The key capabilities of SAN as virtual machine storage

Virtual server and desktop environments demand the use of shared storage for maximum efficiency, but what are the key advantages of SAN as virtual machine storage?

Storage for virtual machines is a key consideration when embarking on a virtual server or virtual desktop project. Most IT organisations opt for shared storage for a virtual machine environment, as it has key advantages over direct-attached server drives. But the choice arises: SAN or NAS?

In the first of two podcast interviews, Bureau Chief Antony Adshead speaks with Mike Laverick, an author, blogger and podcaster, about when it’s best to use a SAN as virtual machine storage and how to split management tasks between storage array software and the hypervisor. In what cases is it best to use SAN as virtual machine storage?

Laverick: Before we begin we should clarify what we mean by "SAN." For a lot of people the SAN has become synonymous with Fibre Channel networking, but nowadays we live in a world of multiprotocol arrays, so a single array might support Fibre Channel [or] iSCSI … and [both] of those are SANs of some description -- storage outside the server rather than being directly attached to the server.

But if we focus on the Fibre Channel side of things, [these devices] are still regarded as offering the lowest latency between the actual host and the storage itself and also offering some of the highest security available as well. Fibre Channel networks don’t directly sit on your Ethernet networks so they’re harder to tap into. And if it’s Fibre Channel it’s photons, and, historically, trying to tap into a Fibre Channel network just to lift data directly from it is quite difficult.

Speaking more specifically about virtual machines, there are two main reasons. Fibre Channel SANs and iSCSI SANs are able to present block-level storage right to the guest operating system or to virtual machine. In the world of VMware this is called raw device mapping, or RDM, and the other vendors who supply virtualisation technologies support features similar to this.

So, there are many reasons why you might want to forego the use of virtual disks and use RDMs, but the main reason is to offer native storage functionality directly into the guest operating system, which is sometimes required for advanced features like clustering inside a virtual machine.

The other side of that is that you may want to use your virtualisation vendor’s own file system. So, in the case of VMware they have their own VMFS file system, which they claim has been designed specifically for the workloads generated inside the virtual disks of virtual machines. Which storage management features should you activate in vSphere and which from the array?

Laverick: There are two or three different areas where advanced management features can be enabled. At the vSphere side, there’s something called VAAI [vStorage APIs for Array Integration] and that’s enabled by default on all ESX 4i and 5i hosts. VAAI enables a series of performance enhancements. The real concern is, Does your firmware on the array support VAAI? So, if you’ve got an older array that hasn’t had recent firmware updates, you might [need to do that] on the array to make VAAI available.

Also there is something called VASA [vStorage APIs for Storage Awareness], which is usually a piece of software or virtual appliance provided by the storage vendor that enables some of the advanced features that have been introduced to vSphere 5. Specifically, these advanced features allow you, from the VirtualCenter environment, to see the type of storage you’ve got; [You can determine whether it is replicated, what RAID levels it has, etc.]. It also enables a feature called Storage Profiles within vSphere 5, which allows you to categorise your LUNs by those attributes.

So, when someone’s going through the process of creating a virtual machine for the first time, you can sort and display the storage by different formats, perhaps separating the storage [that’s] replicated from that that’s not, or the storage that has high levels of data protection like RAID 10 with very good I/O properties such as SAS drives separate from storage [that] may have a lower performance level, like SATA, with a different level of RAID.

The last area of storage management I would recommend is [to] look at your storage vendor’s management plug-ins. NetApp, EMC and Dell provide plug-ins to VirtualCenter that make it incredibly easy for the virtual machine administrator to create new LUNs, clone virtual machines, snapshot virtual machines and revert them back to their previous state.

It’s often the case that virtual machine administrators make a lot of high demands from the storage team, and it’s a good idea to offload some of the day-to-day requests onto the virtual machine administrator if you can, which frees up the storage admins to do things that have a high value rather than just provisioning storage all day.

This was last published in July 2012

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