vMotion is a feature in VMware vSphere that allows live migration of running virtual machines (VMs) between hosts without any downtime or service interruption. vMotion enables workload mobility, load balancing, and hardware maintenance with minimal impact on VM availability. Here’s a deep dive into how vMotion works:

1. Preparing for vMotion:

• Before a VM can be migrated using vMotion, the source and destination hosts must meet certain requirements:
  • Shared Storage: The VM’s virtual disks must reside on shared storage accessible by both the source and destination hosts. This ensures that the VM’s memory and CPU states can be transferred seamlessly.
  • Network Connectivity: The source and destination hosts must be connected over a vMotion network with sufficient bandwidth to handle the migration traffic.
  • Compatible CPUs: The CPUs on the source and destination hosts must be of the same or compatible CPU families to ensure compatibility during the migration.

2. vMotion Process:

The vMotion process involves the following steps:

Step 1: Pre-Copy Phase:

• During the pre-copy phase, the VM’s memory pages are copied from the source host to the destination host.
• While this initial copy is happening, the VM continues to run on the source host and changes to the VM’s memory are tracked using page dirtying.

Step 2: Stop-and-Copy Phase:

• At a certain point during the pre-copy phase, vSphere calculates the remaining memory pages that need to be copied.
• When the number of remaining dirty pages falls below a threshold, the VM’s execution is paused briefly on the source host, and the final memory pages are copied to the destination host.
• After the copy is complete, the VM is resumed on the destination host with the help of a soft “stun” to the VM.

Step 3: Post-Copy Phase:

• During the post-copy phase, the destination host checks for any residual dirty pages that might have changed on the source host since the initial copy.
• If any dirty pages are detected, they are copied from the source host to the destination host in the background.
• The VM remains running on the destination host during this post-copy phase.

3. vMotion Enhancements:

Over the years, VMware has introduced several enhancements to vMotion to improve its performance and capabilities, such as:

• EVC (Enhanced vMotion Compatibility): Allows vMotion across hosts with different CPU generations.
• Cross vCenter vMotion: Enables vMotion across different vCenter Servers for workload mobility across data centers.

Hostd.log and vMotion:

The hostd.log file on the ESXi host provides detailed information about vMotion activities. You can use log analysis tools like grep or tail to monitor the hostd.log for vMotion events. Here are some examples of log entries related to vMotion:

1. Start of vMotion:

[timestamp] vmx| I125: VMotion: 1914: 1234567890123 S: Starting vMotion...

2. Pre-Copy Phase:

[timestamp] vmx| I125: VMotion: 1751: 1234567890123 S: Pre-copy...
[timestamp] vmx| I125: VMotion: 1753: 1234567890123 S: Copied 1000 pages (1MB) in 5 seconds, remaining 5000 pages...

3. Stop-and-Copy Phase:

[timestamp] vmx| I125: VMotion: 1755: 1234567890123 S: Stop-and-copy...

4. Post-Copy Phase:

[timestamp] vmx| I125: VMotion: 1757: 1234567890123 S: Post-copy...
[timestamp] vmx| I125: VMotion: 1760: 1234567890123 S: Copied 2000 pages (2MB) in 10 seconds, remaining 3000 pages...

These are just a few examples of the log entries related to vMotion in the hostd.log file. Analyzing the hostd.log can provide valuable insights into vMotion performance, any issues encountered during the migration, and help in troubleshooting vMotion-related problems.

In the hostd logs, the “vMotion ID” refers to a unique identifier assigned to each vMotion operation that takes place on an ESXi host. This ID is used to track and correlate the various events and activities related to a specific vMotion migration. When a vMotion operation is initiated to migrate a virtual machine from one host to another, a vMotion ID is assigned to that migration.

Detecting the vMotion ID in the hostd logs can be achieved by analyzing the log entries related to vMotion events. The vMotion ID is typically included in the log messages and is used to identify a specific vMotion operation. To detect the vMotion ID, you can use log analysis tools like grep or search functionality in log viewers. Here’s how you can detect the vMotion ID in hostd logs:

1. Using grep (Linux/Unix) or Select-String (PowerShell):

• If you have access to the ESXi host’s shell, you can use the grep command (Linux/Unix) or Select-String cmdlet (PowerShell) to search for vMotion-related log entries and identify the vMotion ID. For example:

grep "Starting vMotion" /var/log/hostd.log

or

Get-Content "C:\vmware\logs\hostd.log" | Select-String "Starting vMotion"

2. Log Analysis Tools:

• If you are using log analysis tools or log management solutions, they usually provide search and filter capabilities to look for specific log entries related to vMotion. You can search for log messages containing phrases like “Starting vMotion” or “Stopping vMotion” to identify the vMotion ID.

3. Manual Inspection:

• If you prefer manual inspection, you can open the hostd.log file in a text editor or log viewer and search for log entries related to vMotion. Each vMotion event should have an associated vMotion ID that you can use to track that specific migration.

The vMotion ID typically appears in log messages that indicate the start, progress, and completion of a vMotion migration. For example, you might see log entries like:

[timestamp] vmx| I125: VMotion: 1914: 1234567890123 S: Starting vMotion...

In this example, “1234567890123” is the vMotion ID assigned to the vMotion operation. By identifying and tracking the vMotion ID in the hostd logs, you can gain insights into the specific details and progress of each vMotion migration, which can be helpful for troubleshooting, performance analysis, and auditing purposes.