We have an iSCSI SAN that will be used as backing store for virtual machines. We use Xen and will directly present the LUNs as block devices of VMs (so there is no file system with files on it for each VM, like VMware does when using VMFS datastores).
I'm wondering if it would be better to provision separate LUNs for operating system and data of each vm (eg. one small lun to install Linux and one bigger lun to keep mailboxes, one small for linux and one big for webserver data, and so on...) or just one big LUN per vm.
Looking at the former, then one could ask "so why not even another one LUN for the swap partition", going on until you just have only one "partition" on each lun.
Keep in mind all these LUNs are from the exact same RAID volume on the SAN so there's no point in differentiating for disk speed or raid level.