FlashVols enhance SSD durability
Enterprises have yet to fully adopt SSDs, being expensive and having a limited lifespan. Optimising them hence to get the best value for money and address the endurance problem is what remains to be explored.
A way to strategically use the disc is by tiering, limiting the type of data stored on flash. Applying compression and deduplication techniques as well can minimise the number of writes to an SSD, thereby prolonging its life.
Tiering does have drawbacks, however, said Tegile Systems VP of marketing Rob Commins, because it operates by sweeping an entire pool of storage to decide which data should be moved to the faster flash storage, and which creates delays.
Tegile has thus added FlashVols to its Metadata Accelerated Storage System architecture to optimise SQL performance on its Zebi storage arrays. In this case, inline deduplication and compression are used to reduce overhead for highly repetitive SQL Server workloads.
Zebi storage arrays comprise eMLC NAND flash and 7,200RPM spinning disks. FlashVols are volumes pinned in SSD so that applications run at maximum performance, Commins explained. Rather than relying on tiering policies or caching algorithms, pinned volumes remain in DRAM or flash close to the SQL Server without any involvement from the database administrator. The data are already predetermined to be frequently accessed enough that they should benefit from flash-grade performance. "It puts a little English on the ball," said Commins.
Deduplication and compression technologies not only help improve performance, but also play a role in lengthening the lifespan of the SSD drive. Commins said each SSD drive in the system can withstand 3.5PB of write data before the drive exhibits signs of write wear.
Jeff Janukowicz, IDC's research director for solid state storage and enabling technologies, said that while SSDs can help accelerate application workloads by providing low, consistent latency and high IOPs, their higher costs compared to spinning disks and their endurance are inherent challenges for enterprise storage adoption. "Compression and deduplication both improve data efficiency rates, and this helps mitigate some of these challenges," he said. Limiting writes to the drive also helps improve endurance.
More specifically, extending the life of the SSD is accomplished by minimising the number of erasures in relation to the writes, said Gartner analyst Stanley Zaffos. "When you're doing a write, you are generally having two operations occurring. You have the erasure and you have the actual write," he said. "Reading is a more benign operation, because you're essentially measuring something as opposed to changing it."
All storage vendors are using techniques such as deduplication and compression to optimise SSDs for performance and endurance, said Zaffos. "The contention is who has done a better implementation from a data flow perspective, and from an algorithm selection process. Some vendors will do compression first, then deduplication. Some will do compression only." There are a number of permutations that can be translated into software and data flows, he said.
Reliability and endurance remains a challenge for NAND flash, and workarounds such as complex architectures and algorithms to address the inherent limitations of NAND flash are starting to hit their limits, particularly when scaling below 25nm, largely due to design constraints.
While flash adoption continues apace in enterprise storage, Commins said customers are generally looking for 3 per cent to 10 per cent of their data to behave in a flash-like manner for specific applications and use cases, such as online transaction processing and supporting virtual desktop infrastructures.
Wikibon predicts that hybrid storage will continue to disrupt traditional disc arrays. The research firm found that the hybrid approach is superior in high-performance environments, both in cost and performance, particularly when IO rates are greater than 700 IOs per terabyte.
- Gary Hilson