Code Block

language	java

-- Add table for reconcile commands
CREATE TABLE IF NOT EXISTS `cloud`.`reconcile_commands` (
    `id` bigint unsigned NOT NULL UNIQUE AUTO_INCREMENT,
    `management_server_id` bigint unsigned NOT NULL COMMENT 'node id of the management server',
    `host_id` bigint unsigned NOT NULL COMMENT 'id of the host',
    `request_sequence` bigint unsigned NOT NULL COMMENT 'sequence of the request',
    `state_by_management` varchar(255) COMMENT 'state of the command updated by management server',
    `state_by_agent` varchar(255) COMMENT 'state of the command updated by cloudstack agent',
    `command_name` varchar(255) COMMENT 'name of the command',
    `command_info` MEDIUMTEXT COMMENT 'info of the command',
    `answer_name` varchar(255) COMMENT 'name of the answer',
    `answer_info` MEDIUMTEXT COMMENT 'info of the answer',
    `created` datetime COMMENT 'date the reconcile command was created',
    `removed` datetime COMMENT 'date the reconcile command was removed',
    `updated` datetime COMMENT 'date the reconcile command was updated',
    `retry_count` bigint unsigned DEFAULT 0 COMMENT 'The retry count of reconciliation',
    PRIMARY KEY(`id`),
    INDEX `i_reconcile_command__host_id`(`host_id`),
    CONSTRAINT `fk_reconcile_command__host_id` FOREIGN KEY (`host_id`) REFERENCES `host`(`id`) ON DELETE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- Add last_id to the volumes table
CALL `cloud`.`IDEMPOTENT_ADD_COLUMN`('cloud.volumes', 'last_id', 'bigint(20) unsigned DEFAULT NULL');

3.3 Core changes

Command.java

Code Block

language	java

    public enum State {
        CREATED,        // Command is created by management server
        STARTED,        // Command is started by agent
        PROCESSING,     // Processing by agent
        PROCESSING_IN_BACKEND,  // Processing in backend by agent
        COMPLETED,      // Operation succeeds by agent or management server
        FAILED,         // Operation fails by agent
           RECONCILE_READYRETRY,        // Ready for reconciliation
        RECONCILING,    // Being reconciled by management server
        RECONCILED,     // Reconciled by management server
        RECONCILE_FAILED,       // Fail to reconcile by management server
        TIMED_OUT,      // Timed out on management server or agent
        INTERRUPTED,    // Interrupted by management server or agent (for example agent is restarted),
        DANGLED_IN_BACKEND     // Backend process which cannot be processed normally (for example agent is restarted)
    }

...

3.4 Considerations

Analysis

Scenarios	Current behavior	How to deal with it
Scenario 1: Connection failures between the agent and the management server	Agent cannot send Answer to management server, therefore timed out on management server	Agent: (for reconcile command only) Save command info as JSON file Update command state in JSON when start/process/complete the command STARTED PROCESSING COMPLETE/FAILED Save Answer in the JSON file Every minute Load command/answer from JSON files Send with with PingCommand Receive the PingAnswer from management server Remove the JSON file if state is COMPLETE/FAILED Management server: (for reconcile command only) When create the reconcile command, insert a record with state CREATED into reconcile_commands table When receive the PingCommand, Update command state and answer in reconcile_commands table When wait for the answer of command (reconcile command only) Every 10 seconds, check answer of the reconcile_commands table If answer is found, parse the answer Returna and continue with the answer No need to wait until timeout, if there are connection failures. if timed out, update command state to TIMED_OUT in reconcile_commands table
Scenario 2: Agent crash (or force killing the agent process)	Agent interrupt the process or processing in backend	Management server When Host Status is determined as Down Update reconcile commands (to the agent) in reconcile_command table to INTERRUPTED state Reconcile the command see "3.6 Reconcile the command"
Scenario 3: Agent restart	Agent interrupt the process or processing in backend	Agent (when restart) ~~When stop the agent, updates state of processes~~ ~~PROCESSING to INTERRUPTED~~ ~~PROCESSING_IN_BACKEND to DANGLED_IN_BACKEND~~ When start the agent, updates state of processes PROCESSING to INTERRUPTED PROCESSING_IN_BACKEND to DANGLED_IN_BACKEND Agent send CommandInfo to management server via PingCommand every minute with new state Management server reconcile commands every minute (from reconcile_commands table) , for commands in state
Scenario 4: Agent has completed but timed out	timed out on management server	Management server Update state_by_management to TIMED_OUT Update state_by_agent to COMPLETED (from PROCESSING) Reconcile the command see "3.6 Reconcile the command" TODO: what should be the correct resource state ?4
Scenario 5: Management restart	Agent process the command has completed and send Answer to management server, but no action on management server has not completed, and is processing command	Management server Update state_by_management to INTERRUPTED when mgmt server is stopped. When another management server is detected DOWN update reconcile_command to INTERRUPTED state by management server

3.5 State transitions (Agent)

Management server (all good): CREATED -> COMPLETED

...

States: INTERRUPTED/TIMED_OUT/RECONCILE_READYRETRY/RECONCILE_FAILED

-> RECONCILING

-> RECONCILED (all good) / RECONCILE_FAILED (failed, will retry) / RECONCILED_READY RETRY (success, but need more information, will retry)

Migrate VM without volumes (If VM is Migrating)

PrepareForMigrationCommand (Update VM state)
MigrateCommand
- check via source host (if source is Up)
- check via destination host (if dest is Up)
- determine the state and update

Migrate VM with volumes (If VM is Migrating)

check VM state and volume states
Check if there are reconcile commands for the VM
- If no, check the state on source host (last_host_id) and destination host (host_id)
- PrepareForMigrationCommand (Update VM state)
- MigrateCommand
  - check via source host (if source is Up)
  - check via destination host (if dest is Up)
  - determine the state and update

Migrate Volumes (If volume is Migrating)

CopyCommand (from primary1 to secondary)
- skipped.
- check if there are other Command on same volume ?
CopyCommand (from secondary to primary2)
- check if volume exists on primary2 (via the host, other host on same cluster or pod or zone, depends on the scope of storage, or cluster of host) TODO
- check if volume is changed on primary2
  - if yes, still Copying
  - if no, update state
CopyCommand (from primary1 to primary2)
- check if volume exists on primary1
- check if volume exists on primary2
- check if volume is changed on primary2
  - if yes, still Copying
  - if no, update state
MigrateVolumeCommand (from primary1 to primary2)
- check if volume exists on primary1
- check if volume exists on primary2
- check if volume is changed on primary2
  - if yes, still Migrating
  - if no, update state

3.7 Limitations

Assumption
- Each request does not have multiple commands with same name
Only support:
- CopyCommand, MigrateCommand, MigrateVolumeCommand
- resources in Migration state (vm, source volume, dest volume)
- Hypervisor: KVM
- Storage: NFS, Local, Powerflex

3.8 To be Discussed (TODO)

How to distribute the reconciiation tasks if there are multiple management servers ?
How to better handle the state DANGLED_IN_BACKEND ?
How to better handle the state COMPLETED and FAILED ? Do not reconcile via hosts if state_by_agent to COMPLETED ?

4. Test cases

Please note:

Migration between NFS and Local requires the fix: https://github.com/apache/cloudstack/pull/10266

See "4.3 Summary of test results" on how to reconcile the command

3.7 Limitations

Assumption
- Each request does not have multiple commands with same name
- for example, 700028267079401857-org.apache.cloudstack.storage.command.CopyCommand
Only support:
- 3 commands
  - CopyCommand
  - MigrateCommand
  - MigrateVolumeCommand
- resources in Migration state (vm, source volume, dest volume)
  - skipped if the resource state is not Migrating
  - skipped if the source or dest host/pool are inconsistent with command
- Hypervisor: KVM
- Storage: NFS, Local, Powerflex

3.8 To be Discussed (TODO)

How to distribute the reconciiation tasks if there are multiple management servers ?
1. now: Reconciliation is processed by first management server
How to better handle the state DANGLED_IN_BACKEND ?
1. now: no special action
How to better handle the state COMPLETED and FAILED ? Do not reconcile via hosts if state_by_agent to COMPLETED ?
1. now: no special action

4. Test cases

4.1 Backend commands of VM and volume migrations

Please note:

Migration between NFS and Local requires the fix: https://github.com/apache/cloudstack/pull/10266

	NFS to NFS	NFS to Local	Local to Local	Local to NFS	Powerflex to Powerflex (different envs)	Powerflex <------> NFS
Migrate VM (NFS or Powerflex)	PrepareForMigrationCommand (dest) MigrateCommand (source)	-	-	-	PrepareForMigrationCommand (dest) MigrateCommand (source)	-
Migrate VM with volumes (NFS only)	CopyCommand (template to primary if needed) CreateObjectCommand (new volume) ModifyTargetsCommand PrepareForMigrationCommand (dest) MigrateCommand (source) DeleteCommand (source)	same as "NFS to NFS"	same as "NFS to NFS"	same as "NFS to NFS"	Migrating a volume online with KVM from managed storage is not currently supported.	Pool [%s] is not compatible with volume [%s], skipping it.
Migrate ROOT Volume (of Running VM) (Powerflex only)	KVM does not support volume live migrationdue to the limited possibility to refresh VM XML domain. Therefore, to live migrate a volume between storage pools, one must migrate the VM to a different host as well to force the VM XML domain update. Use 'migrateVirtualMachineWithVolumes' instead.	same	same	same	MigrateVolumeCommand Same volume ID	Storage pool pr503-t11980-kvm-ol8-kvm-pri3 is not suitable to migrate volume
Migrate ROOT Volume (of Stopped VM) (NFS or Powerflex)	CopyCommand (primary1 to secondary) CopyCommand (secondary to primary2) DeleteCommand (secondary) DeleteCommand (primary1)	same	same	same	CopyCommand (primary1 to primary2) Different volume IDs	same as above

Migrate DATA
NFS to NFS	NFS to Local	Local to Local	Local to NFS	Powerflex to Powerflex	Powerflex <------> NFS
Migrate VM	PrepareForMigrationCommand (dest) MigrateCommand (source)	-	-	-	PrepareForMigrationCommand (dest) MigrateCommand (source)	-
Migrate VM with volumes	CopyCommand (template to primary if needed) CreateObjectCommand (new volume) ModifyTargetsCommand PrepareForMigrationCommand (dest) MigrateCommand (source) DeleteCommand (source)	same as "NFS to NFS"	same as "NFS to NFS"	same as "NFS to NFS"	Migrating a volume online with KVM from managed storage is not currently supported.	Pool [%s] is not compatible with volume [%s], skipping it.
Migrate ROOT Volume (of Running VM)	KVM does not support volume live migrationdue to the limited possibility to refresh VM XML domain. Therefore, to live migrate a volume between storage pools, one must migrate the VM to a different host as well to force the VM XML domain update. Use 'migrateVirtualMachineWithVolumes' instead.	same	same	same	MigrateVolumeCommand Same volume	Storage pool pr503-t11980-kvm-ol8-kvm-pri3 is not suitable to migrate volume	ID	same as above
Migrate DATA Migrate ROOT Volume (of Stopped VM)	CopyCommand (primary1 to secondary) CopyCommand (secondary to primary2) DeleteCommand (secondary) DeleteCommand (primary1)	same	same	same	CopyCommand (primary1 to primary2) Different volume IDs	same as above
Migrate DATA Volume (of Running VM)KVM does not support volume live migrationdue to the limited possibility to refresh VM XML domain. Therefore, to live migrate a volume between storage pools, one must migrate the VM to a different host as well to force the VM XML domain update. Use 'migrateVirtualMachineWithVolumes' instead.unattached)	CopyCommand (primary1 to secondary) CopyCommand (secondary to primary2) DeleteCommand (secondary) DeleteCommand (primary1)	same	same	same	CopyCommand (primary1 to primary2) Different volume IDs MigrateVolumeCommand Same volume	same as above
Migrate DATA Volume (of Stopped VM)	CopyCommand (primary1 to secondary) CopyCommand (secondary to primary2) DeleteCommand (secondary) DeleteCommand (primary1)	same	same	same	CopyCommand (primary1 to primary2) Different volume IDs	same as above
Migrate DATA Volume (unattached)	CopyCommand (primary1 to secondary) CopyCommand (secondary to primary2) DeleteCommand (secondary) DeleteCommand (primary1)	same	same	same	CopyCommand (primary1 to primary2) Different volume IDs	same as above

4.2 How to test

...

systemctl restart cloudstack-agent

...

systemctl restart cloudstack-management

4.2 How to test

agent is restarted
- systemctl restart cloudstack-agent
management server is restarted
- systemctl restart cloudstack-management
agent and management server communication failure
- iptables -I OUTPUT -p tcp -m tcp --dport 8250 -j DROP
- iptables -D OUTPUT -p tcp -m tcp --dport 8250 -j DROP
Agent crash
- pid=$(ps -ef |grep cloudstack-agent |grep -v grep |awk '{print $2}')
- kill -9 $pid
Agent has completed but timed out

4.3 Summary of test results

Action

scenario 1: Connection failures between agent and management server

Scenario 4: Agent has completed but timed out

scenario 2

Agent crash (or force killing the agent process)

scenario 3

Agent is restarted manually

scenarios 5 (Needed ?)

mgmt server is restarted

How to test

...

	iptables -I OUTPUT -p tcp -m

...

tcp --dport 8250 -j DROP

iptables -D OUTPUT -p tcp

...

-m tcp --dport 8250 -j DROP

...

pid=$(ps -ef \|grep cloudstack-agent \|grep -v grep \|awk '{print $2}')

...

&& kill -9 $pid

...

systemctl restart cloudstack-agent

systemctl restart cloudstack-management

1. Migrate VM

MigrateVMCommand
MigrateCommand (internal)

Supported by NFS or Powerflex

intermittent failure

- agent updates answer when connection is back to normal (every reconcile.command.interval in PingRoutingCommand)
  - mgmt server saves the answer into DB
- mgmt server gets the answer from DB every 10 seconds
- mgmt server processes the answer if found
- Operation times out

continuous failure

- agent
  - Migration thread of VM [i-2-1198-VM] finished.
- management
  - Resource [Host:1] is unreachable: Host 1: Operation timed out on migrating VM instance
- Behavior:
  - Active migration command so scheduling a restart
  - Current: vm is Stopped on destination and Running on source (actually it is not if migration is successful)
  - New: vm is Running on destination host if found (via libvirt)
  - (new) check via destination host (if dest is Up)
    - if vm is Running on destination, consider the vm migration is successful.

agent
- agent is started again by systemctl

management server
- Resource [Host:1] is unreachable: Host 1: Operation timed out on migrating VM instance
- (new) check via destination host (if dest is Up)
  - if vm is Running on destination, consider the vm migration is successful.
- Current
  - VM is set to Stopped state on destination host and then Running on the source host
- New:
  - vm is Running on destination if found
  - if not found, set to Stopped state on destination host and then Running on the source host (same as current)

Note:
- - In edge case, if the vm is migrated before agent is restarted (in 10 seconds) , the vm is Stopped (by ACS on destination host)
  - solved

agent
- when agent is restarted or disconnected
- abort the migration job by a disconnect hook (MigrationCancelHook)
  - written by Marcus
  - tested with multiple mgmt servers

management server
- Resource [Host:1] is unreachable: Host 1: Operation timed out on migrating VM instance
- (new) check via destination host (if dest is Up)
  - if vm is Running on destination, consider the vm migration is successful.
- Current: VM is set to Stopped state on destination host and then Running on the source host
- New:
  - vm is Running on destination if found
  - if not found, set to Stopped state on destination host and then Running on the source host (same as current)

Note:
- In edge case, the vm is migrated before aborting the job, the vm is Stopped (by ACS on destination host)
- solved

Issues

mgmt server
- Cancel left-over job-17783
- Cleaning up Instance with Id: 1226
- CleanUp Async Jobs after mgmt server maintenance (#8394)
- vm is Running on source host (this will be updated when mgmt server gets vm report from agent)
agent (new)
- abort the migration job by disconnect hook
- written by Marcus
Edge case: both scenario 1 and 5 happen
- VM is migrated but state is Running on source
- fixed by Abhishek's PR engine/orchestration: fix missing vm powerstate update vm state #10407
- ACS updates the host_id of vm to destination host

No issues found, ALL looks ok

2. Migrate VM with volumes

(between NFS)
MigrateVirtualMachine

WithVolumeCmd

MigrateCommand (internal)

Note: this action is not supported on PowerFlex

Two volumes in DB

source volume
dest volume (last_id = source_volume_id)

intermittent failure

- same as above

continuous failure

- current: vm is Stopped on destination and Running on source and pool (actually it is not if migration is successful)
- New: vm is Running on destination host and pool if found on destination host

management server
- Failed to migrate VM [VM instance xxx along with its volumes due to [com.cloud.utils.exception.CloudRuntimeException: Copy volume(s) to storage(s) xxx failed in StorageSystemDataMotionStrategy.copyAsync. Error message: [Commands 4446460207098233056 to Host 1 timed out after 21600].].
- Current:
  - vm is Running on source host
  - volume is Ready on source pool
  - Bug: new volume is Migrating on destination pool (solved)
- new
  - check via destination host (if dest is Up)
    - if vm is Running on destination, consider the vm migration is successful.
  - (after fix) new volume is Destroy on destination pool if fail

Note:
- In edge case, if the vm is migrated before agent is restarted (in 10 seconds) , the vm is Stopped (by ACS on destination host)
- solved

same as left

Issues

management server
- vm is Running on source host
- Bug: both volumes are stuck at Migrating state (need to reconcile)

Edge case: both scenario 1 and 5 happen
- VM is migrated but state is Running on source

How to reconcile (Verified):

only if
- VM is Running state
- volume is Migrating state
check via host (if source is Up)
check disk path and update volume state
- - if source volume is found but destination is not found, mark source as Ready and dest as Destroy
  - if source volume is not found but destination is found, mark source as Destroy and dest as Ready

3. Migrate Volume (on NFS)

MigrateVolumeCmd (API)

CopyCommand (from primary1 to sec1)
CopyCommand (from sec1 to primary2)

ROOT/DATA volume of Stopped VM

intermittent failure

1st CopyCommand: wait until connection is recoved
2nd CopyCommand: wait until connection is recoved

continuous failure

1st CopyCommand: operation timeout
- Resource [StoragePool:1] is unreachable: Volume [{"name":"ROOT-1159","uuid":"ed322109-8e54-47ca-8183-3c8f2a0f37e6"}] migration failed due to [com.cloud.utils.exception.CloudRuntimeException: com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 5302988561228766380 to Host 1 timed out after 1800].
- Bug: new volume is Creating with empty path on destination pool (solved)
2nd CopyCommand
- Resource [StoragePool:1] is unreachable: Volume [{"name":"ROOT-1159","uuid":"ed322109-8e54-47ca-8183-3c8f2a0f37e6"}] migration failed due to [com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 7457960982925541434 to Host 1 timed out after 2400].
- Bug: new volume is Creating with empty path on destination pool (solved)

New

new volume is Destroy state with empty path on destination pool

1st CopyCommand
- copy object failed: com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 15481123719087037 to Host 1 timed out after 1800
- new volume is Destroy state
  - expungeVolumeAsync is called in copyVolumeCallBack

2nd CopyCommand
- copy failed com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 398850041998999773 to Host 1 timed out after 2400
- new volume is Destroy state
  - expungeVolumeAsync is called in copyVolumeCallBack

No issues found, ALL looks good

same as left

Issues

1st CopyCommand
- new volume is Creating state
- record in volume_store_ref is Creating state (to be cleaned)
2nd CopyCommand
- new volume is Creating state
- record in volume_store_ref is Copying state (to be cleaned)

How to reconcile (verified):

1st CopyCommand
- new volume is Destroy state and removed=now()
- volume_store_ref with Creating state is removed
2nd CopyCommand
- check the size of new volume. If size is changed, the process is still ongoing.
- if size is not changed, new volume is Destroy state and path is set (but not removed)
- volume_store_ref with Copying state is removed

4. Migrate Volume of Running VM (on Powerflex)

MigrateVolumeCommand

agent: succeed
- answer: {"volumePath":"1fe84f0700000002:vol-610-4d48-306f","result":true,"contextMap":{},"wait":0,"bypassHostMaintenance":false}
management server
- volume path is set to new, then set to old when fail
- Bug: Actually, on KVM host, the VM is running with new volume, but the new volume is removed
  - Input/Output error, read-only system
- New:
  - check volume statistics on destination pool via ScaleIO gateway
  - if allocation size = provisioned size, volume migration is done. remove volume on source pool.
  - if allocation size is not same to provisioned size, volume migration failes, remove volume on destination pool.

management
- Resource [StoragePool:12] is unreachable: Migrate volume failed: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 2520889891420635168 to Host 1 timed out after 2400
- Failed to migrate PowerFlex volume
- volume path is set to new, then set to old when fail (looks good)
  - revertBlockCopyVolumeOperations

No issues found, ALL looks good

changes on agent

it uses "dm.blockCopy" (introduced by Hari)
when agent is disconnected or restarted
- abort the block copy job by a disconnect hook (VolumeMigrationCancelHook)
- similar as MigrationCancelHook

same as left

Issue:

one volume in Migrating in DB
- path, iscsi_name, pool_id are info on destination pool
when mgmt server is restarted, it updates all Migrating volume to Ready
- this is disabled if reconcile is enabled

How to reconcile:

only if vm is Running and volume in Migrating state
- now:
The current pool is destination pool of reconcile command
check vm state and vm disks via agent
- if volume is Ready on source pool,
  - revert path, iscsi_name and pool_id to source pool
  - create a dummy volume on dest pool
  - remove the dummy volume
- if volume is not found on source pool
  - update state to Ready
  - create a dummy volume on source pool
  - remove the dummy volume
- if vm is running on source pool (check by fullpath) = volume is ready on source pool
- if vm is running on dest pool, update volume state to Ready, and remove volume from source pool

5. Migrate Volume of Stopped VM (on Powerflex)

CopyCommand

intermittent failure

CopyCommand: wait until connection is recoved

continuous failure

Resource [StoragePool:11] is unreachable: Volume [{"name":"ROOT-533","uuid":"ff8a53a0-d513-4bfc-8920-352948ea066b"}] migration failed due to [com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 8886446489732120778 to Host 1 timed out after 2400]
- 2400 = wait (1200) * 2
- new volume is Expunged
  - due to expungeVolumeAsync in copyManagedVolumeCallBack
- Not an issue since the VM is Stopped so no data loss

management server

- Resource [StoragePool:12] is unreachable: Volume [{"name":"ROOT-531","uuid":"4225516d-52fe-465d-a46d-81f97a2e55b2"}] migration failed due to [com.cloud.utils.exception.CloudRuntimeException: Failed to send command, due to Agent:1, com.cloud.exception.OperationTimedoutException: Commands 3050062847636668430 to Host 1 timed out after 2400].
- new volume is Expunged
  - due to expungeVolumeAsync in copyManagedVolumeCallBack

No issues found, ALL looks good

same as left

Issue: two volumes in DB

source volume: Migrating state
dest volume: Creating state
- dest.last_id = source_volume_id

How to reconcile

check volume information via host
determine state of source and dest volumes
if source volume is Ready
- update source from Migrating to Ready
- update dest volume from Creating to Destroy
if dest volume is Ready
- update dest from Creating to Ready
- update source volume from Migrating to Destroy

TO be fixed

when migrate volume from powerflex to powerflex with same system id, it will not create CopyCommand

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Key

3.3 Core changes

3.4 Considerations

3.5 State transitions (Agent)

3.7 Limitations

3.8 To be Discussed (TODO)

4. Test cases

3.7 Limitations

3.8 To be Discussed (TODO)

4. Test cases

4.1 Backend commands of VM and volume migrations

4.2 How to test

4.2 How to test

4.3 Summary of test results

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Versions Compared

Old Version 20

New Version Current

Key

3.3 Core changes

3.4 Considerations

3.5 State transitions (Agent)

3.7 Limitations

3.8 To be Discussed (TODO)

4. Test cases

3.7 Limitations

3.8 To be Discussed (TODO)

4. Test cases

4.1 Backend commands of VM and volume migrations

4.2 How to test

4.2 How to test

4.3 Summary of test results