Network World has just posted an SSD comparison test written by Logan G. Harbaugh. The test pitted some consumer-class SSDs against enterprise-class SSDs and with an Adaptec ASR5805/512 SSD controller and MaxIQ kit, which uses an attached SSD to accelerate attached drives arrays via flash caching. Overall, Harbaugh found that the SSDs improved system performance by a factor of 2 to 10 depending on the product.

The discussed test results illustrate many key points we’re learning about SSDS used both as direct storage devices and as caches for rotating storage. For example, the article discusses the “write cliff,” which is the sudden loss of write performance sustained by some SSDs after they are initially filled. The cause of the performance loss is the need to scavenge free space from deleted files and the need for “wear leveling,” which prevents premature NAND Flash failure in the SSD. If the SSD capacity is not overprovisioned (larger than the rated size), then writes cannot progress until the SSD’s controller finds, liberates, and organizes the free space. If the SSD capacity is overprovisioned, then free-space “garbage collection” and wear leveling can occur as a background task and will not increase the drive’s write latency. The write latency of an SSD that’s not overprovisioned can range unpredictably from milliseconds to seconds depending on the amount of free space to be liberated. Of course, there’s a cost for capacity overprovisioning and the higher cost of enterprise-class SSDs reflects the cost for added storage. The Network World article notes these characteristics in fine detail.

Harbaugh tested the Adaptec SSD controller/Intel SSD combo and drives or drive arrays from Apricorn, Compellent, Dot Hill, Fusionio, HP, and Ritek. The testbed was an HP ML370G5 server running Windows Server 2003 with external storage connected via Fibre Channel through a 2Gbps HP FC switch and the tests were run with IOmeter. Harbaugh notes that the Fusioio 32-Gbyte ioDrive and the HP StorageWorks IO Accelerator for blade servers (made by Fusionio) achieved the highest throughput in the tests. The Fusionio SSD delivered read and write throughputs of 706 and 456 Mbytes/sec respectively while the HP StorageWorks IO Accelerator delivered read and write throughputs on a blade server of 806 and 618 Mbytes/sec respectively. Both products delivered “excellent IOps, with no write cliff.” Note that the Fusionio drive costs $6,829.99 and the list price for HP StorageWorks IO Accelerator ranges from $4400 to 13,200 on the HP Web site for storage capacities ranging from 80 to 320 Gbytes. The tested 160-Gbyte HP accelerator lists for $7700. So performance comes at a price (nothing new there) that’s substantially higher than the SATA drives configured as HDD replacements.

Only the application developer can determine if high throughput and the absence of a write cliff is worth several thousand dollars. In many enterprise-class situations, the additional hardware cost is irrelevant. Online businesses such as Amazon.com have found that even a 1% increase in response time causes lost sales to the tune of millions of dollars because online customers are impatient and they bore easily. They will not wait long before wandering off, perhaps to a faster competitor. Large investment firms measure millisecond increases in online trading latency in terms of millions of dollars as well. The first trading firm to get a deal leaves competitors choking in the dust with no deal at all. So a few thousand dollars to avoid a sudden latency increase measured in tens or hundreds of milliseconds or even seconds is trivial insurance against large potential business losses.

The Network World article notes that one advantage these faster drives have is their use of the PCIe interface rather than the drive industry’s preferred SATA or SAS interfaces. The PCIe interface is closely coupled to the computer’s or server’s processor and can therefore provide very low latency and very high throughput, which is easily increased simply by adding parallel PCIe lanes. Several of the storage products including those from Fusionio and HP that were tested in this article employ PCIe interfaces to improve the storage subsystem’s performance.