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Inadequate software support for SSD
While significant progress has been made to overcome or at least mitigate the issues related to NAND flash, software support to manage and efficiently take advantage of solid-state storage has evolved at a much slower pace, becoming one of the primary obstacles to more rapid enterprise adoption of SSDs. To counteract the prohibitive effect of the high price of solid-state drives, storage systems need to be able to maximize the use of SSDs by automatically and transparently shuffling data between the fast SSD tier and slower disk tiers. While most storage vendors acknowledge the need and relevance of policy-based data migration between the fast but expensive SSD tier and disk tiers, keeping frequently accessed data in solid-state storage and more static data on disks, only a few can offer an automated solution today.
Leading the pack is Compellent Technologies Inc.'s Storage Center storage-area network (SAN). Its Dynamic Block Architecture tracks the characteristics and usage of every data block; this metadata information is leveraged by the product's Data Progression feature, which automatically moves data from SSDs to disk tiers and vice versa based on how often blocks are accessed.
"Our Data Progression is the killer app for SSD because users can add drives to existing systems and then let automation take
over," said Bob Fine, Compellent's director of product marketing. Contrary to Compellent, the majority of enterprise storage vendors depend on a manual two-way process for migrating data between solid-state drives and disk tiers, first analyzing I/O activity and, in a second step, migrating data to the appropriate tier. Depending on a manual process for now, EMC Corp. has announced Fully Automated Storage Tiering (FAST), which will be available for EMC's Symmetrix V-Max systems later this year. FAST will automate the movement of data across multiple storage tiers based on business policies, predictive models and real-time access patterns. IBM supports automatic data migration to SSD via its Data Facility Storage Management Subsystem (DFSMS), but it's only available on the mainframe z/OS platform with DS8000 storage, with a manual two-way process still required for other systems.
Both Sun Microsystems Inc.'s Sun Storage 7000 Unified Storage Systems and NetApp Inc. filers with Performance Acceleration Modules (PAM) circumvented and solved the software challenge at a storage architecture level by using NAND flash as cache rather than as disk replacements. As a result, SSD is closely woven into their storage architectures and firmware, with the advantage that all data and apps benefit from solid-state drives, eliminating the requirement to shuffle data between tiers. "We want our Storage 7000 customers to have all of their working data in flash," said Michael Cornwell, Sun's lead technologist for flash memory.
Contemporary storage systems have been designed to cope with the limitations of mechanical disk drives, in particular to reduce the impact of high latency and the low number of IOPS mechanical disks can support. With SSDs, this basic truth has changed and capacity limitations of storage controllers have become the limiting factor. Simply replacing disk drives with SSDs can overwhelm storage systems if too many solid-state drives are added. "Storage controllers are just starting to adjust to the new performance requirements of SSD, and today customers need to heed the recommendations and guidelines of storage vendors on how many SSDs they can add," said Greg Schulz, founder and senior analyst at Stillwater, Minn.-based StorageIO Group.
There are currently four methods to complement storage systems with solid-state storage:
- Adding SSD drives in lieu of disk drives
- The use of NAND flash as cache in storage controllers
- The use of NAND flash on servers rather than storage controllers
- Standalone SSD arrays
Adding SSD drives in lieu of disk drives. Adding SSD drives via Fibre Channel, SATA or SAS interfaces to replace disk drives is the easiest and most popular way of adding solid-state drive support to existing arrays. Notwithstanding rigorous testing and qualification procedures, this approach requires few if any changes to storage systems because vendors can leverage what's in place. The lack of automated data migration between SSD and disk tiers, and performance limitations of contemporary storage controllers, are the two biggest drawbacks of this approach. Nevertheless, it's the method adopted by most storage vendors. EMC has been joined by Compellent, Fujitsu, HP, Hitachi Data Systems, IBM, LSI Corp., Pillar Data Systems, Sun and many smaller array vendors, offering SSD drives in addition to hard disks for some of their arrays. The overwhelming majority of these vendors have been using STEC drives as their first generation SSDs, largely because STEC was the first vendor capable of meeting enterprise storage requirements. With disk drive vendors like Seagate Technology LLC, promising startups like Pliant Technology and SandForce, and Intel Corp. targeting the enterprise storage space, STEC's predominance will be challenged.
The use of NAND flash as cache in storage controllers. NetApp and Sun are leveraging NAND flash as cache. By doing so, both vendors have overcome the software issue of automated data migration between SSD and disk tiers, but they have changed their storage architectures to embrace NAND flash, eliminating the possibility of overwhelming their arrays if too much solid-state drive storage is added. By front-ending disk drives with NAND flash instead of replacing disk drives, all data and apps benefit from SSD, not only data that resides within the SSD tier.
This was first published in September 2009