slavkap commented on code in PR #10632:
URL: https://github.com/apache/cloudstack/pull/10632#discussion_r2204502992


##########
engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/snapshot/DefaultSnapshotStrategy.java:
##########
@@ -626,6 +646,30 @@ public StrategyPriority canHandle(Snapshot snapshot, Long 
zoneId, SnapshotOperat
         return StrategyPriority.DEFAULT;
     }
 
+    private StrategyPriority validateVmSnapshot(Snapshot snapshot) {
+        VolumeVO volumeVO = volumeDao.findById(snapshot.getVolumeId());
+        Long instanceId = volumeVO.getInstanceId();
+        if (instanceId == null) {
+            return StrategyPriority.DEFAULT;
+        }
+
+        VMInstanceVO vm = vmInstanceDao.findById(instanceId);
+        if (vm == null) {
+            return StrategyPriority.DEFAULT;
+        }
+
+        for (VMSnapshotVO vmSnapshotVO : 
vmSnapshotDao.findByVmAndByType(vm.getId(), VMSnapshot.Type.Disk)) {
+            List<VMSnapshotDetailsVO> vmSnapshotDetails = 
vmSnapshotDetailsDao.listDetails(vmSnapshotVO.getId());
+            if (vmSnapshotDetails.stream().anyMatch(vmSnapshotDetailsVO -> 
vmSnapshotDetailsVO.getName().equals(VolumeApiServiceImpl.KVM_FILE_BASED_STORAGE_SNAPSHOT)))
 {

Review Comment:
   ```suggestion
               if (vmSnapshotDetails.stream().anyMatch(vmSnapshotDetailsVO -> 
VolumeApiServiceImpl.KVM_FILE_BASED_STORAGE_SNAPSHOT.equals(vmSnapshotDetailsVO.getName())))
 {
   ```



##########
server/src/main/java/com/cloud/storage/VolumeApiServiceImpl.java:
##########
@@ -4043,6 +4069,10 @@ public Snapshot allocSnapshotForVm(Long vmId, Long 
volumeId, String snapshotName
             throw new InvalidParameterValueException("Cannot perform this 
operation, unsupported on storage pool type " + storagePool.getPoolType());
         }
 
+        if 
(vmSnapshotDetailsDao.listDetails(vmSnapshotId).stream().anyMatch(vmSnapshotDetailsVO
 -> vmSnapshotDetailsVO.getName().equals(KVM_FILE_BASED_STORAGE_SNAPSHOT))) {

Review Comment:
   ```suggestion
           if 
(vmSnapshotDetailsDao.listDetails(vmSnapshotId).stream().anyMatch(vmSnapshotDetailsVO
 -> KVM_FILE_BASED_STORAGE_SNAPSHOT.equals(vmSnapshotDetailsVO.getName()))) {
   ```



##########
server/src/main/java/com/cloud/storage/VolumeApiServiceImpl.java:
##########
@@ -1399,6 +1407,29 @@ public VolumeVO resizeVolume(ResizeVolumeCmd cmd) throws 
ResourceAllocationExcep
                 shrinkOk);
     }
 
+    protected void validateNoVmSnapshots(VolumeVO volume) {
+        if (volume.getInstanceId() != null) {
+            if 
(vmHasVmSnapshotsExceptKvmDiskOnlySnapshots(volume.getInstanceId())) {
+                throw new InvalidParameterValueException("The volume is 
attached to a VM with memory&disk VM snapshots; therefore, it cannot be 
resized.");
+            }
+        }
+    }
+
+    protected boolean vmHasVmSnapshotsExceptKvmDiskOnlySnapshots(long 
instanceId) {
+        for (VMSnapshotVO vmSnapshotVO : _vmSnapshotDao.findByVm(instanceId)) {
+            if (VMSnapshot.Type.DiskAndMemory.equals(vmSnapshotVO.getType())) {
+                return true;
+            }
+            List<VMSnapshotDetailsVO> vmSnapshotDetails = 
vmSnapshotDetailsDao.listDetails(vmSnapshotVO.getId());
+            if (vmSnapshotDetails.stream().
+                    noneMatch(vmSnapshotDetailsVO -> 
vmSnapshotDetailsVO.getName().equals(KVM_FILE_BASED_STORAGE_SNAPSHOT))) {

Review Comment:
   ```suggestion
                       noneMatch(vmSnapshotDetailsVO -> 
KVM_FILE_BASED_STORAGE_SNAPSHOT.equals(vmSnapshotDetailsVO.getName()))) {
   ```



##########
engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/vmsnapshot/KvmFileBasedStorageVmSnapshotStrategy.java:
##########
@@ -0,0 +1,689 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.cloudstack.storage.vmsnapshot;
+
+import com.cloud.agent.api.Answer;
+import com.cloud.agent.api.VMSnapshotTO;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.DeleteDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.MergeDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.SnapshotMergeTreeTO;
+import com.cloud.agent.api.to.DataTO;
+import com.cloud.configuration.Resource;
+import com.cloud.event.EventTypes;
+import com.cloud.event.UsageEventUtils;
+import com.cloud.hypervisor.Hypervisor;
+import com.cloud.storage.DataStoreRole;
+import com.cloud.storage.Snapshot;
+import com.cloud.storage.SnapshotVO;
+import com.cloud.storage.Storage;
+import com.cloud.storage.Volume;
+import com.cloud.storage.VolumeVO;
+import com.cloud.user.ResourceLimitService;
+import com.cloud.uservm.UserVm;
+import com.cloud.utils.DateUtil;
+import com.cloud.utils.Pair;
+import com.cloud.utils.exception.CloudRuntimeException;
+import com.cloud.utils.fsm.NoTransitionException;
+import com.cloud.vm.UserVmVO;
+import com.cloud.vm.VirtualMachine;
+import com.cloud.vm.snapshot.VMSnapshot;
+import com.cloud.vm.snapshot.VMSnapshotDetailsVO;
+import com.cloud.vm.snapshot.VMSnapshotVO;
+import org.apache.cloudstack.backup.BackupOfferingVO;
+import org.apache.cloudstack.backup.dao.BackupOfferingDao;
+import 
org.apache.cloudstack.engine.subsystem.api.storage.ObjectInDataStoreStateMachine;
+import org.apache.cloudstack.engine.subsystem.api.storage.SnapshotInfo;
+import org.apache.cloudstack.engine.subsystem.api.storage.StrategyPriority;
+import org.apache.cloudstack.engine.subsystem.api.storage.VMSnapshotOptions;
+import org.apache.cloudstack.engine.subsystem.api.storage.VolumeInfo;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreDao;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreVO;
+import org.apache.cloudstack.storage.datastore.db.StoragePoolVO;
+import org.apache.cloudstack.storage.snapshot.SnapshotObject;
+import org.apache.cloudstack.storage.to.SnapshotObjectTO;
+import org.apache.cloudstack.storage.to.VolumeObjectTO;
+import org.apache.commons.collections.CollectionUtils;
+
+import javax.inject.Inject;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+public class KvmFileBasedStorageVmSnapshotStrategy extends 
StorageVMSnapshotStrategy {
+
+    private static final List<Storage.StoragePoolType> 
supportedStoragePoolTypes = List.of(Storage.StoragePoolType.Filesystem, 
Storage.StoragePoolType.NetworkFilesystem, 
Storage.StoragePoolType.SharedMountPoint);
+
+    @Inject
+    protected SnapshotDataStoreDao snapshotDataStoreDao;
+
+    @Inject
+    protected ResourceLimitService resourceLimitManager;
+
+    @Inject
+    protected BackupOfferingDao backupOfferingDao;
+
+    @Override
+    public VMSnapshot takeVMSnapshot(VMSnapshot vmSnapshot) {
+        Map<VolumeInfo, SnapshotObject> volumeInfoToSnapshotObjectMap = new 
HashMap<>();
+        try {
+            return takeVmSnapshotInternal(vmSnapshot, 
volumeInfoToSnapshotObjectMap);
+        } catch (CloudRuntimeException | NullPointerException | 
NoTransitionException ex) {
+            for (VolumeInfo volumeInfo : 
volumeInfoToSnapshotObjectMap.keySet()) {
+                volumeInfo.stateTransit(Volume.Event.OperationFailed);
+                SnapshotObject snapshot = 
volumeInfoToSnapshotObjectMap.get(volumeInfo);
+                try {
+                    snapshot.processEvent(Snapshot.Event.OperationFailed);
+                } catch (NoTransitionException e) {
+                    logger.error("Failed to change snapshot [{}] state due to 
[{}].", snapshot.getUuid(), e.getMessage(), e);
+                }
+                
snapshot.processEvent(ObjectInDataStoreStateMachine.Event.OperationFailed);
+            }
+            try {
+                vmSnapshotHelper.vmSnapshotStateTransitTo(vmSnapshot, 
VMSnapshot.Event.OperationFailed);
+            } catch (NoTransitionException e) {
+                throw new CloudRuntimeException(e);
+            }
+            throw new CloudRuntimeException(ex);
+        }
+    }
+
+    @Override
+    public boolean deleteVMSnapshot(VMSnapshot vmSnapshot) {
+        logger.info("Starting VM snapshot delete process for snapshot [{}].", 
vmSnapshot.getUuid());
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        VMSnapshotVO vmSnapshotBeingDeleted = (VMSnapshotVO) vmSnapshot;
+        Long hostId = 
vmSnapshotHelper.pickRunningHost(vmSnapshotBeingDeleted.getVmId());
+        long virtualSize = 0;
+        boolean isCurrent = vmSnapshotBeingDeleted.getCurrent();
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.ExpungeRequested);
+
+        List<VolumeObjectTO> volumeTOs = 
vmSnapshotHelper.getVolumeTOList(vmSnapshotBeingDeleted.getVmId());
+        List<VMSnapshotVO> snapshotChildren = 
vmSnapshotDao.listByParentAndStateIn(vmSnapshotBeingDeleted.getId(), 
VMSnapshot.State.Ready, VMSnapshot.State.Hidden);
+
+        long realSize = getVMSnapshotRealSize(vmSnapshotBeingDeleted);
+        int numberOfChildren = snapshotChildren.size();
+
+        List<SnapshotVO> volumeSnapshotVos = new ArrayList<>();
+        if (isCurrent && numberOfChildren == 0) {
+            volumeSnapshotVos = 
mergeCurrentDeltaOnSnapshot(vmSnapshotBeingDeleted, userVm, hostId, volumeTOs);
+        } else if (numberOfChildren == 0) {
+            logger.debug("Deleting VM snapshot [{}] as no snapshots/volumes 
depend on it.", vmSnapshot.getUuid());
+            volumeSnapshotVos = deleteSnapshot(vmSnapshotBeingDeleted, hostId);
+            
mergeOldSiblingWithOldParentIfOldParentIsDead(vmSnapshotDao.findByIdIncludingRemoved(vmSnapshotBeingDeleted.getParent()),
 userVm, hostId, volumeTOs);
+        } else if (!isCurrent && numberOfChildren == 1) {
+            VMSnapshotVO childSnapshot = snapshotChildren.get(0);
+            volumeSnapshotVos = mergeSnapshots(vmSnapshotBeingDeleted, 
childSnapshot, userVm, volumeTOs, hostId);
+        }
+
+        for (SnapshotVO snapshotVO : volumeSnapshotVos) {
+            snapshotVO.setState(Snapshot.State.Destroyed);
+            snapshotDao.update(snapshotVO.getId(), snapshotVO);
+        }
+
+        for (VolumeObjectTO volumeTo : volumeTOs) {
+            publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_DELETE, 
vmSnapshotBeingDeleted, userVm, volumeTo);
+            virtualSize += volumeTo.getSize();
+        }
+
+        publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_OFF_PRIMARY, 
vmSnapshotBeingDeleted, userVm, realSize, virtualSize);
+
+        if (numberOfChildren > 1 || (isCurrent && numberOfChildren == 1)) {
+            transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.Hide);
+            return true;
+        }
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.OperationSucceeded);
+
+        vmSnapshotDetailsDao.removeDetails(vmSnapshotBeingDeleted.getId());
+
+        vmSnapshotBeingDeleted.setRemoved(DateUtil.now());
+        vmSnapshotDao.update(vmSnapshotBeingDeleted.getId(), 
vmSnapshotBeingDeleted);
+
+        return true;
+    }
+
+    @Override
+    public boolean revertVMSnapshot(VMSnapshot vmSnapshot) {
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        if (!userVm.getState().equals(VirtualMachine.State.Stopped)) {

Review Comment:
   ```suggestion
           if (!VirtualMachine.State.Stopped.equals(userVm.getState())) {
   ```



##########
plugins/backup/nas/src/main/java/org/apache/cloudstack/backup/NASBackupProvider.java:
##########
@@ -400,6 +413,14 @@ public Backup 
createNewBackupEntryForRestorePoint(Backup.RestorePoint restorePoi
 
     @Override
     public boolean assignVMToBackupOffering(VirtualMachine vm, BackupOffering 
backupOffering) {
+        for (VMSnapshotVO vmSnapshotVO : 
vmSnapshotDao.findByVmAndByType(vm.getId(), VMSnapshot.Type.Disk)) {
+            List<VMSnapshotDetailsVO> vmSnapshotDetails = 
vmSnapshotDetailsDao.listDetails(vmSnapshotVO.getId());
+            if (vmSnapshotDetails.stream().anyMatch(vmSnapshotDetailsVO -> 
vmSnapshotDetailsVO.getName().equals(VolumeApiServiceImpl.KVM_FILE_BASED_STORAGE_SNAPSHOT)))
 {

Review Comment:
   ```suggestion
               if (vmSnapshotDetails.stream().anyMatch(vmSnapshotDetailsVO -> 
VolumeApiServiceImpl.KVM_FILE_BASED_STORAGE_SNAPSHOT.equals(vmSnapshotDetailsVO.getName())))
 {
   ```



##########
engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/vmsnapshot/KvmFileBasedStorageVmSnapshotStrategy.java:
##########
@@ -0,0 +1,689 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.cloudstack.storage.vmsnapshot;
+
+import com.cloud.agent.api.Answer;
+import com.cloud.agent.api.VMSnapshotTO;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.DeleteDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.MergeDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.SnapshotMergeTreeTO;
+import com.cloud.agent.api.to.DataTO;
+import com.cloud.configuration.Resource;
+import com.cloud.event.EventTypes;
+import com.cloud.event.UsageEventUtils;
+import com.cloud.hypervisor.Hypervisor;
+import com.cloud.storage.DataStoreRole;
+import com.cloud.storage.Snapshot;
+import com.cloud.storage.SnapshotVO;
+import com.cloud.storage.Storage;
+import com.cloud.storage.Volume;
+import com.cloud.storage.VolumeVO;
+import com.cloud.user.ResourceLimitService;
+import com.cloud.uservm.UserVm;
+import com.cloud.utils.DateUtil;
+import com.cloud.utils.Pair;
+import com.cloud.utils.exception.CloudRuntimeException;
+import com.cloud.utils.fsm.NoTransitionException;
+import com.cloud.vm.UserVmVO;
+import com.cloud.vm.VirtualMachine;
+import com.cloud.vm.snapshot.VMSnapshot;
+import com.cloud.vm.snapshot.VMSnapshotDetailsVO;
+import com.cloud.vm.snapshot.VMSnapshotVO;
+import org.apache.cloudstack.backup.BackupOfferingVO;
+import org.apache.cloudstack.backup.dao.BackupOfferingDao;
+import 
org.apache.cloudstack.engine.subsystem.api.storage.ObjectInDataStoreStateMachine;
+import org.apache.cloudstack.engine.subsystem.api.storage.SnapshotInfo;
+import org.apache.cloudstack.engine.subsystem.api.storage.StrategyPriority;
+import org.apache.cloudstack.engine.subsystem.api.storage.VMSnapshotOptions;
+import org.apache.cloudstack.engine.subsystem.api.storage.VolumeInfo;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreDao;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreVO;
+import org.apache.cloudstack.storage.datastore.db.StoragePoolVO;
+import org.apache.cloudstack.storage.snapshot.SnapshotObject;
+import org.apache.cloudstack.storage.to.SnapshotObjectTO;
+import org.apache.cloudstack.storage.to.VolumeObjectTO;
+import org.apache.commons.collections.CollectionUtils;
+
+import javax.inject.Inject;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+public class KvmFileBasedStorageVmSnapshotStrategy extends 
StorageVMSnapshotStrategy {
+
+    private static final List<Storage.StoragePoolType> 
supportedStoragePoolTypes = List.of(Storage.StoragePoolType.Filesystem, 
Storage.StoragePoolType.NetworkFilesystem, 
Storage.StoragePoolType.SharedMountPoint);
+
+    @Inject
+    protected SnapshotDataStoreDao snapshotDataStoreDao;
+
+    @Inject
+    protected ResourceLimitService resourceLimitManager;
+
+    @Inject
+    protected BackupOfferingDao backupOfferingDao;
+
+    @Override
+    public VMSnapshot takeVMSnapshot(VMSnapshot vmSnapshot) {
+        Map<VolumeInfo, SnapshotObject> volumeInfoToSnapshotObjectMap = new 
HashMap<>();
+        try {
+            return takeVmSnapshotInternal(vmSnapshot, 
volumeInfoToSnapshotObjectMap);
+        } catch (CloudRuntimeException | NullPointerException | 
NoTransitionException ex) {
+            for (VolumeInfo volumeInfo : 
volumeInfoToSnapshotObjectMap.keySet()) {
+                volumeInfo.stateTransit(Volume.Event.OperationFailed);
+                SnapshotObject snapshot = 
volumeInfoToSnapshotObjectMap.get(volumeInfo);
+                try {
+                    snapshot.processEvent(Snapshot.Event.OperationFailed);
+                } catch (NoTransitionException e) {
+                    logger.error("Failed to change snapshot [{}] state due to 
[{}].", snapshot.getUuid(), e.getMessage(), e);
+                }
+                
snapshot.processEvent(ObjectInDataStoreStateMachine.Event.OperationFailed);
+            }
+            try {
+                vmSnapshotHelper.vmSnapshotStateTransitTo(vmSnapshot, 
VMSnapshot.Event.OperationFailed);
+            } catch (NoTransitionException e) {
+                throw new CloudRuntimeException(e);
+            }
+            throw new CloudRuntimeException(ex);
+        }
+    }
+
+    @Override
+    public boolean deleteVMSnapshot(VMSnapshot vmSnapshot) {
+        logger.info("Starting VM snapshot delete process for snapshot [{}].", 
vmSnapshot.getUuid());
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        VMSnapshotVO vmSnapshotBeingDeleted = (VMSnapshotVO) vmSnapshot;
+        Long hostId = 
vmSnapshotHelper.pickRunningHost(vmSnapshotBeingDeleted.getVmId());
+        long virtualSize = 0;
+        boolean isCurrent = vmSnapshotBeingDeleted.getCurrent();
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.ExpungeRequested);
+
+        List<VolumeObjectTO> volumeTOs = 
vmSnapshotHelper.getVolumeTOList(vmSnapshotBeingDeleted.getVmId());
+        List<VMSnapshotVO> snapshotChildren = 
vmSnapshotDao.listByParentAndStateIn(vmSnapshotBeingDeleted.getId(), 
VMSnapshot.State.Ready, VMSnapshot.State.Hidden);
+
+        long realSize = getVMSnapshotRealSize(vmSnapshotBeingDeleted);
+        int numberOfChildren = snapshotChildren.size();
+
+        List<SnapshotVO> volumeSnapshotVos = new ArrayList<>();
+        if (isCurrent && numberOfChildren == 0) {
+            volumeSnapshotVos = 
mergeCurrentDeltaOnSnapshot(vmSnapshotBeingDeleted, userVm, hostId, volumeTOs);
+        } else if (numberOfChildren == 0) {
+            logger.debug("Deleting VM snapshot [{}] as no snapshots/volumes 
depend on it.", vmSnapshot.getUuid());
+            volumeSnapshotVos = deleteSnapshot(vmSnapshotBeingDeleted, hostId);
+            
mergeOldSiblingWithOldParentIfOldParentIsDead(vmSnapshotDao.findByIdIncludingRemoved(vmSnapshotBeingDeleted.getParent()),
 userVm, hostId, volumeTOs);
+        } else if (!isCurrent && numberOfChildren == 1) {
+            VMSnapshotVO childSnapshot = snapshotChildren.get(0);
+            volumeSnapshotVos = mergeSnapshots(vmSnapshotBeingDeleted, 
childSnapshot, userVm, volumeTOs, hostId);
+        }
+
+        for (SnapshotVO snapshotVO : volumeSnapshotVos) {
+            snapshotVO.setState(Snapshot.State.Destroyed);
+            snapshotDao.update(snapshotVO.getId(), snapshotVO);
+        }
+
+        for (VolumeObjectTO volumeTo : volumeTOs) {
+            publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_DELETE, 
vmSnapshotBeingDeleted, userVm, volumeTo);
+            virtualSize += volumeTo.getSize();
+        }
+
+        publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_OFF_PRIMARY, 
vmSnapshotBeingDeleted, userVm, realSize, virtualSize);
+
+        if (numberOfChildren > 1 || (isCurrent && numberOfChildren == 1)) {
+            transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.Hide);
+            return true;
+        }
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.OperationSucceeded);
+
+        vmSnapshotDetailsDao.removeDetails(vmSnapshotBeingDeleted.getId());
+
+        vmSnapshotBeingDeleted.setRemoved(DateUtil.now());
+        vmSnapshotDao.update(vmSnapshotBeingDeleted.getId(), 
vmSnapshotBeingDeleted);
+
+        return true;
+    }
+
+    @Override
+    public boolean revertVMSnapshot(VMSnapshot vmSnapshot) {
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        if (!userVm.getState().equals(VirtualMachine.State.Stopped)) {
+            throw new CloudRuntimeException("VM must be stopped to revert 
disk-only VM snapshot.");
+        }
+
+        VMSnapshotVO vmSnapshotBeingReverted = (VMSnapshotVO) vmSnapshot;
+        Long hostId = 
vmSnapshotHelper.pickRunningHost(vmSnapshotBeingReverted.getVmId());
+
+        transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.RevertRequested);
+
+        List<SnapshotDataStoreVO> volumeSnapshots = 
getVolumeSnapshotsAssociatedWithVmSnapshot(vmSnapshotBeingReverted);
+        List<SnapshotObjectTO> volumeSnapshotTos = volumeSnapshots.stream()
+                .map(snapshot -> (SnapshotObjectTO) 
snapshotDataFactory.getSnapshot(snapshot.getSnapshotId(), 
snapshot.getDataStoreId(), DataStoreRole.Primary).getTO())
+                .collect(Collectors.toList());
+
+        RevertDiskOnlyVmSnapshotCommand revertDiskOnlyVMSnapshotCommand = new 
RevertDiskOnlyVmSnapshotCommand(volumeSnapshotTos, userVm.getName());
+        Answer answer = agentMgr.easySend(hostId, 
revertDiskOnlyVMSnapshotCommand);
+
+        if (answer == null || !answer.getResult()) {
+            transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.OperationFailed);
+            logger.error(answer != null ? answer.getDetails() : 
String.format("Communication failure with host [%s].", hostId));
+            throw new CloudRuntimeException(String.format("Error reverting VM 
snapshot [%s].", vmSnapshot.getUuid()));
+        }
+
+        RevertDiskOnlyVmSnapshotAnswer revertDiskOnlyVMSnapshotAnswer = 
(RevertDiskOnlyVmSnapshotAnswer) answer;
+
+        for (VolumeObjectTO volumeObjectTo : 
revertDiskOnlyVMSnapshotAnswer.getVolumeObjectTos()) {
+            VolumeVO volumeVO = 
volumeDao.findById(volumeObjectTo.getVolumeId());
+            volumeVO.setPath(volumeObjectTo.getPath());
+            updateSizeIfNeeded(volumeSnapshots, volumeVO, volumeObjectTo);
+
+            volumeDao.update(volumeVO.getId(), volumeVO);
+            publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_REVERT, 
vmSnapshotBeingReverted, userVm, volumeObjectTo);
+        }
+
+        transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.OperationSucceeded);
+
+        VMSnapshotVO currentVmSnapshot = 
vmSnapshotDao.findCurrentSnapshotByVmId(userVm.getId());
+        currentVmSnapshot.setCurrent(false);
+        vmSnapshotBeingReverted.setCurrent(true);
+
+        vmSnapshotDao.update(currentVmSnapshot.getId(), currentVmSnapshot);
+        vmSnapshotDao.update(vmSnapshotBeingReverted.getId(), 
vmSnapshotBeingReverted);
+
+        mergeOldSiblingWithOldParentIfOldParentIsDead(currentVmSnapshot, 
userVm, hostId, vmSnapshotHelper.getVolumeTOList(userVm.getId()));
+        return true;
+    }
+
+    /**
+     * Updates the volume size if it changed due to the snapshot reversion.
+     * */
+    private void updateSizeIfNeeded(List<SnapshotDataStoreVO> volumeSnapshots, 
VolumeVO volumeVO, VolumeObjectTO volumeObjectTO) {
+        SnapshotDataStoreVO snapshotRef = 
volumeSnapshots.stream().filter(snapshotDataStoreVO -> 
snapshotDataStoreVO.getVolumeId() == volumeVO.getId()).
+                findFirst().
+                orElseThrow(() -> new 
CloudRuntimeException(String.format("Unable to map any snapshot to volume 
[%s].", volumeVO)));
+
+        if (volumeVO.getSize() == snapshotRef.getSize()) {
+            logger.debug("No need to update the volume size and launch a 
resize event as the snapshot [{}] and volume [{}] size are equal.", 
snapshotRef.getSnapshotId(), volumeVO.getUuid());
+            return;
+        }
+
+        long delta = volumeVO.getSize() - snapshotRef.getSize();
+        if (delta < 0) {
+            
resourceLimitManager.incrementResourceCount(volumeVO.getAccountId(), 
Resource.ResourceType.primary_storage, -delta);
+        } else {
+            
resourceLimitManager.decrementResourceCount(volumeVO.getAccountId(), 
Resource.ResourceType.primary_storage, delta);
+        }
+        volumeVO.setSize(snapshotRef.getSize());
+        volumeObjectTO.setSize(snapshotRef.getSize());
+        volumeDao.update(volumeVO.getId(), volumeVO);
+        UsageEventUtils.publishUsageEvent(EventTypes.EVENT_VOLUME_RESIZE, 
volumeVO.getAccountId(), volumeVO.getDataCenterId(), volumeVO.getId(), 
volumeVO.getName(),
+                volumeVO.getDiskOfferingId(), volumeVO.getTemplateId(), 
volumeVO.getSize(), Volume.class.getName(), volumeVO.getUuid());
+    }
+
+    private void mergeOldSiblingWithOldParentIfOldParentIsDead(VMSnapshotVO 
oldParent, UserVmVO userVm, Long hostId, List<VolumeObjectTO> volumeTOs) {
+        if (oldParent == null || oldParent.getRemoved() != null || 
!VMSnapshot.State.Hidden.equals(oldParent.getState())) {
+            return;
+        }
+
+        List<SnapshotVO> snapshotVos;
+
+        if (oldParent.getCurrent()) {
+            snapshotVos = mergeCurrentDeltaOnSnapshot(oldParent, userVm, 
hostId, volumeTOs);
+        } else {
+            List<VMSnapshotVO> oldSiblings = 
vmSnapshotDao.listByParentAndStateIn(oldParent.getId(), VMSnapshot.State.Ready, 
VMSnapshot.State.Hidden);
+
+            if (oldSiblings.size() > 1) {
+                logger.debug("The old snapshot [{}] is dead and still has more 
than one live child snapshot. We will keep it on storage still.", 
oldParent.getUuid());
+                return;
+            }
+
+            if (oldSiblings.isEmpty()) {
+                logger.warn("The old snapshot [{}] is dead, but it only had 
one child. This is an inconsistency and should be analysed/reported.", 
oldParent.getUuid());
+                return;
+            }
+
+            VMSnapshotVO oldSibling = oldSiblings.get(0);
+            logger.debug("Merging VM snapshot [{}] with [{}] as the former was 
hidden and only the latter depends on it.", oldParent.getUuid(), 
oldSibling.getUuid());
+
+            snapshotVos = mergeSnapshots(oldParent, oldSibling, userVm, 
volumeTOs, hostId);
+        }
+
+        for (SnapshotVO snapshotVO : snapshotVos) {
+            snapshotVO.setState(Snapshot.State.Destroyed);
+            snapshotDao.update(snapshotVO.getId(), snapshotVO);
+        }
+
+        vmSnapshotDetailsDao.removeDetails(oldParent.getId());
+
+        oldParent.setRemoved(DateUtil.now());
+        vmSnapshotDao.update(oldParent.getId(), oldParent);
+
+        transitStateWithoutThrow(oldParent, VMSnapshot.Event.ExpungeRequested);
+        transitStateWithoutThrow(oldParent, 
VMSnapshot.Event.OperationSucceeded);
+    }
+
+    @Override
+    public StrategyPriority canHandle(VMSnapshot vmSnapshot) {
+        if (!VMSnapshot.State.Allocated.equals(vmSnapshot.getState())) {
+            List<VMSnapshotDetailsVO> vmSnapshotDetails = 
vmSnapshotDetailsDao.findDetails(vmSnapshot.getId(), 
KVM_FILE_BASED_STORAGE_SNAPSHOT);
+            if (CollectionUtils.isEmpty(vmSnapshotDetails)) {
+                logger.debug("KVM file based storage VM snapshot strategy 
cannot handle [{}] as it is not a KVM file based storage VM snapshot.",
+                        vmSnapshot.getUuid());
+                return StrategyPriority.CANT_HANDLE;
+            }
+            return StrategyPriority.HIGHEST;
+        }
+
+        long vmId = vmSnapshot.getVmId();
+        boolean memorySnapshot = 
VMSnapshot.Type.DiskAndMemory.equals(vmSnapshot.getType());
+        return canHandle(vmId, null, memorySnapshot);
+    }
+
+    @Override
+    public StrategyPriority canHandle(Long vmId, Long rootPoolId, boolean 
snapshotMemory) {
+        VirtualMachine vm = userVmDao.findById(vmId);
+
+        String cantHandleLog = String.format("KVM file based storage VM 
snapshot strategy cannot handle VM snapshot for [%s]", vm);
+        if (snapshotMemory) {
+            logger.debug("{} as a snapshot with memory was requested.", 
cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        if (!Hypervisor.HypervisorType.KVM.equals(vm.getHypervisorType())) {
+            logger.debug("{} as the hypervisor is not KVM.", cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        if (CollectionUtils.isNotEmpty(vmSnapshotDao.findByVmAndByType(vmId, 
VMSnapshot.Type.DiskAndMemory))) {
+            logger.debug("{} as there is already a VM snapshot with disk and 
memory.", cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        List<VolumeVO> volumes = volumeDao.findByInstance(vmId);
+        for (VolumeVO volume : volumes) {
+            StoragePoolVO storagePoolVO = 
storagePool.findById(volume.getPoolId());
+            if 
(!supportedStoragePoolTypes.contains(storagePoolVO.getPoolType())) {
+                logger.debug(String.format("%s as the VM has a volume that is 
in a storage with unsupported type [%s].", cantHandleLog, 
storagePoolVO.getPoolType()));
+                return StrategyPriority.CANT_HANDLE;
+            }
+            List<SnapshotVO> snapshots = 
snapshotDao.listByVolumeIdAndTypeNotInAndStateNotRemoved(volume.getId(), 
Snapshot.Type.GROUP);
+            if (CollectionUtils.isNotEmpty(snapshots)) {
+                logger.debug("{} as VM has a volume with snapshots {}. Volume 
snapshots and KvmFileBasedStorageVmSnapshotStrategy are not compatible, as 
restoring volume snapshots will erase VM " +
+                        "snapshots and cause data loss.", cantHandleLog, 
snapshots);
+                return StrategyPriority.CANT_HANDLE;
+            }
+        }
+
+        BackupOfferingVO backupOffering = 
backupOfferingDao.findById(vm.getBackupOfferingId());
+        if (backupOffering != null) {
+            logger.debug("{} as the VM has a backup offering. This strategy 
does not support snapshots on VMs with current backup providers.", 
cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        return StrategyPriority.HIGHEST;
+    }
+
+    private List<SnapshotVO> deleteSnapshot(VMSnapshotVO vmSnapshotVO, Long 
hostId) {
+        List<SnapshotDataStoreVO> volumeSnapshots = 
getVolumeSnapshotsAssociatedWithVmSnapshot(vmSnapshotVO);
+        List<DataTO> volumeSnapshotTOList = volumeSnapshots.stream()
+                .map(snapshotDataStoreVO -> 
snapshotDataFactory.getSnapshot(snapshotDataStoreVO.getSnapshotId(), 
snapshotDataStoreVO.getDataStoreId(), DataStoreRole.Primary).getTO())
+                .collect(Collectors.toList());
+
+        DeleteDiskOnlyVmSnapshotCommand deleteSnapshotCommand = new 
DeleteDiskOnlyVmSnapshotCommand(volumeSnapshotTOList);
+        Answer answer = agentMgr.easySend(hostId, deleteSnapshotCommand);
+        if (answer == null || !answer.getResult()) {
+            logger.error("Failed to delete VM snapshot [{}] due to {}.", 
vmSnapshotVO.getUuid(), answer != null ? answer.getDetails() : "Communication 
failure");
+            throw new CloudRuntimeException(String.format("Failed to delete VM 
snapshot [%s].", vmSnapshotVO.getUuid()));
+        }
+
+        logger.debug("Updating metadata of VM snapshot [{}].", 
vmSnapshotVO.getUuid());
+        List<SnapshotVO> snapshotVOList = new ArrayList<>();
+        for (SnapshotDataStoreVO snapshotDataStoreVO : volumeSnapshots) {
+            snapshotDataStoreDao.remove(snapshotDataStoreVO.getId());
+            
snapshotVOList.add(snapshotDao.findById(snapshotDataStoreVO.getSnapshotId()));
+        }
+        return snapshotVOList;
+    }
+
+    private List<SnapshotVO> mergeSnapshots(VMSnapshotVO vmSnapshotVO, 
VMSnapshotVO childSnapshot, UserVmVO userVm, List<VolumeObjectTO> 
volumeObjectTOS, Long hostId) {
+        logger.debug("Merging VM snapshot [{}] with its child [{}].", 
vmSnapshotVO.getUuid(), childSnapshot.getUuid());
+
+        List<VMSnapshotVO> snapshotGrandChildren = 
vmSnapshotDao.listByParentAndStateIn(childSnapshot.getId(), 
VMSnapshot.State.Ready, VMSnapshot.State.Hidden);
+
+        if (userVm.getState().equals(VirtualMachine.State.Running) && 
!snapshotGrandChildren.isEmpty()) {
+            logger.debug("Removing VM snapshots that are part of the VM's [{}] 
current backing chain from the list of snapshots to be rebased.", 
userVm.getUuid());
+            
removeCurrentBackingChainSnapshotFromVmSnapshotList(snapshotGrandChildren, 
userVm);
+        }
+
+        List<SnapshotMergeTreeTO> snapshotMergeTreeToList = 
generateSnapshotMergeTrees(vmSnapshotVO, childSnapshot, snapshotGrandChildren);
+
+        if (childSnapshot.getCurrent() && 
!userVm.getState().equals(VirtualMachine.State.Running)) {

Review Comment:
   ```suggestion
           if (childSnapshot.getCurrent() && 
!VirtualMachine.State.Running.equals(userVm.getState()) {
   ```



##########
engine/storage/snapshot/src/main/java/org/apache/cloudstack/storage/vmsnapshot/KvmFileBasedStorageVmSnapshotStrategy.java:
##########
@@ -0,0 +1,689 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.cloudstack.storage.vmsnapshot;
+
+import com.cloud.agent.api.Answer;
+import com.cloud.agent.api.VMSnapshotTO;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.CreateDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.DeleteDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.MergeDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotAnswer;
+import com.cloud.agent.api.storage.RevertDiskOnlyVmSnapshotCommand;
+import com.cloud.agent.api.storage.SnapshotMergeTreeTO;
+import com.cloud.agent.api.to.DataTO;
+import com.cloud.configuration.Resource;
+import com.cloud.event.EventTypes;
+import com.cloud.event.UsageEventUtils;
+import com.cloud.hypervisor.Hypervisor;
+import com.cloud.storage.DataStoreRole;
+import com.cloud.storage.Snapshot;
+import com.cloud.storage.SnapshotVO;
+import com.cloud.storage.Storage;
+import com.cloud.storage.Volume;
+import com.cloud.storage.VolumeVO;
+import com.cloud.user.ResourceLimitService;
+import com.cloud.uservm.UserVm;
+import com.cloud.utils.DateUtil;
+import com.cloud.utils.Pair;
+import com.cloud.utils.exception.CloudRuntimeException;
+import com.cloud.utils.fsm.NoTransitionException;
+import com.cloud.vm.UserVmVO;
+import com.cloud.vm.VirtualMachine;
+import com.cloud.vm.snapshot.VMSnapshot;
+import com.cloud.vm.snapshot.VMSnapshotDetailsVO;
+import com.cloud.vm.snapshot.VMSnapshotVO;
+import org.apache.cloudstack.backup.BackupOfferingVO;
+import org.apache.cloudstack.backup.dao.BackupOfferingDao;
+import 
org.apache.cloudstack.engine.subsystem.api.storage.ObjectInDataStoreStateMachine;
+import org.apache.cloudstack.engine.subsystem.api.storage.SnapshotInfo;
+import org.apache.cloudstack.engine.subsystem.api.storage.StrategyPriority;
+import org.apache.cloudstack.engine.subsystem.api.storage.VMSnapshotOptions;
+import org.apache.cloudstack.engine.subsystem.api.storage.VolumeInfo;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreDao;
+import org.apache.cloudstack.storage.datastore.db.SnapshotDataStoreVO;
+import org.apache.cloudstack.storage.datastore.db.StoragePoolVO;
+import org.apache.cloudstack.storage.snapshot.SnapshotObject;
+import org.apache.cloudstack.storage.to.SnapshotObjectTO;
+import org.apache.cloudstack.storage.to.VolumeObjectTO;
+import org.apache.commons.collections.CollectionUtils;
+
+import javax.inject.Inject;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.stream.Collectors;
+
+public class KvmFileBasedStorageVmSnapshotStrategy extends 
StorageVMSnapshotStrategy {
+
+    private static final List<Storage.StoragePoolType> 
supportedStoragePoolTypes = List.of(Storage.StoragePoolType.Filesystem, 
Storage.StoragePoolType.NetworkFilesystem, 
Storage.StoragePoolType.SharedMountPoint);
+
+    @Inject
+    protected SnapshotDataStoreDao snapshotDataStoreDao;
+
+    @Inject
+    protected ResourceLimitService resourceLimitManager;
+
+    @Inject
+    protected BackupOfferingDao backupOfferingDao;
+
+    @Override
+    public VMSnapshot takeVMSnapshot(VMSnapshot vmSnapshot) {
+        Map<VolumeInfo, SnapshotObject> volumeInfoToSnapshotObjectMap = new 
HashMap<>();
+        try {
+            return takeVmSnapshotInternal(vmSnapshot, 
volumeInfoToSnapshotObjectMap);
+        } catch (CloudRuntimeException | NullPointerException | 
NoTransitionException ex) {
+            for (VolumeInfo volumeInfo : 
volumeInfoToSnapshotObjectMap.keySet()) {
+                volumeInfo.stateTransit(Volume.Event.OperationFailed);
+                SnapshotObject snapshot = 
volumeInfoToSnapshotObjectMap.get(volumeInfo);
+                try {
+                    snapshot.processEvent(Snapshot.Event.OperationFailed);
+                } catch (NoTransitionException e) {
+                    logger.error("Failed to change snapshot [{}] state due to 
[{}].", snapshot.getUuid(), e.getMessage(), e);
+                }
+                
snapshot.processEvent(ObjectInDataStoreStateMachine.Event.OperationFailed);
+            }
+            try {
+                vmSnapshotHelper.vmSnapshotStateTransitTo(vmSnapshot, 
VMSnapshot.Event.OperationFailed);
+            } catch (NoTransitionException e) {
+                throw new CloudRuntimeException(e);
+            }
+            throw new CloudRuntimeException(ex);
+        }
+    }
+
+    @Override
+    public boolean deleteVMSnapshot(VMSnapshot vmSnapshot) {
+        logger.info("Starting VM snapshot delete process for snapshot [{}].", 
vmSnapshot.getUuid());
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        VMSnapshotVO vmSnapshotBeingDeleted = (VMSnapshotVO) vmSnapshot;
+        Long hostId = 
vmSnapshotHelper.pickRunningHost(vmSnapshotBeingDeleted.getVmId());
+        long virtualSize = 0;
+        boolean isCurrent = vmSnapshotBeingDeleted.getCurrent();
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.ExpungeRequested);
+
+        List<VolumeObjectTO> volumeTOs = 
vmSnapshotHelper.getVolumeTOList(vmSnapshotBeingDeleted.getVmId());
+        List<VMSnapshotVO> snapshotChildren = 
vmSnapshotDao.listByParentAndStateIn(vmSnapshotBeingDeleted.getId(), 
VMSnapshot.State.Ready, VMSnapshot.State.Hidden);
+
+        long realSize = getVMSnapshotRealSize(vmSnapshotBeingDeleted);
+        int numberOfChildren = snapshotChildren.size();
+
+        List<SnapshotVO> volumeSnapshotVos = new ArrayList<>();
+        if (isCurrent && numberOfChildren == 0) {
+            volumeSnapshotVos = 
mergeCurrentDeltaOnSnapshot(vmSnapshotBeingDeleted, userVm, hostId, volumeTOs);
+        } else if (numberOfChildren == 0) {
+            logger.debug("Deleting VM snapshot [{}] as no snapshots/volumes 
depend on it.", vmSnapshot.getUuid());
+            volumeSnapshotVos = deleteSnapshot(vmSnapshotBeingDeleted, hostId);
+            
mergeOldSiblingWithOldParentIfOldParentIsDead(vmSnapshotDao.findByIdIncludingRemoved(vmSnapshotBeingDeleted.getParent()),
 userVm, hostId, volumeTOs);
+        } else if (!isCurrent && numberOfChildren == 1) {
+            VMSnapshotVO childSnapshot = snapshotChildren.get(0);
+            volumeSnapshotVos = mergeSnapshots(vmSnapshotBeingDeleted, 
childSnapshot, userVm, volumeTOs, hostId);
+        }
+
+        for (SnapshotVO snapshotVO : volumeSnapshotVos) {
+            snapshotVO.setState(Snapshot.State.Destroyed);
+            snapshotDao.update(snapshotVO.getId(), snapshotVO);
+        }
+
+        for (VolumeObjectTO volumeTo : volumeTOs) {
+            publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_DELETE, 
vmSnapshotBeingDeleted, userVm, volumeTo);
+            virtualSize += volumeTo.getSize();
+        }
+
+        publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_OFF_PRIMARY, 
vmSnapshotBeingDeleted, userVm, realSize, virtualSize);
+
+        if (numberOfChildren > 1 || (isCurrent && numberOfChildren == 1)) {
+            transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.Hide);
+            return true;
+        }
+
+        transitStateWithoutThrow(vmSnapshotBeingDeleted, 
VMSnapshot.Event.OperationSucceeded);
+
+        vmSnapshotDetailsDao.removeDetails(vmSnapshotBeingDeleted.getId());
+
+        vmSnapshotBeingDeleted.setRemoved(DateUtil.now());
+        vmSnapshotDao.update(vmSnapshotBeingDeleted.getId(), 
vmSnapshotBeingDeleted);
+
+        return true;
+    }
+
+    @Override
+    public boolean revertVMSnapshot(VMSnapshot vmSnapshot) {
+        UserVmVO userVm = userVmDao.findById(vmSnapshot.getVmId());
+        if (!userVm.getState().equals(VirtualMachine.State.Stopped)) {
+            throw new CloudRuntimeException("VM must be stopped to revert 
disk-only VM snapshot.");
+        }
+
+        VMSnapshotVO vmSnapshotBeingReverted = (VMSnapshotVO) vmSnapshot;
+        Long hostId = 
vmSnapshotHelper.pickRunningHost(vmSnapshotBeingReverted.getVmId());
+
+        transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.RevertRequested);
+
+        List<SnapshotDataStoreVO> volumeSnapshots = 
getVolumeSnapshotsAssociatedWithVmSnapshot(vmSnapshotBeingReverted);
+        List<SnapshotObjectTO> volumeSnapshotTos = volumeSnapshots.stream()
+                .map(snapshot -> (SnapshotObjectTO) 
snapshotDataFactory.getSnapshot(snapshot.getSnapshotId(), 
snapshot.getDataStoreId(), DataStoreRole.Primary).getTO())
+                .collect(Collectors.toList());
+
+        RevertDiskOnlyVmSnapshotCommand revertDiskOnlyVMSnapshotCommand = new 
RevertDiskOnlyVmSnapshotCommand(volumeSnapshotTos, userVm.getName());
+        Answer answer = agentMgr.easySend(hostId, 
revertDiskOnlyVMSnapshotCommand);
+
+        if (answer == null || !answer.getResult()) {
+            transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.OperationFailed);
+            logger.error(answer != null ? answer.getDetails() : 
String.format("Communication failure with host [%s].", hostId));
+            throw new CloudRuntimeException(String.format("Error reverting VM 
snapshot [%s].", vmSnapshot.getUuid()));
+        }
+
+        RevertDiskOnlyVmSnapshotAnswer revertDiskOnlyVMSnapshotAnswer = 
(RevertDiskOnlyVmSnapshotAnswer) answer;
+
+        for (VolumeObjectTO volumeObjectTo : 
revertDiskOnlyVMSnapshotAnswer.getVolumeObjectTos()) {
+            VolumeVO volumeVO = 
volumeDao.findById(volumeObjectTo.getVolumeId());
+            volumeVO.setPath(volumeObjectTo.getPath());
+            updateSizeIfNeeded(volumeSnapshots, volumeVO, volumeObjectTo);
+
+            volumeDao.update(volumeVO.getId(), volumeVO);
+            publishUsageEvent(EventTypes.EVENT_VM_SNAPSHOT_REVERT, 
vmSnapshotBeingReverted, userVm, volumeObjectTo);
+        }
+
+        transitStateWithoutThrow(vmSnapshotBeingReverted, 
VMSnapshot.Event.OperationSucceeded);
+
+        VMSnapshotVO currentVmSnapshot = 
vmSnapshotDao.findCurrentSnapshotByVmId(userVm.getId());
+        currentVmSnapshot.setCurrent(false);
+        vmSnapshotBeingReverted.setCurrent(true);
+
+        vmSnapshotDao.update(currentVmSnapshot.getId(), currentVmSnapshot);
+        vmSnapshotDao.update(vmSnapshotBeingReverted.getId(), 
vmSnapshotBeingReverted);
+
+        mergeOldSiblingWithOldParentIfOldParentIsDead(currentVmSnapshot, 
userVm, hostId, vmSnapshotHelper.getVolumeTOList(userVm.getId()));
+        return true;
+    }
+
+    /**
+     * Updates the volume size if it changed due to the snapshot reversion.
+     * */
+    private void updateSizeIfNeeded(List<SnapshotDataStoreVO> volumeSnapshots, 
VolumeVO volumeVO, VolumeObjectTO volumeObjectTO) {
+        SnapshotDataStoreVO snapshotRef = 
volumeSnapshots.stream().filter(snapshotDataStoreVO -> 
snapshotDataStoreVO.getVolumeId() == volumeVO.getId()).
+                findFirst().
+                orElseThrow(() -> new 
CloudRuntimeException(String.format("Unable to map any snapshot to volume 
[%s].", volumeVO)));
+
+        if (volumeVO.getSize() == snapshotRef.getSize()) {
+            logger.debug("No need to update the volume size and launch a 
resize event as the snapshot [{}] and volume [{}] size are equal.", 
snapshotRef.getSnapshotId(), volumeVO.getUuid());
+            return;
+        }
+
+        long delta = volumeVO.getSize() - snapshotRef.getSize();
+        if (delta < 0) {
+            
resourceLimitManager.incrementResourceCount(volumeVO.getAccountId(), 
Resource.ResourceType.primary_storage, -delta);
+        } else {
+            
resourceLimitManager.decrementResourceCount(volumeVO.getAccountId(), 
Resource.ResourceType.primary_storage, delta);
+        }
+        volumeVO.setSize(snapshotRef.getSize());
+        volumeObjectTO.setSize(snapshotRef.getSize());
+        volumeDao.update(volumeVO.getId(), volumeVO);
+        UsageEventUtils.publishUsageEvent(EventTypes.EVENT_VOLUME_RESIZE, 
volumeVO.getAccountId(), volumeVO.getDataCenterId(), volumeVO.getId(), 
volumeVO.getName(),
+                volumeVO.getDiskOfferingId(), volumeVO.getTemplateId(), 
volumeVO.getSize(), Volume.class.getName(), volumeVO.getUuid());
+    }
+
+    private void mergeOldSiblingWithOldParentIfOldParentIsDead(VMSnapshotVO 
oldParent, UserVmVO userVm, Long hostId, List<VolumeObjectTO> volumeTOs) {
+        if (oldParent == null || oldParent.getRemoved() != null || 
!VMSnapshot.State.Hidden.equals(oldParent.getState())) {
+            return;
+        }
+
+        List<SnapshotVO> snapshotVos;
+
+        if (oldParent.getCurrent()) {
+            snapshotVos = mergeCurrentDeltaOnSnapshot(oldParent, userVm, 
hostId, volumeTOs);
+        } else {
+            List<VMSnapshotVO> oldSiblings = 
vmSnapshotDao.listByParentAndStateIn(oldParent.getId(), VMSnapshot.State.Ready, 
VMSnapshot.State.Hidden);
+
+            if (oldSiblings.size() > 1) {
+                logger.debug("The old snapshot [{}] is dead and still has more 
than one live child snapshot. We will keep it on storage still.", 
oldParent.getUuid());
+                return;
+            }
+
+            if (oldSiblings.isEmpty()) {
+                logger.warn("The old snapshot [{}] is dead, but it only had 
one child. This is an inconsistency and should be analysed/reported.", 
oldParent.getUuid());
+                return;
+            }
+
+            VMSnapshotVO oldSibling = oldSiblings.get(0);
+            logger.debug("Merging VM snapshot [{}] with [{}] as the former was 
hidden and only the latter depends on it.", oldParent.getUuid(), 
oldSibling.getUuid());
+
+            snapshotVos = mergeSnapshots(oldParent, oldSibling, userVm, 
volumeTOs, hostId);
+        }
+
+        for (SnapshotVO snapshotVO : snapshotVos) {
+            snapshotVO.setState(Snapshot.State.Destroyed);
+            snapshotDao.update(snapshotVO.getId(), snapshotVO);
+        }
+
+        vmSnapshotDetailsDao.removeDetails(oldParent.getId());
+
+        oldParent.setRemoved(DateUtil.now());
+        vmSnapshotDao.update(oldParent.getId(), oldParent);
+
+        transitStateWithoutThrow(oldParent, VMSnapshot.Event.ExpungeRequested);
+        transitStateWithoutThrow(oldParent, 
VMSnapshot.Event.OperationSucceeded);
+    }
+
+    @Override
+    public StrategyPriority canHandle(VMSnapshot vmSnapshot) {
+        if (!VMSnapshot.State.Allocated.equals(vmSnapshot.getState())) {
+            List<VMSnapshotDetailsVO> vmSnapshotDetails = 
vmSnapshotDetailsDao.findDetails(vmSnapshot.getId(), 
KVM_FILE_BASED_STORAGE_SNAPSHOT);
+            if (CollectionUtils.isEmpty(vmSnapshotDetails)) {
+                logger.debug("KVM file based storage VM snapshot strategy 
cannot handle [{}] as it is not a KVM file based storage VM snapshot.",
+                        vmSnapshot.getUuid());
+                return StrategyPriority.CANT_HANDLE;
+            }
+            return StrategyPriority.HIGHEST;
+        }
+
+        long vmId = vmSnapshot.getVmId();
+        boolean memorySnapshot = 
VMSnapshot.Type.DiskAndMemory.equals(vmSnapshot.getType());
+        return canHandle(vmId, null, memorySnapshot);
+    }
+
+    @Override
+    public StrategyPriority canHandle(Long vmId, Long rootPoolId, boolean 
snapshotMemory) {
+        VirtualMachine vm = userVmDao.findById(vmId);
+
+        String cantHandleLog = String.format("KVM file based storage VM 
snapshot strategy cannot handle VM snapshot for [%s]", vm);
+        if (snapshotMemory) {
+            logger.debug("{} as a snapshot with memory was requested.", 
cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        if (!Hypervisor.HypervisorType.KVM.equals(vm.getHypervisorType())) {
+            logger.debug("{} as the hypervisor is not KVM.", cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        if (CollectionUtils.isNotEmpty(vmSnapshotDao.findByVmAndByType(vmId, 
VMSnapshot.Type.DiskAndMemory))) {
+            logger.debug("{} as there is already a VM snapshot with disk and 
memory.", cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        List<VolumeVO> volumes = volumeDao.findByInstance(vmId);
+        for (VolumeVO volume : volumes) {
+            StoragePoolVO storagePoolVO = 
storagePool.findById(volume.getPoolId());
+            if 
(!supportedStoragePoolTypes.contains(storagePoolVO.getPoolType())) {
+                logger.debug(String.format("%s as the VM has a volume that is 
in a storage with unsupported type [%s].", cantHandleLog, 
storagePoolVO.getPoolType()));
+                return StrategyPriority.CANT_HANDLE;
+            }
+            List<SnapshotVO> snapshots = 
snapshotDao.listByVolumeIdAndTypeNotInAndStateNotRemoved(volume.getId(), 
Snapshot.Type.GROUP);
+            if (CollectionUtils.isNotEmpty(snapshots)) {
+                logger.debug("{} as VM has a volume with snapshots {}. Volume 
snapshots and KvmFileBasedStorageVmSnapshotStrategy are not compatible, as 
restoring volume snapshots will erase VM " +
+                        "snapshots and cause data loss.", cantHandleLog, 
snapshots);
+                return StrategyPriority.CANT_HANDLE;
+            }
+        }
+
+        BackupOfferingVO backupOffering = 
backupOfferingDao.findById(vm.getBackupOfferingId());
+        if (backupOffering != null) {
+            logger.debug("{} as the VM has a backup offering. This strategy 
does not support snapshots on VMs with current backup providers.", 
cantHandleLog);
+            return StrategyPriority.CANT_HANDLE;
+        }
+
+        return StrategyPriority.HIGHEST;
+    }
+
+    private List<SnapshotVO> deleteSnapshot(VMSnapshotVO vmSnapshotVO, Long 
hostId) {
+        List<SnapshotDataStoreVO> volumeSnapshots = 
getVolumeSnapshotsAssociatedWithVmSnapshot(vmSnapshotVO);
+        List<DataTO> volumeSnapshotTOList = volumeSnapshots.stream()
+                .map(snapshotDataStoreVO -> 
snapshotDataFactory.getSnapshot(snapshotDataStoreVO.getSnapshotId(), 
snapshotDataStoreVO.getDataStoreId(), DataStoreRole.Primary).getTO())
+                .collect(Collectors.toList());
+
+        DeleteDiskOnlyVmSnapshotCommand deleteSnapshotCommand = new 
DeleteDiskOnlyVmSnapshotCommand(volumeSnapshotTOList);
+        Answer answer = agentMgr.easySend(hostId, deleteSnapshotCommand);
+        if (answer == null || !answer.getResult()) {
+            logger.error("Failed to delete VM snapshot [{}] due to {}.", 
vmSnapshotVO.getUuid(), answer != null ? answer.getDetails() : "Communication 
failure");
+            throw new CloudRuntimeException(String.format("Failed to delete VM 
snapshot [%s].", vmSnapshotVO.getUuid()));
+        }
+
+        logger.debug("Updating metadata of VM snapshot [{}].", 
vmSnapshotVO.getUuid());
+        List<SnapshotVO> snapshotVOList = new ArrayList<>();
+        for (SnapshotDataStoreVO snapshotDataStoreVO : volumeSnapshots) {
+            snapshotDataStoreDao.remove(snapshotDataStoreVO.getId());
+            
snapshotVOList.add(snapshotDao.findById(snapshotDataStoreVO.getSnapshotId()));
+        }
+        return snapshotVOList;
+    }
+
+    private List<SnapshotVO> mergeSnapshots(VMSnapshotVO vmSnapshotVO, 
VMSnapshotVO childSnapshot, UserVmVO userVm, List<VolumeObjectTO> 
volumeObjectTOS, Long hostId) {
+        logger.debug("Merging VM snapshot [{}] with its child [{}].", 
vmSnapshotVO.getUuid(), childSnapshot.getUuid());
+
+        List<VMSnapshotVO> snapshotGrandChildren = 
vmSnapshotDao.listByParentAndStateIn(childSnapshot.getId(), 
VMSnapshot.State.Ready, VMSnapshot.State.Hidden);
+
+        if (userVm.getState().equals(VirtualMachine.State.Running) && 
!snapshotGrandChildren.isEmpty()) {

Review Comment:
   ```suggestion
           if (VirtualMachine.State.Running.equals(userVm.getState()) && 
!snapshotGrandChildren.isEmpty()) {
   ```



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