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Desktop storage in-depth: Explaining diskless VDI

It's time to become familiar with diskless VDI. This type of virtual desktop storage may be the answer to many administrators' prayers -- especially if you're struggling with VDI storage capacity.

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Doing diskless virtual desktop infrastructure (VDI) allows you to ditch the SAN and instead use RAM for virtual desktop storage, making VDI projects cheaper and giving them better performance.

Early last year, we were introduced to the concept of in-memory VDI storage by Atlantis Computing Inc.'s Diskless VDI. My first inclination was to smack my forehead in disbelief that nobody had done this sooner. The technology could very well be the next iteration of virtualization storage.

What is diskless VDI?

Diskless VDI, in its simplest sense, is like a RAM disk for your virtual desktops -- but it's so much more!

Atlantis implements diskless VDI by running a storage appliance virtual machine (VM) on each VDI host that acts as the interface between the host and the storage system. This virtual desktop storage appliance isn't new technology; you may be familiar with it from one of the many other vendors that do software-based storage optimization. With these solutions, storage operations are sent to the virtual appliance, which deduplicates, compresses, sequentializes and/or otherwise optimizes both reads and writes.

Through this optimization, it's possible not only to reduce the amount of virtual desktop storage capacity required to support VMs, but also to reduce the amount of I/O required. What is unique about Atlantis' Diskless VDI is that instead of optimizing physical storage, it uses each host's RAM to store all the compressed, deduped data. Imagine: No SAN or tiered storage -- just super-fast, super-cheap RAM.

Pulling in persistent desktops

Diskless VDI supported only nonpersistent VDI environments. This was essentially the low-hanging fruit for a new technology, because nonpersistent VDI is how the virtual desktop storage system only needs to provide one (or very few) OS images to all the users. Since there is a common image, it's very easy (in the context of storage virtualization) to deduplicate that data and make deltas of it available in memory to individual users. There is only one image, so no SAN is required because all that is needed is to place a copy on (or make a copy available to) each virtual host so it can be loaded into memory.

Atlantis' Persistent VDI was announced earlier this year, extending the technology to support persistent desktops. The challenge with persistent VMs is that each OS image is different, so building deltas off a single OS image is not possible since there's no such thing as a single OS image. There is, however, an incredible amount of overlap between Windows images, and that overlap is fairly easy to deal with. The tough part is figuring out what to do with the differences between images.

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To address this challenge, Atlantis developed what it calls a replication host. The replication host, which is a separate, centralized VM, catalogs and maintains a master copy of each user's OS image blocks. When a user is directed to a VM on a VDI host, the VM is created using the blocks stored in memory on the VDI host while the replication host kicks in any delta blocks that don't already reside locally. These delta blocks are copied from other VDI hosts that contain those unique blocks and are delivered to the user's virtual machine in real time.

You can now deliver persistent virtual desktops to users with better performance than almost any other disk-based tool on the market, and for much less money (no SAN). According to Brian Madden's testing, a single VM leveraging Diskless VDI achieved a PassMark score of 4355, high enough to be the seventh fastest hard drive in the world.

Why is diskless VDI so important?

This storage appliance is software, so the only requirement is to buy a bunch of memory -- and even then we're not talking astronomical amounts. In fact, the deduplication principles apply in memory as much as they do in physical storage. The fewer differences between your images, the less memory you need.

Of course, nonpersistent VDI requires less memory than persistent because there are significantly fewer different blocks. But even in a persistent desktop environment, adding memory to servers is much cheaper than buying big-iron storage.

All this means that to scale up your environment, you simply add more server hardware. You'll never exceed the capacity of your SAN or have to worry about tiered storage (where the more stagnant information lives on slow storage and the dynamic data lives on solid-state drives).

Diskless virtual desktop storage just might be the revolution in storage that we've been waiting for. That doesn't mean big data or data warehousing is going away, but for desktops and servers, it could potentially be the next big thing. Atlantis may be the first to the party, but I expect to see others follow with similar approaches.

This was first published in March 2013

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