last column on DIY storage, I talked about engineering redundancy for inexpensive systems built on SAS direct-attached storage. These arrays are great because of the huge savings involved. But what if you just need a few terabytes of auxiliary storage for backups or archive and don’t want to purchase a dedicated piece of server hardware to run a storage management OS?
One innovative way to tackle auxiliary storage demands on a shoestring budget is to buy a direct-attached storage chassis and an interface card and retrofit it into an existing hypervisor host in your data center. You can pack that chassis full of near-line SAS or commodity SATA disks to further economize and then spin up a storage management VM, connect it to the DAS box and provision access to that storage over inexpensive gigabit iSCSI to the other hosts in the cluster.
Voila, in about two hours you could have an extra 10, 20 or even 200 gigabytes of inexpensive archival, backup, or scratch storage available to the entire virtualization cluster. And if 1GbE isn’t enough to satisfy the I/O requirements, you can simply add more NICs for multipath, put a 10GbE adapter or two into the hypervisor host, or even add a couple FC adapters to switch that storage directly into an existing fiber mesh.
However, the problem with this approach is that it brings our inexpensive storage expansion plan back into the realm of single point of failure storage, rendering it immediately unsuitable for tier-one applications. So let’s expand our thinking a little. Suppose instead of connecting the new DAS chassis to only a single hypervisor in the data center, we connect it to two. The only prerequisites to this approach is that you have a couple of hypervisor hosts with a spare PCIe slot or two to accommodate a dedicated SAS card and maybe an extra network card or FC adapter for interconnect to the rest of the data center.
Now, instead of running only one storage management VM, we’ll run two: one on each host, either of which can provide access to the DAS array. With hardware in hand, all we’ll need is a compatible storage management OS, like Nexenta or Windows Storage Spaces on Server 2012 that’s capable of operating in an high-availability (HA) cluster.
Nexenta is by far the more feature rich, high-performance and robust option of the two, although to use HA with Nexenta, we need the commercial HA Plugin for the Nexenta product, which adds some cost but enables some significant performance features, such as shared ZIL SSD caching. Alternatively, Windows Storage Spaces is less robust and poorer performing, but still perfectly production ready with the added benefit that we don’t need anything additional to our Windows licensing to provision a clustered storage space.
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There are a few caveats to this approach as well. First, as I’ve mentioned in an earlier column, SAS is a dual-channel interconnect protocol. This means that the DAS box has to be a dual expander so that there are two discrete paths to the storage, one from each machine. Second, since we will need both of those channels to go directly to each disk, single-channel SATA disks are out of the running and we’re into commodity SAS to fill our new chassis. Third, there are some hypervisor and storage software specific features that we need to be aware of to configure high availability across hosts in our storage management VMs.
On VMware, we’re going to want to map both the SAS interface card and whatever storage mesh interface card we’re using directly to the storage management VM with DirectPath to ensure optimum performance. On Hyper-V, we’re a little bit more limited because virtualizing a ZFS-driven product such as Nexenta properly is problematic. Without VMDirectPath, we can’t pass storage interfaces directly through to the Nexenta VM, which can cause problems and dramatically limit performance.
If we need a truly enterprise storage software feature set like Nexenta, but need to run it on Hyper-V hosts, we could also choose a product like DataCore’s SANsymphony-V, which uses replication and active/active load balancing between high-availability node pairs that have their own local storage. This makes SANsymphony-V much less sensitive to guest operating system storage configurations than Nexenta, while providing a similarly enterprise feature set with features like asynchronous replication, PCIe SSD cache and Metro Mirroring.
The ultimate cost of acquiring storage this way varies. I/O requirements dictate base hardware costs and feature set does the same for storage management software licensing. But whether you’re engineering for massive scale data or just bringing some additional capacity online, leveraging commodity hardware and competitively priced or free storage software can yield savings of greater than 50% versus traditional enterprise storage vendors. In my final column in this series, I'll look at the risks, costs and benefits of DIY storage configurations.
Find out about the impact of VDI on storage infrastructures and innovative technologies that use RAM, flash and clever software to tame the VDI storage beast in Howard Marks' session, Storage Solutions For VDI at Interop New York this October.