Hello,
I'm currently setting up a virtualization infrastructure based on CentOS 5.4 x86_64 and KVM.
I want to use LVM because of its flexibility, especially the ability to add more disk space to a guest if needed. We are considering using an hardware RAID1 with BBU as the lowest layer.
My question is: where is it recommended to put the LVM?
1. On the host only In that case, the guests would see logical volumes as hard drives and would be installed with "plain" partitions (ext3 / swap) When more space is needed the logical volume (from host perspective) / drive (from guest perspective) would simply become bigger. This feels simple, but I wonder whether there may be side effects, since real harddrive cannot grow, and maybe some parts of the system expect them to stay of a given size.
2. On the guest only On the host, there would only be "plain" raw partitions, which the guest would see as hard drives. The guest would be installed with an LVM and when more space is needed, an additional partition (from host perspective) / drive (from guest perspective) would be added to the guest volume group, which would then become bigger, allowing logical volumes to grow. I'm not too comfortable with that, since my understanding of the role of the host is to manage resources for the guests: memory, CPUs so I'd like to have some powerful abstraction there for disk space as well, which is one purpose of LVM. The guest would then need to be restarted when space is added.
3. On both host and guest LVM would be installed on the host, which would then provide the logical volumes as hard drives for the guest. The guest would use them as additional drives to be added to its own volume group when it needs to grow. This feels the most flexible, but I wonder whether it is not wrong to add LVM layers upon LVM layers, from a performance/stability/simplicity perspective. The guest would probably have to be restarted when more space is needed (I've not tested this extensively yet)
I would be very interested to have your opinion / experience about this before I go further in my testing!
Cheers,
Mathieu