Add details about the batch-migrations

Signed-off-by: Jian Wang <[email protected]>

Author: w13915984028

docs/host/host.md

@@ -21,15 +21,15 @@ Because Harvester is built on top of Kubernetes and uses etcd as its database, t
 
 ## Node Maintenance
 
-Admin users can enable Maintenance Mode (select **⋮ > Enable Maintenance Mode**) to automatically evict all virtual machines from a node. This mode leverages the **live migration** feature to migrate the virtual machines to other nodes, which is useful when you need to reboot, upgrade firmware, or replace hardware components. At least two active nodes are required to use this feature.
+Admin users can enable Maintenance Mode (select **⋮ > Enable Maintenance Mode**) to automatically evict all virtual machines from a node. This mode leverages the [batch-migrations](../vm/live-migration.md#automatically-triggered-batch-migrations) feature to migrate the [live-migratable virtual machines](../vm/live-migration.md#live-migratable-vms) to other nodes, which is useful when you need to reboot, upgrade firmware, or replace hardware components. At least two active nodes are required to use this feature.
 
 Check if there are any [non-migratable VMs](../vm/live-migration.md#non-migratable-vms) and take essential actions.
 
 :::warning
 
 A [bug](https://github.com/harvester/harvester/issues/7128) may cause an I/O error to occur in virtual machines while Maintenance Mode is enabled on the underlying node. To mitigate the issue, you can set a taint on the node before enabling Maintenance Mode.
 
-1. Set the taint on the target node.
+1. Set the taint on the target node. It triggers the [batch-migrations](../vm/live-migration.md#automatically-triggered-batch-migrations) automatically.
 
     ```sh
     kubectl taint node <NODE> --overwrite kubevirt.io/drain=draining:NoSchedule

docs/upgrade/automatic.md

@@ -81,7 +81,7 @@ When a Rancher version reaches its End of Maintenance (EOM) date, Harvester only
 
 ### Live-migratable VMs
 
-Those VMs are migrated to other nodes automatically when the hosting node is to be upgraded, they have zero down-time through the upgrade.
+Those [live-migratable VMs](../vm/live-migration.md#live-migratable-vms) are live migrated to other nodes automatically via [batch-migrations](../vm/live-migration.md#automatically-triggered-batch-migrations) when the hosting node is to be upgraded, they have zero down-time through the upgrade.
 
 ### Non-migratable VMs
 

docs/vm/live-migration.md

@@ -15,7 +15,15 @@ description: Live migration means moving a virtual machine to a different host w
   <link rel="canonical" href="https://docs.harvesterhci.io/v1.5/vm/live-migration"/>
 </head>
 
-Live migration means moving a virtual machine to a different host without downtime.
+Live migration means moving a virtual machine to a different host without downtime. A couple of comprehensive processes and tasks are done under the hood to fulfill the live migration.
+
+The general requirements are:
+
+- The cluster has at least one additional scheduable node which can meet all the scheduling rules of the VM besides the current host node.
+
+- The destination node has enough spare resources to host the VM.
+
+- The requested CPU, memory, [volumes](./create-vm.md#volumes), devices and other resources of the VM can be copied or rebuilt on the destination host while the source VM is still running.
 
 ## Non-migratable VMs
 
@@ -49,6 +57,10 @@ Following conditions are checked on the runtime (e.g. before an upgrade) to mark
 
 - Other [node scheduling](./create-vm.md#node-scheduling) rules.
 
+## Live-migratable VMs
+
+Besides `non-migratable VMs`, the rest of the running VMs are considered `live-migratable`.
+
 ## How Migration Works
 
 Each node has multiple CPU models that are labeled with different keys.
@@ -100,10 +112,26 @@ When you have [node scheduling rules](./create-windows-vm.md#node-scheduling-tab
 
 - The `Abort Migration` menu is available when the VM already has a running or pending migration process.
 
-- Don't click `Abort Migration` if it is triggered by the [Harvester upgrade](../upgrade/automatic.md#live-migratable-vms) or [node maintenance](../host/host.md#node-maintenance).
+- Don't click `Abort Migration` if it is created by the [batch-migrations](#automatically-triggered-batch-migrations)
 
 :::
 
+## Automatically triggered batch-migrations
+
+Both [Harvester upgrade](../upgrade/automatic.md#live-migratable-vms) and [node maintenance](../host/host.md#node-maintenance) benefit from the **Live Migration**, and the process is slightly different with above [Starting a Migration](#starting-a-migration). It is called `batch-migrations`.
+
+The general process is:
+
+1. The controller watchs a dedicated taint on the node object.
+
+1. The controller creates a `virtualmachineinstancemigration` object for each [live-migratable VM](#live-migratable-vms) on the current node.
+
+1. The migrations are queued, scheduled internally, and are process in batch mode. UI shows `Pending migration` or `Migrating` according to their status.
+
+1. The controller monitors the processing and waits until all of them are done or time-out.
+
+![batch-migrations](/img/v1.6/vm/batch-migrations.png)
+
 ## Migration Timeouts
 
 ### Completion Timeout