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Flash storage vs. hard drive: Which is best for virtual servers?

Virtual servers add a new dimension to the flash storage vs. hard drive question. Size, speed and connectivity all play a role in the outcome.

In the flash storage vs. hard drive debate, which should I choose for my virtualized server? Does form factor affect my decision? Are there any particular considerations for disk controllers on a virtualized server?

Solid-state drive (SSD) use is growing now that the technology is well developed, reliable and less expensive than earlier generations. Although SSDs typically offer less total capacity than conventional magnetic media, SSD performance is unmatched. Servers with mission-critical storage performance needs can potentially benefit from SSD deployment. It is also possible to mix drives on the server, using SSDs for critical applications and SAS -- or even SATA -- drives for more mediocre storage tasks.

An alternative to pure SSD technology is the SSD hybrid -- basically a magnetic hard drive with a huge solid-state cache between the platters and the system interface. For example, data the system reads regularly can reside in the cache where it is available almost instantly, while the magnetic platters can seek other data used less often. For some virtual servers, these hybrid SSDs can perform as well as pure SSDs.

Disk form factor comes down to the physical size of the drives, and organizations can typically choose either common 3.5-inch disks or small 2.5-inch disks. Because of the platter size, the 3.5-inch disks can potentially hold more data, but they require more energy to spin. The outermost tracks also experience more latency than the smaller disks at the same RPM thanks to that larger platter circumference, so using smaller disks can provide a slight performance boost and a slight energy savings. And if areal density is greater on the 2.5-inch disk, there is little real loss in total storage capacity.

Comparing flash and disk memory controllers

Where pure SSDs, or even hybrid SSDs, may still be cost prohibitive, consider cache on the storage controller. A large pool of cache memory on the storage controller can vastly accelerate disk writes and return control to the virtual machines (VMs) quickly, then take as much time as needed to commit the writes to the disk media. This may require upgrading the server's disk controller with a production-grade controller designed for heavy caching and ample battery backup protection for the cache.

Another issue with controllers is the potential for bottlenecks when multiple disks run on the same controller channel. For example, a modern SATA disk controller exchanges data at about 300 MB per second, and a SATA disk can burst about 150 MB per second, so putting more than two SATA disks on the same controller channel may cause a bottleneck that diminishes storage performance. This can happen very easily because a principal performance goal is to have multiple spindles reading and writing at the same time. Benchmarking can help to identify storage bottlenecks that you can usually correct by distributing disks across multiple controller channels or upgrading the controller to provide additional channels.

Centralized SAN storage is common in enterprise data centers, but it is hardly universal. Organizations can deploy storage local to each rack or blade to hold workloads and production data. Local storage presents a series of potential pitfalls and limitations that administrators need to consider, which can involve issues ranging from physical disk and interface selection to data protection strategies and even controller characteristics.

Regardless of the specific storage technology, it is important to benchmark the storage subsystem and gauge VM performance with the local storage architecture. Testing can help to identify the most appropriate disk technology, interface technology and storage system configuration for your unique environment.

Dig Deeper on Enterprise data storage strategies

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A couple of comments on this article, great place to start if you are totally oblivious to storage and disk, the desktop can survive well on any disk type as they usually don't require performance. SSD performance when generalised may look excellent by this article however for sequential data read is only 3 times faster than conventional enterprise hard drives, IF your load is sequential read. Also write performance degrades in SSD after the first couple of cell passes. Back end administration to commit data to a cell adds overhead. SSD offer great random read, low write performance, used as a next step in the disk drive evolution it has a place in history but a short lifespan in the enterprise in comparison to spinning disk. The flash themselves are low powered however the processes required to run them in arrays are not. Technology will take the lessons of SSD to create the next generation drives quite soon I think.

Cache is a sticking plaster over a technological wound, if you can't affect the end to end commit put it in cache. Once cache fills you are still left with a bottleneck and using read ahead cache again is only really affective if data requests are sequential.

In VDI environments and virtual world people should look at their workload first and not the technology. If you decide to race you don't buy a car then sit down and decide what type of racing you will participate in. Your workload should dictate your technology choice every time and budget will then compromise your decision.
Great bit of info on the options available though.
Paul brings up what I was going to mention, the aging problem with SSDs. If you're going to be doing a lot of back-and-forth on the same real estate, performance is going to go south in a hurry.
Excellent article... as a Sr Technical Documentation Specialist, I have been away from hardware for a while (process taking up most of my time) so this is great review as I am now working with a Client's Data Center group...