If you have vSAN, then you’ll know that the Virtual SAN vCenter Server provides an excellent way to connect the vSphere environment to the vSAN datastore in the cloud, including both the cluster (which is used to connect multiple nodes) and the pool (which is used to provide the storage). You might not know that you can also use vSphere to connect vSAN virtual machines to existing physical disks directly. This can be useful in a number of scenarios including the following.
If a disk in a VM gets damaged (e.g. a VM crash or a power outage), you can recover that disk from the VM and then plug that disk back into the host to restore service. This way, you don’t need to move the VM and its data to another host. This is just a win-win scenario when it comes to using (vSAN) virtual SAN.
Disks are shared in vSAN to offer an easily scalable solution to provide I/O performance. If a disk from a VM becomes too slow, you can disconnect it and plug in a new disk. This method can also be used for any type of problem in a VM, such as the need to update a critical driver or software component. You can add a new disk to the cluster, and vSAN will replicate the data across all the hosts in that very same cluster, ensuring that there is always a good copy of the data available. How do you add disks to the vSAN cluster? In this post, we’ll demonstrate how to do this from the vSphere client, as well as with PowerShell and the API.
Working With Virtual Disks
The Virtual Disks are what vSAN uses to store data in the storage pool or shared pool. Each Virtual Disk can store data, such as virtual machines and their VMs, as well as other virtual appliances and cloud services that are not virtual machines. Virtual Disks are also used by Storage vMotion, for instance, to migrate VMs between hosts. Virtual Disks also function as an abstraction for storage devices, such as physical disks, as you can think of a Virtual Disk being a file that is accessed like a physical disk.
Each Virtual Disk consists of a set of LUNs (logical unit numbers) that provide storage to the VM. Each LUN can be provisioned into an existing physical disk or a new physical disk (when it’s a blank disk that doesn’t have any data). This last option provides the most efficient use of storage since it leverages the raw capacity of the new physical disk (that is, zeroed out). However, since LUNs are just files and are just identified by a number, it’s possible to have a single physical disk with multiple LUNs. As such, LUNs are shared to provide storage. This is useful in a couple of scenarios:
- If a VM loses its disk, it can lose access to its data. In the event you share the disk across multiple VMs, the data in that disk can be accessed by any VM on the vSAN cluster (which can potentially be in any other location);
- If a physical disk becomes unusable (for example, due to physical damage or disk failure), you can quickly replace it with a new disk and continue to run the VM.
Using Storage vMotion
Storage vMotion is a way to migrate a VM between hosts in a cluster or even between clusters. If a VM is moved from host A to host B, the data will not necessarily be on the same disk. In that case, the data will also be moved since the Virtual Disks are just files. The good news is that Storage vMotion is very efficient in terms of how it handles data migration. If you use it in th right way, you’ll never need to worry about the data being moved and re-mapped.
You can use Storage vMotion to transfer a VM between hosts or clusters. To do so, you create a Virtual Disk for the VM, use Storage vMotion to move the VM and its data to the new host or cluster, and then use Storage vMotion to transfer the Virtual Disk from the old host or cluster to the new host or cluster. In vSAN 6.7 and later, you can also create a Virtual Disk from the moved VM, allowing you to transfer it multiple times.
Using a Virtual Disk for storage vMotion can be helpful for several reasons. First, using a Virtual Disk allows you to create an abstraction layer for the storage, making it easier to troubleshoot and work on problems. Second, the amount of space used on a Virtual Disk can be much less than that on a physical disk since a Virtual Disk uses LUNs to store data. This means you can have many more Virtual Disks per physical disk, allowing you to use a bigger physical disk to store more data.