If you are running a server, what will happen if your root device (on /sda) goes down?
It is normal to have at least a RAID1 configuration for your root partition, to help keep the server up should a single disk fail. If uptime isn't an issue perhaps this isn't a consideration. Note that you have to be careful to get grub installed on both disks' boot sectors if you are using software raid for a boot partition.
Raid 5 for your data storage is sensible, although Raid 6 is better! You asked
Is it usefull to create a LVM on the raid5 /dev/md0?
If you want to use up the whole of the software raid device, it isn't useful to create LVM logical devices. If you think you might want to, for instance, move
/var off your boot device to a new device created in a logical volume, then yes. Naturally if you are going to do that you should be careful not to use the whole of your md0 device with a single logical volume as then the reason for using lvm is somewhat obviated.
Other important stuff to know about lvm:
- While it is fine to fill up your PV with a VG, don't fill up your VG with LVs if you want flexibility in future (!)
- Choose a filesystem that can expand if you choose to extend an LV (XFS is a good choice)
- Filesystems in general aren't good at contracting to match an LV contraction
- LVM snapshots are best avoided as they have terrible IO.
Check out the tldp Howto for some more interesting info.