means stripe size in kilobytes. For best productivity, (at least in this configuration) the size of the partitions should average out to be the same. The default value is 4KB, however a higher value--about 32KB--will give more productiviy. It should be like the size of disk cylinders. The calculation of disk caches in modern hard drives can sometimes vary, sometimes becoming more like cache size. Creating, starting, and stopping RAID uses identical methods as those describe above.
/etc/raidtab for RAID 1 will read:
raiddev /dev/md raid-level 1 . nr-raid-disks 2 nr-spare-disks 1 chunk-size 4 # doesn't matter persistent-superblock 1 # as usual device /dev/sdb6 raid-disk 0 device /dev/sdc5 raid-disk 1 # description of drives of "hot reserve" device /dev/sdd5 spare-disk 0
When we have "hot reserve" disks and if one of the "mirror" disks fails, a process of reconstruction of disk information from the proper disk in the array will start in the background. After that, the "hot reserve" disk will be exchanged with the broken disk.
Finally, for RAID 5, the /etc/raidtab file will read:
raiddev /dev/md0 raid-level 5 nr-raid-disks 3 nr-spare-disks 1 persistent-superblock 1 parity-algorithm left-symmetric # it should be this way chunk-size 128 # "good" value for the beginning # device /dev/sda3 raid-disk 0 device /dev/sdb1 raid-disk 1 device /dev/sdc1 raid-disk 2 device /dev/sdd5 # reserve disk spare-disk 0
This situation is like what we find in RAID 1. Array productiviy depends on chunk-size, so in this case you should increase that value, more than what it is in RAID 0. 128-256KB usually gives good results.
It is important to remember that while formatting the file system with
mke2fs command, you have special argument
which affects records placement on disks. Usually, the best value of this argument is
chunk size/inode size, i.e., with chunk-size = 256 and block-size =
4096 bytes, stride = 32.
You point it this way:
mke2fs -b 4096 -R stride=32
Only run this command for RAID levels 0, 4, or 5. For Linear-mode and RAID 1, it doesn't make any sense.
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