After snapshot-delete, the qcow2 image file size doesn't decrease, isn't that a waste of disk space?
Would someone please tell me how to decrease the file size when snapshot-delete, if that's possible?


The image file name of my virtual machine is d0.qcow
As follows:
[root@test1 ]# virsh list 
 Id    Name                           State
----------------------------------------------------

 32    bfbe8ca8-8579-11e2-844a-001018951f48 running


[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 3.6G
cluster_size: 65536
[root@test1 ]# ls -lh
total 3.6G
-rw------- 1 qemu qemu 3.6G Apr 12 17:13 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-list bfbe8ca8-8579-11e2-844a-001018951f48
 Name                 Creation Time             State
------------------------------------------------------------

[root@test1 8]# virsh snapshot-create bfbe8ca8-8579-11e2-844a-001018951f48
Domain snapshot 1365758005 created
[root@test1 ]# virsh snapshot-create bfbe8ca8-8579-11e2-844a-001018951f48
Domain snapshot 1365758022 created
[root@test1 bfbe8ca8-8579-11e2-844a-001018951f48]# virsh snapshot-list bfbe8ca8-8579-11e2-844a-001018951f48
 Name                 Creation Time             State
------------------------------------------------------------
 1365758005           2013-04-12 17:13:25 +0800 running
 1365758022           2013-04-12 17:13:42 +0800 running

[root@test1 ]# qemu-img info d0.qcow
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 3.8G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         1365758005             127M 2013-04-12 17:13:25   00:00:53.141
2         1365758022             127M 2013-04-12 17:13:42   00:01:09.508
[root@test1 ]# ls -lh
total 3.9G
-rw------- 1 qemu qemu 3.9G Apr 12 17:14 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-delete bfbe8ca8-8579-11e2-844a-001018951f48 1365758022
Domain snapshot 1365758022 deleted

[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 3.8G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
                                    0 1970-01-01 08:00:00   00:00:00.000
[root@test1 ]# ls -lh
total 3.9G
-rw------- 1 qemu qemu 3.9G Apr 12 17:14 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-delete bfbe8ca8-8579-11e2-844a-001018951f48 1365758005
Domain snapshot 1365758005 deleted

[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 3.8G
cluster_size: 65536
[root@test1 ]# ls -lh
total 3.9G
-rw------- 1 qemu qemu 3.9G Apr 12 17:14 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-create bfbe8ca8-8579-11e2-844a-001018951f48
Domain snapshot 1365758311 created
[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 3.8G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         1365758311             252M 2013-04-12 17:18:32   00:05:58.136
[root@test1 ]# ls -lh
total 3.9G
-rw------- 1 qemu qemu 3.9G Apr 12 17:18 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-create bfbe8ca8-8579-11e2-844a-001018951f48
Domain snapshot 1365758338 created
[root@test1 ]# qemu-img info d0.qcow
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 4.1G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         1365758311             252M 2013-04-12 17:18:32   00:05:58.136
2         1365758338             252M 2013-04-12 17:18:58   00:06:24.272
[root@test1 ]# ls -lh
total 4.1G
-rw------- 1 qemu qemu 4.1G Apr 12 17:19 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-list bfbe8ca8-8579-11e2-844a-001018951f48
 Name                 Creation Time             State
------------------------------------------------------------
 1365758311           2013-04-12 17:18:31 +0800 running
 1365758338           2013-04-12 17:18:58 +0800 running

[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 4.1G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         1365758311             252M 2013-04-12 17:18:32   00:05:58.136
2         1365758338             252M 2013-04-12 17:18:58   00:06:24.272
[root@test1 ]# virsh snapshot-delete bfbe8ca8-8579-11e2-844a-001018951f48 1365758338
Domain snapshot 1365758338 deleted

[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 4.1G
cluster_size: 65536
Snapshot list:
ID        TAG                 VM SIZE                DATE       VM CLOCK
1         1365758311             252M 2013-04-12 17:18:32   00:05:58.136
[root@test1 ]# ls -lh
total 4.1G
-rw------- 1 qemu qemu 4.1G Apr 12 17:20 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-delete bfbe8ca8-8579-11e2-844a-001018951f48 1365758311
Domain snapshot 1365758311 deleted

[root@test1 ]# qemu-img info d0.qcow 
image: d0.qcow
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 4.1G
cluster_size: 65536
[root@test1 ]# ls -lh
total 4.1G
-rw------- 1 qemu qemu 4.1G Apr 12 17:20 d0.qcow
drwx------ 2 root root 4.0K Apr 12 17:12 held
[root@test1 ]# virsh snapshot-list bfbe8ca8-8579-11e2-844a-001018951f48
 Name                 Creation Time             State
------------------------------------------------------------