*** Snapshots and Storage Information ***

So, I converted an existing 13G RAW disk image to qcow2.
#root@~ $qemu-img -f raw convert -O qcow2 /var/lib/libvirt/images/cs81test.img /var/lib/libvirt/images/cs81test.qcow2

#File system notes
root@~ $du -sh /var/lib/libvirt/images/lovelock1.dsk 
2.4G/var/lib/libvirt/images/lovelock1.dsk

#Note: Preallocation is faster(when using qemu-img)
##Ex for creating:  qemu-img create -f qcow2 -o preallocation=metadata /var/lib/libvirt/images/fedora15.qcow2 8G
##Ex for conversion: qemu-img convert -c -f raw -O qcow2 -o preallocation=metadata /var/lib/libvirt/images/honeycomb.dsk /var/lib/libvirt/images/honeycombq.qcow2
##################
root@~ $qemu-img convert -c -f raw -O qcow2 /var/lib/libvirt/images/lovelock1.dsk /var/lib/libvirt/images/lovelock1q.qcow2

root@~ $du -sh /var/lib/libvirt/images/lovelock1q.qcow2
785M/var/lib/libvirt/images/lovelock1q.qcow2
root@~ $
##################

== References/Notes ==
- sparse-files: http://intgat.tigress.co.uk/rmy/uml/sparsify.html
- performance: http://www.ilsistemista.net/index.php/virtualization/11-kvm-io-slowness-on-rhel-6.html?start=4
- http://wiki.qemu.org/Features/Snapshots2
- http://wiki.qemu.org/QMP


#'write-through cache'  (research more on this)
	+ Every write to the cache causes a synchronous write to the backing store.
	+ For RAW Images -- write-through cache should *always* be disabled. (cache=none) ;  (preallocation doesn't have influence)
		EX to disbale cache:  <driver name='qemu' type='raw' cache='none'/>
	+ For QCOW2 Images -- And enable 'metadata preallocation'


NOTES:
####################### Examples of creating images with, without preallocation #######################

[root@sunrise temp]# qemu-img create -f qcow2 -o preallocation=metadata fedora15tbx.qcow2 8G
Formatting 'fedora15tbx.qcow2', fmt=qcow2 size=8589934592 encryption=off cluster_size=0 preallocation='metadata' 
##################
[root@sunrise temp]# du -sh fedora15tbx.qcow2
1.4M	fedora15tbx.qcow2
[root@sunrise temp]# 
##################
[root@sunrise temp]# qemu-img create -f qcow2  fedora15-without-preall.qcow2 8G
Formatting 'fedora15-without-preall.qcow2', fmt=qcow2 size=8589934592 encryption=off cluster_size=0 
##################
[root@sunrise temp]# du -sh fedora15-without-preall.qcow2
136K	fedora15-without-preall.qcow2
[root@sunrise temp]# 
##################
[root@sunrise temp]# ls -lash fedora15tbx.qcow2 fedora15-without-preall.qcow2
1.4M -rw-r--r--. 1 root root 8.1G Aug 11 15:36 fedora15tbx.qcow2
136K -rw-r--r--. 1 root root 193K Aug 11 15:36 fedora15-without-preall.qcow2
[root@sunrise temp]# 
##################
[root@sunrise temp]# qemu-img create -f qcow2 -o size=10000000000,preallocation=metadata testfile.qcow2 8G
Formatting 'testfile.qcow2', fmt=qcow2 size=10000000000 encryption=off cluster_size=0 preallocation='metadata' 
##################
[root@sunrise temp]# ls -lash testfile.qcow2
1.6M -rw-r--r--. 1 root root 9.4G Aug 11 15:40 testfile.qcow2
[root@sunrise temp]# 
##################


############### Converting an existing disk image to Qcow2 with preallocation ; modifying your virtual machine XML file########################
=> 1/ convert your RAW image to qcow2
root@images $ls -lash /var/lib/libvirt/images/honeycomb.dsk 
1.1G -rw-r--r--. 1 root root 11G Aug 11 14:56 /var/lib/libvirt/images/honeycomb.dsk
###############
## This is INCORRCET
root@images $qemu-img convert -c -f raw -O qcow2 -o preallocation=metadata /var/lib/libvirt/images/honeycomb.dsk /var/lib/libvirt/images/honeycombq.qcow2
###############
root@images $ls -lash /var/lib/libvirt/images/honeycombq.qcow2
335M -rw-r--r--. 1 root root 11G Aug 11 15:56 /var/lib/libvirt/images/honeycombq.qcow2
root@images $
###############

=> 2/ Edit the xml file --  /etc/libvirt/qemu/honeycomb.xml and ensure change the 'disk' parameters -- 'type' , and correct 'source file'

.
.
.

 <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/honeycombq.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
###############

=> 3/ (Re-)Define the xml file

root@images $virsh define /etc/libvirt/qemu/honeycomb.xml
Domain honeycomb defined from /etc/libvirt/qemu/honeycomb.xml

root@images $
############################################################################



## IMPORTANT NOTE:'compression' and 'preallocation' *cannot* be used at the same time.
## The really slow part of qcow2 is allocating clusters, and you don't have to do that any more with preallocation.
## qcow2 with preallocation is about the best thing you can do to extract best performance out of qcow2.
## preallocation=full  -- Ref: http://lists.gnu.org/archive/html/qemu-devel/2011-01/msg03017.html

=>1/ Convert convert your RAW image to qcow2 (with preallocation)
##############
root@oz-test $qemu-img convert -f raw -O qcow2 -o preallocation=metadata /var/lib/libvirt/images/cs81test.img /var/lib/libvirt/images/cs81test.qcow2
root@oz-test $
##############
root@oz-test $ls -lash /var/lib/libvirt/images/cs81test.img
13G -rw-------. 1 root root 12G Jul 22 15:53 /var/lib/libvirt/images/cs81test.img
root@oz-test $
##############
root@oz-test $ls -lash /var/lib/libvirt/images/cs81test.qcow2
1.6G -rw-r--r--. 1 root root 13G Aug 11 16:59 /var/lib/libvirt/images/cs81test.qcow2
root@oz-test $

=>2/ change your libvirt xml
##############
.
.

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/var/lib/libvirt/images/cs81test.qcow2'/>
      <target dev='vda' bus='virtio'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
##############

=>3/ (Re-)Define the xml file
##############
root@~ $virsh define /etc/libvirt/qemu/cs81test.xml
Domain cs81test defined from /etc/libvirt/qemu/cs81test.xml
##############


=>4/ Snapshots work(with libvirt)
##############
root@~ $virsh snapshot-current cs81test

############## CREATE A SNAPSHOT TEMPLATE ##############
root@~ $cat /etc/libvirt/qemu/snapshot-cs81test.xml
<domainsnapshot>
	<name>cs81-inst-aug11</name>
	<description>up2date cs81 packages installed. As of Aug-11 </description>
</domainsnapshot>

root@~ $
############## CREATE SNAPSHOT ##############
root@~ $virsh snapshot-create cs81test /etc/libvirt/qemu/snapshot-cs81test.xml 
Domain snapshot cs81-inst-aug11 created from '/etc/libvirt/qemu/snapshot-cs81test.xml'

root@~ $
############## LIST THE SNAPSHOT ##############
root@~ $virsh snapshot-list cs81test
 Name                 Creation Time             State
---------------------------------------------------
 cs81-inst-aug11      2011-08-11 18:43:24 +0530 shutoff

root@~ $
##############

=>5/ After the above, weirdly, it fails
root@~ $virsh snapshot-create cs81test
Domain snapshot 1313069252 created

root@~ $virsh snapshot-list cs81test
 Name                 Creation Time             State
---------------------------------------------------
 1313069252           2011-08-11 18:57:32 +0530 shutoff
 cs81-inst-aug11      2011-08-11 18:43:24 +0530 shutoff

root@~ $
##############
root@~ $virsh snapshot-delete cs81test 1313069252

root@~ $virsh snapshot-list cs81test
 Name                 Creation Time             State
---------------------------------------------------
 cs81-inst-aug11      2011-08-11 18:43:24 +0530 shutoff

root@~ $
##############


=CREATING A SECOND SNAPSHOT WITH A NAME=
##############
root@~ $cat /var/tmp/snap2-cs81test.xml
<domainsnapshot>
	<name>2nd snapshot</name>
	<description>with a bit of text </description>
</domainsnapshot>

root@~ $
##############
root@~ $virsh snapshot-create cs81test /var/tmp/snap2-cs81test.xml 
Domain snapshot 2nd snapshot created from '/var/tmp/snap2-cs81test.xml'

##############
root@~ $virsh snapshot-list cs81test
 Name                 Creation Time             State
---------------------------------------------------
 2nd snapshot         2011-08-11 19:10:07 +0530 shutoff
 cs81-inst-aug11      2011-08-11 18:43:24 +0530 shutoff
##############