Using the Sanbolic Scale-Out Platform, virtual machines (hosted on any hypervisor) can share concurrent read-and-write access to logical volumes comprised of one or more virtual disks in various storage configurations (i.e., RAID0, 1, 5, 6, 10, 50 or 60) to achieve high availability and dynamic scale-out of virtualized workloads as well as centralized management and protection of business-critical data.

This article explains how to provision VMDKs so they can be attached to multiple VMware ESX (5.x) virtual machines (VMs) in order to create logical volumes whose data can be accessed concurrently by applications running on each of the VMs using the Sanbolic Scale-Out Platform.

Note: When sharing storage across multiple VMs (hosted on any hypervisor), Sanbolic best practices recommend using iSCSI for the following reasons:

• Compared to other block storage protocols, iSCSI is relatively simple to implement and maintain.
• Depending on the configuration (number of virtual network connections, available bandwidth, MPIO implementation, etc.), high levels of performance can be achieved.
• There is no loss in connectivity to shared volumes during VM migration between host machines.

Note: It is important to highlight some other reasons why iSCSI is the preferred method for sharing storage across ESX VMs. In order to attach a RDM (Raw Device Mapped) LUN or VMDK to multiple VMs, bus-sharing must be enabled within the hypervisor. When this parameter is enabled, certain features such as vMotion are disabled on the VMs to which these devices are attached. Thus, for customers who want or need to maintain this functionality, sharing RDM LUNs or VMDKs across VMs may not be viable options.

However, you need iSCSI storage in order to present iSCSI LUNs to the VMs. For customers who do not have a hardware-based iSCSI SAN, a software-based  iSCSI Target may be used to present iSCSI disks to the VMs for shared access.

If you do not have or cannot use iSCSI, shared RDM LUNs or shared VMDKs are other options. Out of these options, Sanbolic recommends sharing RDM LUNs rather than VMDKs, primarily for performance reasons.

If you choose not to present iSCSI storage or RDM LUNs to the VMs, you will need to share VMDKs.  Regardless of whether VMDKs are provisioned from SAN-based VMFS data stores or server-side (internal storage) VMFS data stores, they must be configured in such a way as to allow them to be attached to multiple VMs concurrently.

Note: Sanbolic recommends users contact VMware technical support for best practices regarding sharing VMDKs across VMs.

Instructions

In the vCenter management console, perform the following:

1. In the properties of the first virtual machine, add a new SCSI Controller and configure its mode of operation (“virtual” compatibility mode when attaching a VMDK to VMs running on the same host machine; “physical” compatibility mode when attaching a VMDK to VMs running on different host machines).

   

2. Create a virtual disk (VMDK) and assign it to the new SCSI Controller by selecting the Virtual Device Node ID (ex. SCSI 1:1) from the drop-down list.

   

3. Go to the second virtual machine and add the new SCSI Controller created in step #1.

4. Add the virtual disk created in step #2 to the second virtual machine and assign it to the new SCSI Controller by selecting the Virtual Device Node ID (ex. SCSI 1:1) from the drop-down list.

5. Turn on both virtual machines.

6. Launch Diskpart from the command line on each virtual machine and type the command list disk to confirm the virtual disk is seen by Windows.

7. Type the command select disk # (where # equals the disk number of the virtual disk) and then type the command attributes disk to ensure the virtual disk is not in “Read-only” mode. If the disk is in “Read-only” mode, type the command attributes disk clear readonly to disable “Read-only” mode.

8. Open the Sanbolic management dashboard and follow the instructions found at http://sanbolic.com/melio5-quick-start-guide/ to create a logical volume on the virtual disk that can be accessed (read-and-write) by both VMs concurrently.

   When you have created the logical volume, test read-and-write access by both VMs.