[*** Example-1: Thin provisioning using QCOW2 overlays]
+++++++++++++++++++++++++++++++++++++++++++++++++++++++

        (1) List the root base image which has installed plain vanilla minimal F17 OS on it.
                ======================
                # qemu-img info /home/kashyap/vmimages/base-f17.qcow2 
                image: /home/kashyap/vmimages/base-f17.qcow2
                file format: qcow2
                virtual size: 5.0G (5368709120 bytes)
                disk size: 5.0G
                cluster_size: 65536
                ======================

        (2) To see if it's working, we can do a quick check using qemu-kvm to
            boot the disk image on the shell (side note: for this to run successfully, the
            guest kernel should have serial console enabled on kernel cmd-line -  ‘console=tty0
            console=ttyS0,115200′):

                ======================
                # qemu-kvm -enable-kvm -m 1024 base-f17.qcow2 -nographic
                ======================

        (3) Now, using our above base image as a backing file, create a
            Fedora-17 machine(overlay) using 'qemu-img'

                ======================
                # qemu-img create -o 'backing_file=/home/kashyap/vmimages/base-f17.qcow2,backing_fmt=qcow2' -f qcow2 /home/kashyap/vmimages/overlay-1.qcow2
                Formatting '/home/kashyap/vmimages/overlay-1.qcow2', fmt=qcow2 size=5368709120 backing_file='/home/kashyap/vmimages/base-f17.qcow2' backing_fmt='qcow2' eancryption=off cluster_size=65536 lazy_refcounts=off 
                ======================

        (4) Run, qemu-img on the overlay file to see its
            information(specifically backing file; & the actual disk image size):

                ======================
                # qemu-img info /home/kashyap/vmimages/overlay-1.qcow2
                image: /home/kashyap/vmimages/overlay-1.qcow2
                file format: qcow2
                virtual size: 5.0G (5368709120 bytes)
                disk size: 196K
                cluster_size: 65536
                backing file: /home/kashyap/vmimages/base-f17.qcow2
                ======================
            Mention: Once we have a image chain with a depth of more than 1, the ONLY file we 
                     can safely edit is the 'leaf' image in the chain.
  
        (5) Now, boot into the overlay image using 'qemu-kvm' 
            (or via qemu git - qemu-system-x86_64-oct21-git  -enable-kvm -m 2048 /kashyap/home/vmimages/base-f17.img -nographic)

                 ======================
                 # qemu-kvm -enable-kvm -m 1024 overlay-1.qcow2 -nographic
                 ======================

        (5.1) Optionally, create another overlay-2 using base-f17.qcow2 as its backing file & boot into it.
 
        (6) Note that, officially, the right way to boot the guest(overlays or
            otherwise) is via Libvirt('virsh') ( & not via invoking 'qemu-kvm'
            directly). To exemplify:
                - Create/re-purpose a libvirt xml file /etc/libvirt/qemu/overlay-1.xml
                - Edit the overlay-1.xml - Update 'name', 'uuid', 'source file'
                  path, & 'mac address' to reflect the overlay-1 attributes.
                - Then define & startthe overlay :
                    # virsh define overlay-1
                    # virsh start overlay-1 --console
                



[*** Example-2: Snapshot creation(External, live): ***]
+++++++++++++++++++++++++++++++++++++++++++++++++++++++

        (1) List the block device associated with a guest. 
                ======================
                # virsh domblklist f17-base
                Target     Source
                ---------------------------------------------
                vda        /export/vmimages/f17-base.qcow2

                #
                ======================


        (2) Create external disk-only snapshot (while the guest is running).
                ======================
                # virsh snapshot-create-as f18-base snap1 "snap1-desc"
                --disk-only --atomic 
                ======================
                
                More specific command:
                =====================
                # virsh snapshot-create-as --domain f17-base snap1 snap1-desc \
                --disk-only --diskspec vda,snapshot=external,file=/export/vmimages/sn1-of-f17-base.qcow2 \
                --atomic
                Domain snapshot snap1 created
                #
                ======================

                    * Once the above command is issued, the original disk-image
                    of f17-base will become the backing_file & a new overlay
                    image is created to track the new changes. Here on, libvirt
                    will use this overlay for further write operations(while
                    using the original image as a read-only backing_file).

        (3) Now, list the block device associated(use cmd from step-1, above)
            with the guest,again, to ensure it reflects the new overlay image as
            the current block device in use. 
                ======================
                # virsh domblklist f17-base
                Target     Source
                ----------------------------------------------------
                vda        /export/vmimages/sn1-of-f17-base.qcow2

                #
                ======================
               


*** Example-3: Reducing the snapshot chain with "blockcommit" ***
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

[1] Create disk-only live snapshots :

    - Before taking a snapshot, Aad a textfile (say, file1.txt before taking
      snap1, file2.txt before taking snap2...in /export), to distinguish
      content between snapshots:

                ======================
                - Add a text file(file1.txt) by logging into the live guest - (# virsh
                  console commitraw-base).

                [root@moon qemu]# virsh snapshot-create-as --domain commitraw-base snap1 snap1-desc --disk-only --diskspec vda,snapshot=external,file=/export/vmimgs2/snap1-c-rawbase.qcow2 --atomic
                Domain snapshot snap1 created
                [root@moon qemu]# 
                
                - Add file2.txt in the live guest.

                [root@moon qemu]# virsh snapshot-create-as --domain commitraw-base snap2 snap2-desc --disk-only --diskspec vda,snapshot=external,file=/export/vmimgs2/snap2-c-rawbase.qcow2 --atomic
                Domain snapshot snap2 created
                [root@moon qemu]# 

                - Add file3.txt in the live guest.

                [root@moon qemu]# virsh snapshot-create-as --domain commitraw-base snap3 snap3-desc --disk-only --diskspec vda,snapshot=external,file=/export/vmimgs2/snap3-c-rawbase.qcow2 --atomic
                Domain snapshot snap3 created
                [root@moon qemu]# 
                
                - Add file4.txt in the live guest.

                [root@moon qemu]# virsh snapshot-create-as --domain commitraw-base snap4 snap4-desc --disk-only --diskspec vda,snapshot=external,file=/export/vmimgs2/snap4-c-rawbase.qcow2 --atomic
                Domain snapshot snap4 created
                [root@moon qemu]# 
                ======================

[2] List the backing chain, BEFORE blockcommit (it can be noticed that the Base image is RAW)

                ======================
                [kashyap@moon qemu-n]$ sudo ./qemu-img info --backing-chain /export/vmimgs2/snap4-c-rawbase.qcow2 
                image: /export/vmimgs2/snap4-c-rawbase.qcow2
                file format: qcow2
                virtual size: 1.0G (1073741824 bytes)
                disk size: 520K
                cluster_size: 65536
                backing file: /export/vmimgs2/snap3-c-rawbase.qcow2
                backing file format: qcow2
                
                image: /export/vmimgs2/snap3-c-rawbase.qcow2
                file format: qcow2
                virtual size: 1.0G (1073741824 bytes)
                disk size: 776K
                cluster_size: 65536
                backing file: /export/vmimgs2/snap2-c-rawbase.qcow2
                backing file format: qcow2
                
                image: /export/vmimgs2/snap2-c-rawbase.qcow2
                file format: qcow2
                virtual size: 1.0G (1073741824 bytes)
                disk size: 712K
                cluster_size: 65536
                backing file: /export/vmimgs2/snap1-c-rawbase.qcow2
                backing file format: qcow2
                
                image: /export/vmimgs2/snap1-c-rawbase.qcow2
                file format: qcow2
                virtual size: 1.0G (1073741824 bytes)
                disk size: 392K
                cluster_size: 65536
                backing file: /export/vmimgs2/commitraw-base-f17.img
                backing file format: raw
                
                image: /export/vmimgs2/commitraw-base-f17.img
                file format: raw
                virtual size: 1.0G (1073741824 bytes)
                disk size: 699M
                [kashyap@moon qemu-n]$ 
                ======================

[3] List files in /export in base & all snapshots BEFORE 'blockcommit'

                ======================
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/commitraw-base-f17.img ls /export
                file1.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap1-c-rawbase.qcow2 ls /export
                file1.txt
                file2.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap2-c-rawbase.qcow2 ls /export
                file1.txt
                file2.txt
                file3.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap3-c-rawbase.qcow2 ls /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap4-c-rawbase.qcow2 ls /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                ======================

[4] List the disk image sizes BEFORE 'blockcommit'

                ======================
                [root@moon qemu]# ls -lash /export/vmimgs2/*rawbase*
                392K -rw-------. 1 qemu qemu 448K Oct 21 18:38 /export/vmimgs2/snap1-c-rawbase.qcow2
                712K -rw-------. 1 qemu qemu 768K Oct 21 18:39 /export/vmimgs2/snap2-c-rawbase.qcow2
                776K -rw-------. 1 qemu qemu 832K Oct 21 18:39 /export/vmimgs2/snap3-c-rawbase.qcow2
                712K -rw-------. 1 qemu qemu 768K Oct 21 18:40 /export/vmimgs2/snap4-c-rawbase.qcow2
                [root@moon qemu]# ls -lash /export/vmimgs2/commitraw-base-f17.img
                700M -rw-r--r--. 1 qemu qemu 1.0G Oct 21 18:35 /export/vmimgs2/commitraw-base-f17.img
                [root@moon qemu]# 
                ======================

[5] Recursively merge overlays into their spective backing files:  sn3 into sn2 ; sn2 into sn1 ; sn1 into base(RAW)

                ======================
                [root@moon qemu]# virsh blockcommit --domain commitraw-base vda --base /export/vmimgs2/snap2-c-rawbase.qcow2 --top /export/vmimgs2/snap3-c-rawbase.qcow2 --wait --verbose
                Block Commit: [100 %]
                Commit complete
                ======================
                [root@moon qemu]# virsh blockcommit --domain commitraw-base vda --base /export/vmimgs2/snap1-c-rawbase.qcow2 --top /export/vmimgs2/snap2-c-rawbase.qcow2 --wait --verbose                                                                                                                               
                Block Commit: [100 %]
                Commit complete
                ======================
                [root@moon qemu]# virsh blockcommit --domain commitraw-base vda --base /export/vmimgs2/commitraw-base-f17.img --top /export/vmimgs2/snap1-c-rawbase.qcow2 --wait --verbose
                Block Commit: [100 %]
                Commit complete
                [root@moon qemu]# 
                ======================


[6] Now, list the backing chain info AFTER 'blockcommit' of snap4-c-rawbase.qcow2 (the current active-layer) to reflect the reduced backing chain.
                ======================
                [kashyap@moon qemu-n]$ sudo ./qemu-img info --backing-chain /export/vmimgs2/snap4-c-rawbase.qcow2 
                image: /export/vmimgs2/snap4-c-rawbase.qcow2
                file format: qcow2
                virtual size: 1.0G (1073741824 bytes)
                disk size: 904K
                cluster_size: 65536
                backing file: /export/vmimgs2/commitraw-base-f17.img
                backing file format: raw
                
                image: /export/vmimgs2/commitraw-base-f17.img
                file format: raw
                virtual size: 1.0G (1073741824 bytes)
                disk size: 699M
                [kashyap@moon qemu-n]$ 
                ======================

[7] List files in base & all snapshots AFTER 'blockcommit'
                ======================
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/commitraw-base-f17.img ls
                /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap1-c-rawbase.qcow2 ls
                /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap2-c-rawbase.qcow2 ls
                /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap3-c-rawbase.qcow2 ls
                /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]# guestfish --ro -i -a /export/vmimgs2/snap4-c-rawbase.qcow2 ls
                /export
                file1.txt
                file2.txt
                file3.txt
                file4.txt
                [root@moon qemu]#
                ======================

[8] List the disk image sizes AFTER 'blockcommit'
                ======================
                [root@moon qemu]# ls -lash /export/vmimgs2/commitraw-base-f17.img
                700M -rw-r--r--. 1 qemu qemu 1.0G Oct 21 18:48 /export/vmimgs2/commitraw-base-f17.img
                [root@moon qemu]# 
                
                [root@moon qemu]# ls -lash /export/vmimgs2/*rawbase*
                904K -rw-------. 1 qemu qemu 960K Oct 21 18:47 /export/vmimgs2/snap1-c-rawbase.qcow2
                840K -rw-------. 1 qemu qemu 896K Oct 21 18:46 /export/vmimgs2/snap2-c-rawbase.qcow2
                776K -rw-------. 1 qemu qemu 832K Oct 21 18:39 /export/vmimgs2/snap3-c-rawbase.qcow2
                904K -rw-------. 1 qemu qemu 960K Oct 21 18:48 /export/vmimgs2/snap4-c-rawbase.qcow2
                [root@moon qemu]# 
                ======================

[9] Snapshot metadata can be cleaned to reflect the new reality of disk image snapshots:

                # virsh snapshot-delete --domain commitraw-base snap3 --metadata  
                # virsh snapshot-delete --domain commitraw-base snap2 --metadata  
                # virsh snapshot-delete --domain commitraw-base snap1 --metadata



[*** Example-4: Reducing snapshot image chain using "blockpull" ***
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

    (0) Assuming 4 external disk-only (live) snapshots were created.
        

    (1) Let's check the snapshot overlay images size *before* blockpull operation (note the image of 'Active'):
           ======================
            # ls -lash /var/lib/libvirt/images/RootBase.img 
            608M -rw-r--r--. 1 qemu qemu 1.0G Oct 11 17:54 /var/lib/libvirt/images/RootBase.img

            # ls -lash /var/lib/libvirt/images/*Snap*
            840K -rw-------. 1 qemu qemu 896K Oct 11 17:56 /var/lib/libvirt/images/Snap-1.qcow2
            392K -rw-------. 1 qemu qemu 448K Oct 11 17:56 /var/lib/libvirt/images/Snap-2.qcow2
            456K -rw-------. 1 qemu qemu 512K Oct 11 17:56 /var/lib/libvirt/images/Snap-3.qcow2
            2.9M -rw-------. 1 qemu qemu 3.0M Oct 11 18:10 /var/lib/libvirt/images/Active.qcow2
           ======================

    (2) Also, check the disk image information of 'Active'. It can noticed that
        'Active' has Snap-3 as its backing file. 
           ======================
            [kashyap@moon qemu-n]$ sudo ./qemu-img info --backing-chain /var/lib/libvirt/images/Active.qcow2
            image: /var/lib/libvirt/images/Active.qcow2
            file format: qcow2
            virtual size: 1.0G (1073741824 bytes)
            disk size: 520K
            cluster_size: 65536
            backing file: /var/lib/libvirt/images/snap3.qcow2
            backing file format: qcow2

            image: /var/lib/libvirt/images/snap3.qcow2
            file format: qcow2
            virtual size: 1.0G (1073741824 bytes)
            disk size: 776K
            cluster_size: 65536
            backing file: /var/lib/libvirt/images/snap2.qcow2
            backing file format: qcow2

            image: /var/lib/libvirt/images/snap2.qcow2
            file format: qcow2
            virtual size: 1.0G (1073741824 bytes)
            disk size: 712K
            cluster_size: 65536
            backing file: /var/lib/libvirt/images/snap1.qcow2
            backing file format: qcow2

            image: /var/lib/libvirt/images/snap1.qcow2
            file format: qcow2
            virtual size: 1.0G (1073741824 bytes)
            disk size: 392K
            cluster_size: 65536
            backing file: /var/lib/libvirt/images/RootBase.img
            backing file format: raw

            image: /var/lib/libvirt/images/commitraw-base-f17.img
            file format: raw
            virtual size: 1.0G (1073741824 bytes)
            disk size: 699M
            [kashyap@moon qemu-n]$
           ======================


    (3) Run the **blockpull** command (without specifying --base). So that the backing chain is merged(inclduing RootBase)
           ======================
            # virsh blockpull --domain ptest2-base --path /var/lib/libvirt/images/Active.qcow2 --wait --verbose
            Block Pull: [100 %]
            Pull complete
           ======================


    (4) Let's again check the snapshot overlay images size *after*
        blockpull operation. It can be noticed, 'Active' is now considerably larger. 
            ======================
            # ls -lash /var/lib/libvirt/images/*Snap*
             840K -rw-------. 1 qemu qemu 896K Oct 11 17:56 /var/lib/libvirt/images/Snap-1.qcow2
             392K -rw-------. 1 qemu qemu 448K Oct 11 17:56 /var/lib/libvirt/images/Snap-2.qcow2
             456K -rw-------. 1 qemu qemu 512K Oct 11 17:56 /var/lib/libvirt/images/Snap-3.qcow2
            1011M -rw-------. 1 qemu qemu 3.0M Oct 11 18:29 /var/lib/libvirt/images/Active.qcow2
            ======================


    (5) Also, check the disk image information of 'Active'. It can now be
        noticed that 'Active' is flattened to a standalone image without any backing file.
            ======================
            # qemu-img info /var/lib/libvirt/images/Active.qcow2
            image: /var/lib/libvirt/images/Active.qcow2
            file format: qcow2
            virtual size: 1.0G (1073741824 bytes)
            disk size: 1.0G
            cluster_size: 65536
            ======================


    (6) We can now clean-up the snapshot tracking metadata by libvirt to reflect the new reality
            ======================
            # virsh snapshot-delete --domain RootBase Snap-3 --metadata
            ======================

        Optionally, one can check, the guest disk contents by invoking guestfish tool(part of libguestfs) READ-ONLY (--ro option below does it) as below
            ======================
            # guestfish --ro -i -a /var/lib/libvirt/images/Active.qcow2
            ======================