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Making the move to solid-state disk drives in a SAN/NAS

SSDDs can boost storage performance and conserve power significantly – if you’re willing to pay the price.

Magnetic storage has been a staple of computing for more than 50 years, providing a cost-effective mix of capacity and performance that meets the fundamental storage needs of almost every application. But solid-state memory devices provide an order of magnitude improvement in storage performance, and this has long captured the interest of IT administrators as a long-term data storage medium.

Solid-state disk drives (SSDDs) package huge amounts of memory (dynamic random access or flash) into a drive device that allows organizations to push the limits of applications and infrastructure. Let's examine the tradeoffs of SSDDs and consider their implications in the Windows data center.

Tradeoffs of solid-state disk drives

The principal advantage of a solid-state disk drive is not what it includes, but what it removes. By foregoing the traditional magnetic platters and the electromechanical devices required to spin and seek them, a SSDD reads and writes directly to an array of dense memory chips. Mechanical delays involved with track seek time, latency and moving data to and from the platters themselves are gone, and memory devices are accessed at much faster speeds. This results in far higher IOPS (input/output operations per second) than with a traditional magnetic drive.

Lately, you can get a 400 GB SSD device [at prices] comparable to a 400 GB or 600 GB high-performance disk drive device.

Ray Lucchesi, president and founder of Silverton Consulting

Power consumption is also more efficient with SSDD devices. Since power is not needed to drive spindle motors and head servos, the power-per-IOPS is lower. Still, the total power demands of the SSDD have the potential to be much lower than a magnetic drive. For example, the 200 GB SATA version of Seagate's Pulsar flash memory SSD reports a sustained random read rate of 25,000 IOPS (only 5,200 IOPS for sustained write due to the slower writing nature of non-volatile flash memory), but consumes only 1.3 watts of average operating power.

The Pulsar reports only an average burst data transfer speed of 300 MB/s limited by the SATA interface. By comparison, Seagate's 160 GB Constellation SATA drive calls for burst data transfers up to 300 MB/s at an average operating power of 2.8 watts (IOPS are not specified for the Constellation). The idle power demands are also lower for SSDD devices, resulting in lower power bills and cooling requirements.

But the benefits of solid-state disk drives have downsides. The first issue is cost. While SSD technology is certainly not new, it's only started to gain mainstream acceptance in recent years, and solid-state drives can still cost 10 times more per GB than their magnetic counterparts with similar capacities and interface types. Capacity can be another concern, though the cost and capacity gap is narrowing. "Lately, you can get a 400 GB SSD device [at prices] comparable to a 400 GB or 600 GB high-performance disk drive device," said Ray Lucchesi, president and founder of Silverton Consulting in Broomfield, Colo.

Greg Schulz, founder and senior analyst at the Server and Storage IO Group in Stillwater, Minn., is quick to point out that the benefit of time -- the ability to handle more storage transactions over a given period -- can often justify the higher cost and lower capacity of SSDD devices. The trick is to ensure that the application(s) using the solid-state storage can truly benefit from the additional performance. For example, storing Word documents on a SSDD is probably not the best way to utilize solid-state media.

Schulz also explains that SSDDs with flash memory will experience significantly lower write performance than those based on dynamic RAM. This may make flash SSDDs less desirable than dynamic RAM SSDDs for write-centric tasks such as storing Windows paging/swap files. Since flash memory also tends to wear out (limiting the number of write cycles available), the choice of flash SSDD controller should also include features that support flash memory. "You need to have it paired with a good controller that can do write optimization and wear leveling," he said. Any other concerns about flash SSDD reliability are usually addressed with availability features such as RAID protection and snapshots.

Solid-state disk drives in the Windows data center

Generally speaking, solid-state disk drives are compatible with Windows environments without substantial changes to hardware, software or tools. In virtually all cases, the SSDD device is packaged and presented to the Windows server as a conventional hard drive, and you might need to be concerned with drivers or OS support for your specific version of Windows Server. If the SSDD device is installed in a storage sub-system or RAID array, there is additional abstraction between the SSDD and the Windows server or the applications being supported, and Windows servers will simply take advantage of the additional space. More support consideration may be needed, however, for other operating systems. "There are products that are optimized for non-Windows platforms," Schulz said. "And their drivers, if they exist, may not be optimized for Windows."

The principal issue is not Windows OS support, but application benefit. Organizations adding solid-state storage in the environment should provision that storage to applications that can take advantage of the performance benefit. That's where the cost/benefit of SSDD devices makes the most sense. Allocating SSDD storage to non-transactional or other applications that are not time-sensitive is almost always a wasted investment. Some SAN/NAS storage sub-systems are designed to automatically move data between tiers based on access patterns over time, so deploying SSDDs in storage arrays (rather than installing directly into Windows servers) can potentially enhance the SSDD performance benefit by ensuring that the most frequently accessed data is located on SSDD LUNs, while less essential data is migrated to lower tiers.

Server virtualization and consolidation has placed a renewed emphasis on solid-state storage. As physical servers host more virtual machines (VMs), there is added I/O contention with network and storage resources, and the I/O potential available with SSDD devices can help to ease storage bottlenecks that might impair storage access on busy virtualized servers. There are only a limited number of SSDD devices that can be installed in a given storage sub-system, however, before their performance benefit starts to fall off due to throughput limitations. "Let's say a sub-system can support 250,000 I/Os per second, and an SSD can do 10,000 I/Os," Lucchesi said. "Maybe 25 SSDs is the maximum amount that you could have behind that storage sub-system."

Existing server and storage management tools should readily accommodate solid-state storage devices (especially since SSDDs are typically packaged with standard drive interfaces like Fibre Channel or SATA), but it's always worth testing the tools in advance to ensure compatibility between the specific tool and particular SSDD model. For example, wear leveling and other storage management features for flash-based SSD drives may be lacking, while older tools may incorrectly flag a problem with unexpectedly high IOPS, but Schulz notes that the situation is always improving. "We saw a lot of that with some of the earlier generations of solid-states when some of the Windows tools were a lot newer," he said.

Finally, admins should take the time to look past just provisioning solid-state storage and consider some of the other possibilities that solid-state disk drive devices might enable. As an example, solid-state storage might not provide a substantial benefit to any business-critical applications, but it might offer an entirely new performance dimension to other important functions such as server backups or more frequent VM snapshots.

Stephen J. Bigelow, senior features writer, has more than 15 years of technical writing experience in the PC/technology industry. He holds a BSEE, CompTIA A+, Network+, Security+ and Server+ certifications and has written hundreds of articles and more than 15 feature books on computer troubleshooting. Contact him at [email protected].

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