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The failure is because iSCSI needs an implementation of bdrv_reopen() in the block driver layer. The "NotSupported" that is returned back is from the block commit call attempting to do a reopen(), only to have it fail due to no implementation in the iSCSI driver.
It works when the snapshot chain is created live, because each chain stays r/w because without reopen(), none of the images are made r/o after a subsequent snapshot.
This should mean that this is limited to iSCSI, and I believe iSCSI just needs a stub implementation of reopen() (i.e. essentially a no-op), so this should be an easy fix.
Actually, this is not due to needing iSCSI reopen support, but due to missing hdev reopen support. This makes this bug and bug #877339 duplicates, so I will close this bug as it is the lower-priority bug.
*** This bug has been marked as a duplicate of bug 877339 ***
Description of problem: ------------------------ Live commit with previously created snapshot chain failed. Version-Release number of selected component (if applicable): ------------------------------------------------------------- qemu-kvm-rhev-0.12.1.2-2.334.el6.x86_64 How reproducible: ------------------ 100% Steps to Reproduce: -------------------- 1. create a snapshot chain with qemu-img, like base->sn1->sn2->sn3->sn4 qemu-img create -f qcow2 -F qcow2 -b base sn1 qemu-img create -f qcow2 -F qcow2 -b sn1 sn2 qemu-img create -f qcow2 -F qcow2 -b sn2 sn3 qemu-img create -f qcow2 -F qcow2 -b sn3 sn4 2. boot guest with sn4, do live commit, error prompt: { "execute": "__com.redhat_block-commit", "arguments": { "device": "drive-virtio-disk0", "base": "sn1", "top": "sn3", "speed": 1000000000 } } {"error": {"class": "NotSupported", "desc": "Not supported", "data": {}}} Additional info: ------------------------ if boot guest with base, create live snapshot sn1 sn2 sn3 sn4, then do live commit, it works well. above command is simplified, in my real test, related commands are: [root@localhost ~]# qemu-img create -f qcow2 -F qcow2 -b /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-1 /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-2 Formatting '/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-2', fmt=qcow2 size=10737418240 backing_file='/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-1' backing_fmt='qcow2' encryption=off cluster_size=65536 [root@localhost ~]# qemu-img create -f qcow2 -F qcow2 -b /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-2 /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-3 Formatting '/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-3', fmt=qcow2 size=10737418240 backing_file='/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-2' backing_fmt='qcow2' encryption=off cluster_size=65536 [root@localhost ~]# qemu-img create -f qcow2 -F qcow2 -b /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-3 /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-4 Formatting '/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-4', fmt=qcow2 size=10737418240 backing_file='/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-3' backing_fmt='qcow2' encryption=off cluster_size=65536 [root@localhost ~]# qemu-img create -f qcow2 -F qcow2 -b /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-4 /dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-5 Formatting '/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-5', fmt=qcow2 size=10737418240 backing_file='/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-4' backing_fmt='qcow2' encryption=off cluster_size=65536 /usr/libexec/qemu-kvm -enable-kvm -M rhel6.4.0 -m 4G -name rhel6.3-64 -rtc base=utc,clock=host,driftfix=slew -no-kvm-pit-reinjection -uuid 3f2ea5cd-3d29-48ff-aab2-23df1b6ae213 -device virtio-scsi-pci,id=scsi0 -drive file=/dev/disk/by-path/ip-10.66.7.91\:3260-iscsi-iqn.2001-04.com.example\:storage.disk1.amiens.sys1.xyz-lun-5,cache=none,if=none,rerror=stop,werror=stop,id=drive-virtio-disk0,format=qcow2 -device scsi-hd,bus=scsi0.0,drive=drive-virtio-disk0,id=device-virtio-disk0 -netdev tap,script=/etc/qemu-ifup,id=netdev0 -device virtio-net-pci,netdev=netdev0,id=device-net0,mac=00:24:7E:11:22:33 -boot order=cd -monitor stdio -chardev socket,id=s1,path=/tmp/s1,server,nowait -device isa-serial,chardev=s1 -vnc :10 -monitor tcp::1234,server,nowait -smp 4 -qmp tcp:0:5555,server,nowait -chardev socket,id=qmp_monitor_id_qmpmonitor1,path=/tmp/qmp,server,nowait -mon chardev=qmp_monitor_id_qmpmonitor1,mode=control Escape character is '^]'. {"QMP": {"version": {"qemu": {"micro": 1, "minor": 12, "major": 0}, "package": "(qemu-kvm-0.12.1.2)"}, "capabilities": []}} { 'execute' : 'qmp_capabilities' } {"return": {}} { "execute": "__com.redhat_block-commit", "arguments": { "device": "drive-virtio-disk0", "base": "/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-2", "top": "/dev/disk/by-path/ip-10.66.7.91:3260-iscsi-iqn.2001-04.com.example:storage.disk1.amiens.sys1.xyz-lun-4", "speed": 1000000000 } } {"error": {"class": "NotSupported", "desc": "Not supported", "data": {}}}