xen支持的块设备
1. 物理块设备（Dom0中的物理块设备，如逻辑卷）
phy:/dev/cdrom
phy:/dev/xenvg/vm1
2. 磁盘镜像
file:/root/tst.iso（使用loopback）
nbd: ip_port （网络块设备）
tap:cdrom:/dev/sr0（CD设备）
tap:aio:/path/file （blktap）
tap:qcow:/mnt/disks/sles10sp1.iso.qcow（QEMU磁盘镜像）
tap:vmdk:/mnt/disks/sles10sp1.iso.vmdk（Vmware磁盘镜像）
3. ISCSI设备：
iscsi:iqn.2001-04.com.acme@0ac47ee2-216e-452a-a341-a12624cd0225
4. NPIV设备：
npiv:210400e08b80c40f
file与tap的区别
file: uses dom0 kernel page cache, and thus might give better performance than phy: or tap:aio:,
but it's also more insecure because of the caching.
tap:aio: uses direct IO, so it bypasses dom0 kernel caches, and works like phy: in that sense.
blktap1 provides also other modes than :aio, like: qcow, but those were not very stable or fast,
disk = ['tap:aio:<FILENAME>,sda1,w']
disk = ['tap:qcow:<FILENAME>,sda1,w']
so blktap2 was developed. for blktap1 see: http://wiki.xensource.com/xenwiki/blktap .
blktap2 also has vhd image support including snapshots and cloning.
disk = [ "tap:tapdisk:vhd:/path/images/disk.vhd,xvda,w" ]
disk = [ "tap2:tapdisk:vhd:/path/images/disk.vhd,xvda,w" ]
See here: http://wiki.xensource.com/xenwiki/blktap2 .