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How to choose the right virtual server storage

It's no secret that virtual server storage needs to meet different requirements than storage in a physical environment. The randomization of I/O that occurs in virtualized environments means virtual server storage needs to offer a higher level of performance than storage for physical servers. On top of greater performance needs, storage administrators need to be aware of how storage systems and functions such as replication and snapshots work with the hypervisor to effectively manage it. In this video from Storage Decisions 2013, DeepStorage.net founder Howard Marks sat down with Todd Erickson, news and features writer for the Storage Media Group, to discuss how virtualizing servers affects storage and why flash is one of the best virtual server storage options.

What storage factors should administrators consider when moving to a virtualized environment?

Howard Marks: Well, virtualized environments generally have a much higher random I/O demand than physical environments because the workloads of multiple virtual machines [VMs] on the same host get multiplexed together. By the time it reaches the storage system, it looks very much like random I/O. So, the storage system that may have been barely adequate for a set of workloads when they were physical workloads will turn out to be overloaded, and you'll need more random IOPS. That generally means you need to introduce some sort of flash.

What are virtual server hypervisor providers doing to address virtual server storage issues?

Marks: In the short term, virtual server hypervisor vendors -- specifically VMware and Microsoft -- are building extensions to the SCSI command set to offload common tasks to the storage system. So, things [are different for] zeroing-out a new volume or copying a template to create a new VM. Instead of reading the data from the storage system up into the host and then writing a new copy back, they can now -- through VAAI [vStorage APIs for Array Integration] for VMware or ODX [Offloaded Data Transfer] for Hyper-V -- send a single command to the storage system and have the storage system perform that task.

What are your top three tips for managing virtual server storage?

Marks: No. 1 is to look for that VAAI or ODX support. A storage system that is at least aware that it's being connected to a virtualization host will run substantially more effectively because there's a lot less load being placed on it by the host.

The second thing is to look for some effective mix of flash and spinning disks. Most systems, especially systems from the next generation of startups that designed their storage systems in the flash era, can, with about 10% of flash, get about a four times performance advantage. So, a little bit of flash well used can give you a big performance boost.

The third thing is to look at how you use storage system functionality like replication and especially snapshots. In the current generation of hypervisors and storage systems, very few of them let you really do snapshots on a per-VM basis. And when you are going to use snapshots or replication, you have to carefully assign a set of virtual servers to a volume so that when you snapshot or replicate that volume you're not taking VMs you don't care about and increasing the load.

Can solid-state flash storage improve virtual server storage?

Marks: Yes. The I/O blender means virtual servers present a random workload. Disk drives are really good at sequential I/O. They're not so good at random I/O. Solid-state storage is really good at random I/O. In fact, today's SSDs are a tiny bit faster for random I/O than they are for sequential I/O. So, the two fit really well.

But the cardinal sin in a spinning disk environment is moving the heads. You don't want to do random I/O because moving the heads takes a long time.

The cardinal sin in the SSD world is writing because flash has a limited write endurance. Now, by limited I mean that in the average data center that flash device will last seven to 10 years, and you're only likely to use it [for] three to five years. So, we need to eventually start thinking about flash drives, like the timing belt on your car. Yes, it will eventually wear out, but we know when it's going to wear out and we should be prepared for that rather than being afraid of the write endurance problem.

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