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Solid-state storage update

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NetApp offers the Performance Acceleration Module (PAM), which can be added to any NetApp filer with available PCI Express slots. Depending on the controller, up to five modules can be installed for a unified cache as large as 80 GB today and up to 512 GB later in the year when a higher density PAM card will become available. PAM is used to cache metadata only. "By storing a copy of the metadata in flash memory on the storage controller, we're seeing a 30% to 50% performance gain for typical workloads," said Patrick Rogers, vice president, solutions marketing at NetApp. "Filers with PAM and SATA drives have become a viable alternative, replacing filers with FC drives, because of comparable performance at a significantly lower cost," he said.

Unlike NetApp, Sun uses flash memory in its Sun Storage 7000 Unified Storage Systems to cache all reads and writes -- not only metadata -- and therefore has one of the most advanced architectures to support flash memory.

The Sun Storage 7000 Unified Storage Systems run Solaris on an x86 platform with an optimized storage stack and the Zettabyte File System (ZFS) that supports a Hybrid Storage Pool of DRAM cache, SSD and mechanical disks. The solid-state drive is situated between the DRAM-based Adaptive Replacement Cache (ARC) and SATA drives. The ZFS Intent Log (ZIL), which holds the write journal to allow the file system to recover from system

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failures, is written to a write-optimized SSD. The L2ARC cache comprises read-optimized SSDs to extend the DRAM-based ARC cache for read operations; L2ARC can be hundreds of gigabytes in size, and its purpose is to keep working data in memory to minimize disk access. This Hybrid Storage Pool enables the Sun Storage 7000 Unified Storage Systems to support more than 800,000 IOPS, according to Sun.

The use of NAND flash on servers rather than storage controllers. Although the Sun Storage 7000 Unified Storage Systems is a standalone storage system, it makes the point for those who argue that flash memory belongs in the server rather than the storage controller. "Just like L2 cache extends memory on the CPU and DRAM extends L2 cache, flash memory is intended to extend DRAM," explained David Flynn, chief technology officer at Fusion-io. The Fusion-io ioDrive and ioDrive Duo NAND flash PCI Express cards provide direct-attached storage (DAS) for servers. Being a server company that also sells storage, Sun concurs that servers are the right place for flash memory. "Flash memory is a game-changer for server architectures, and next-generation servers will extend DRAM caches with flash memory," Sun's Cornwell said.

Standalone SSD arrays. Complementing disk arrays with SSD-based storage systems that run parallel to traditional storage arrays is the least-disruptive method of adding solid-state storage to a storage environment. The leading vendor of standalone SSD arrays is Texas Memory Systems. Offering both DRAM and NAND flash-based SSD arrays, the company sells its RamSan family of products directly and through OEM relationships with BlueArc Corp., NetApp and others. On the downside, standalone solid-state systems aren't able to leverage existing array components and are therefore likely to be more expensive. Moreover, they're less integrated with the disk tier than other architectural approaches, making it even more difficult to overcome the data migration challenge between the solid-state drive and disk tiers.

Solid-state outlook

Solid-state storage has just begun to play a role in enterprise-level systems, but it's apparent that its rise is unstoppable. Enterprise storage systems are moving toward two-tier architectures, namely, a solid-state drive tier for transactional and changing data, and a large capacity SATA disk tier for more static data. With the continuous innovation that has overcome some of the limitations of NAND flash, as well as newer memory technologies like magnetoresistive random access memory (MRAM) on the horizon to eventually replace NAND flash, the real challenge to rapid adoption of SSDs is the lack of storage architectures that are capable of seamlessly integrating and efficiently taking advantage of solid-state drives.

BIO: Jacob Gsoedl is a freelance writer and a corporate director for business systems. He can be reached at jgsoedl@yahoo.com.

This was first published in September 2009

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