Disk Image

First, word of wisdom: backup if you care, whenever handle partitions

References

http://en.wikibooks.org/wiki/QEMU/Images

IBM - KVM Best Practice - best practice for block I/O performance

http://pic.dhe.ibm.com/infocenter/lnxinfo/v3r0m0/index.jsp?topic=%2Fliaat%2Fliaatbpkvmasynchio.htm

Improve Disk IO Performance

First we can try VirtIO. IDE has popular support from all guest systems, while VirtIO has better performance but require driver on Guest system. Refer to below for download of driver and enable VirtIO on guest system.

http://www.linux-kvm.org/page/Boot_from_virtio_block_device

Steps (on installed win7)

https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Virtualization_Host_Configuration_and_Guest_Installation_Guide/form-Virtualization_Host_Configuration_and_Guest_Installation_Guide-Para_virtualized_drivers-Mounting_the_image_with_virt_manager.html

1. add new virtio disk
2. start windows, open device manager
3. find new drive, update driver, manually find path to CD
4. make sure it works
5. shutdown
6. change main disk to virtio
7. start

Other settings to improve performance according to readings from Internet (http://www.ilsistemista.net/index.php/virtualization/11-kvm-io-slowness-on-rhel-6.html?showall=1&start=1):

• pre-allocate entire disk
• disable cache

Note:

I cannot get Windows XP to work with VirtIO. (forget it, I tried)

Win7 works fine.

I can get the Fedora VirtIO storage driver work on Win7 and experience (not tested) performance improvement.

I cannot get the Fedora VirtIO network driver work on Win7.

Tips from IBM (see reference):

• use raw partitions
• use ext3 file system (for guest or host? not clear...)
• set cache mode to balance data integrity and performance

Image Types

Using QCOW2 Image Type

Convert a raw image to qcow2:

sudo qemu-img convert -O qcow2 -p /var/lib/libvirt/images/xxx.img /var/lib/libvirt/images/xxx.qcow2.img

Handling snapshot (quote qemu-img manual):

snapshot [-l | -a snapshot | -c snapshot | -d snapshot ] filename

List, apply, create or delete snapshots in image filename.

Re-sizing Image

qemu-img creates, converts and resizes qemu image. For example:

qemu-img resize XP-nuxeo.img +10G

Note that qemu-img does not deal with the partitions inside the image. Don't shrink size before shrinking partition, otherwise the partition may be destroyed and data lost. To increase partition, first extend image then extend partition.

Use GParted live cd to handle partition while the host OS is down.

http://gparted.sourceforge.net/

Example

My laptop has a small boot partition and one big partition for Ubuntu. Now I want to place my guest Windows workstation to a separate partition to improve performance. Here's what I am doing.

1. Create a USB boot disk with GParted live cd
2. Reboot machine, shrink the big Ubuntu partition, and add an unformated partition behind the big Ubuntu partition
3. Reboot to Ubuntu
4. Find out the path to the current raw image file (xxx.img). Its current size = 21474836480 (ls -l)
5. Check out how large the partition exactly is: use GParted tool, see how much sectors the partition has, see sector size from device information, multiply the two. Its size = 512 X 102400000 = 52428800000
6. Calculate the difference: 52428800000 - 21474836480 = 30953963520
7. Increase the disk size: sudo qemu-img resize /var/lib/libvirt/images/xxx.img +30953963520
8. Check carefully to make sure which partition is the one you are working on. Copy to wrong partition completely erases it!
9. Copy the disk image to the partition: sudo dd if=/var/lib/libvirt/images/xxx.img of=/dev/sda4
10. Now the wait, for hours. If wander the progress of the copy, follow this document: http://linuxcommando.blogspot.com/2008/06/show-progress-during-dd-copy.html
11. Done. Now modify the virtual machine to use the partition for its disk image. Start the Windows machine.
12. Use the windows partition manager to expand the windows partition.
13. After-thought: now I realised that maybe it's best to just copy the unexpended image to the partition - same effect and less copying time. Much of the time is only spent on copying non-data.