Disk Format
The disk format of a virtual machine image is the format of the underlying disk image. Virtual appliance vendors have different formats for laying out the information contained in a virtual machine disk image.
You can set your image’s disk format to one of the following:
- raw: This is an unstructured disk image format
- vhd: This is the VHD disk format, a common disk format used by virtual machine monitors from VMWare, Xen, Microsoft, VirtualBox, and others
- vmdk: Another common disk format supported by many common virtual machine monitors
- vdi: A disk format supported by VirtualBox virtual machine monitor and the QEMU emulator
- iso: An archive format for the data contents of an optical disc (e.g. CDROM).
- qcow2: A disk format supported by the QEMU emulator that can expand dynamically and supports Copy on Write
- aki: This indicates what is stored in Glance is an Amazon kernel image
- ari: This indicates what is stored in Glance is an Amazon ramdisk image
- ami: This indicates what is stored in Glance is an Amazon machine image
Container Format
The container format refers to whether the virtual machine image is in a file format that also contains metadata about the actual virtual machine. You can set your image’s container format to one of the following:
- bare: This indicates there is no container or metadata envelope for the image
- ovf: This is the OVF container format
- aki: This indicates what is stored in Glance is an Amazon kernel image
- ari: This indicates what is stored in Glance is an Amazon ramdisk image
- ami: This indicates what is stored in Glance is an Amazon machine image
Ubuntu Cloud images
Create a qcow2 disk image
First, let’s create a qcow2 disk image using ‘qemu-img’ tool
$ /usr/bin/qemu-img create -f qcow2 -o preallocation=metadata /export/vmimgs/glacier.qcow2 8G
NOTE: At this point in time, preallocation=metadata option is the best we can do to extract max. possible (near RAW) I/O performance out of QCOW2 format. (hint from Kevin Wolf – Qemu/Qcow2 developer )