Details about CAW and ATS

The VMWare CAW SCSI command and ATS (Atomic Test and Set) dependency are related to the management of virtual machine disk images in a VMware vSphere environment.

The CAW (Compare and Write) SCSI command is used by VMware vSphere to implement the ATS feature, which provides a way for multiple virtual machines to access the same disk image concurrently without the risk of data corruption or conflicts. The ATS feature is used with VMFS (Virtual Machine File System) and NFS (Network File System) datastores to ensure consistent access to the same disk image.

When multiple virtual machines access the same disk image, they need to coordinate their access to avoid conflicts. ATS enables this coordination by providing a way for virtual machines to inform each other of their intent to modify the disk image. This is done using the CAW SCSI command, which allows a virtual machine to read a block of data from the disk, compare it to the data it has in memory, and write it back to the disk only if it matches.

The ATS dependency means that virtual machines that use ATS must support the CAW SCSI command. This includes the guest operating system and any applications that access the disk image. If a virtual machine does not support the CAW SCSI command, it may experience performance issues or data corruption when accessing a disk image that is also accessed by other virtual machines.

In summary, the VMWare CAW SCSI command and ATS dependency are important components of the VMware vSphere environment, enabling multiple virtual machines to access the same disk image concurrently while maintaining data consistency and avoiding conflicts.

Is it possible to disable CAW  SCSI command for certain virtual disks?

Yes, it is possible to disable the use of the Compare and Write (CAW) SCSI command for certain virtual disks in a VMware vSphere environment.

By default, the ATS (Atomic Test and Set) feature is enabled for all virtual disks residing on VMFS and NFS datastores. However, you can disable ATS for a specific virtual disk by configuring its Advanced Settings in the vSphere Web Client or vSphere Client.

To disable ATS for a virtual disk, follow these steps:

1. Power off the virtual machine that contains the virtual disk.
2. In the vSphere Web Client or vSphere Client, navigate to the virtual machine and select it.
3. Go to the “Configure” tab and select “VM Options”.
4. Expand the “Advanced” section and click “Edit Configuration”.
5. In the “Advanced Configuration Parameters” dialog box, click “Add Row” to create a new parameter.
6. In the “Name” field, enter “disk.EnableUUID” (without quotes).
7. In the “Value” field, enter “FALSE” (without quotes).
8. Click “OK” to save the changes and close the dialog box.
9. Power on the virtual machine.

After completing these steps, the virtual disk will no longer use the ATS feature, and the CAW SCSI command will not be used for that disk.

It’s important to note that disabling ATS for a virtual disk may impact its performance, especially if the disk is accessed by multiple virtual machines concurrently. Therefore, it’s recommended to evaluate the impact before disabling ATS for a virtual disk.