RAID devices cannot be partitioned, like ordinary disks can. This can be a real annoyance on systems where one wants to run, for example, two disks in a RAID-1, but divide the system onto multiple different filesystems. A horror example could look like:
# df -h Filesystem Size Used Avail Use% Mounted on /dev/md2 3.8G 640M 3.0G 18% / /dev/md1 97M 11M 81M 12% /boot /dev/md5 3.8G 1.1G 2.5G 30% /usr /dev/md6 9.6G 8.5G 722M 93% /var/www /dev/md7 3.8G 951M 2.7G 26% /var/lib /dev/md8 3.8G 38M 3.6G 1% /var/spool /dev/md9 1.9G 231M 1.5G 13% /tmp /dev/md10 8.7G 329M 7.9G 4% /var/www/html
If a RAID device could be partitioned, the administrator could simply
have created one single
/dev/md0 device device, partitioned
it as he usually would, and put the filesystems there. Instead, with
today's Software RAID, he must create a RAID-1 device for every single
filesystem, even though there are only two disks in the system.
There have been various patches to the kernel which would allow partitioning of RAID devices, but none of them have (as of this writing) made it into the kernel. In short; it is not currently possible to partition a RAID device - but luckily there is another solution to this problem.
The solution to the partitioning problem is LVM, Logical Volume Management. LVM has been in the stable Linux kernel series for a long time now - LVM2 in the 2.6 kernel series is a further improvement over the older LVM support from the 2.4 kernel series. While LVM has traditionally scared some people away because of its complexity, it really is something that an administrator could and should consider if he wishes to use more than a few filesystems on a server.
We will not attempt to describe LVM setup in this HOWTO, as there
already is a fine HOWTO for exactly this purpose. A small example of a
RAID + LVM setup will be presented though. Consider the
output below, of such a system:
# df -h Filesystem Size Used Avail Use% Mounted on /dev/md0 942M 419M 475M 47% / /dev/vg0/backup 40G 1.3M 39G 1% /backup /dev/vg0/amdata 496M 237M 233M 51% /var/lib/amanda /dev/vg0/mirror 62G 56G 2.9G 96% /mnt/mirror /dev/vg0/webroot 97M 6.5M 85M 8% /var/www /dev/vg0/local 2.0G 458M 1.4G 24% /usr/local /dev/vg0/netswap 3.0G 2.1G 1019M 67% /mnt/netswap"What's the difference" you might ask... Well, this system has only two RAID-1 devices - one for the root filesystem, and one that cannot be seen on the
dfoutput - this is because
/dev/md1is used as a "physical volume" for LVM. What this means is, that
/dev/md1acts as "backing store" for all "volumes" in the "volume group" named
All this "volume" terminology is explained in the LVM HOWTO - if you do not completely understand the above, there is no need to worry - the details are not particularly important right now (you will need to read the LVM HOWTO anyway if you want to set up LVM). What matters is the benefits that this setup has over the many-md-devices setup:
/dev/md2device a physical volume and add it to your volume group. That's it! You now have more free space in your volume group for either growing your existing logical volumes, or for adding new ones.
All in all - for servers with many filesystems, LVM (and LVM2) is definitely a fairly simple solution which should be considered for use on top of Software RAID. Read on in the LVM HOWTO if you want to learn more about LVM.