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SSDs: the future of enterprise storage

Akhtar Pasha looks at the new approach to enterprise storage technology—NAND-based flash SSDs in the wake of DRAM-based SSDs that are still going strong

The debate for supremacy in disk technologies continues to center around DRAM-based Solid State Disks or SSDs and NAND-based flash SSDs. While the former have been used in blade servers and enterprise storage arrays for quite some time, flash SSDs are non-volatile (they retain data without a power source), have no mechanical parts and are used in consumer electronic gadgets such as phones, PDAs and MP3 devices.

While HDDs have been the workhorses of enterprise storage for nearly 50 years, DRAM SSDs and now flash SSDs are getting more attention than ever. There are important speed differences between these two technologies—flash SSDs are much faster than spinning disks, but they are not as fast as DRAM-based SSD when handling I/O. So lets understand how both these two technologies work and where they differ from one another as well as how these two will be used in future.

Disk is the bottleneck

To understand this let us examine the disparity that exists between server processors, memory and disk. Intel’s latest technology innovations, including 6-core Intel Xeon 7400 series (formerly code named Dunnington) processors only highlight a significant gap in the IT ecosystem: multi-socket, multi-core servers have far outpaced the performance of traditional disk drives resulting in expensive and complex architectures, which use massive amounts of expensive DRAM or disk drives—designed to maximize CPU utilization.

Dave Hitz, Founder, NetApp in his blog puts it nicely how DRAM-based SSD and NAND flash-based SSD technologies differ from one another, and what is driving the innovation in the NAND-based SSDs. According to him, almost every measure of computer performance increases exponentially with the important exception of disk drives that keep getting bigger but are not getting much faster. As a result, the number of seeks-per-second available for each gigabyte of data (seeks/second/GB) is plummeting. He explained, from a human perspective, that seeks/second/GB have gone down by a factor of five hundred, but from the CPU perspective, it is five hundred thousand times slower.

Surajit Sen, Director-Marketing & Alliances, NetApp, said, “The disk technology has been lagging behind the rest of the computing system such as the server CPU and memory. While CPU performance has doubled every 18 months, disk technology performance per gigabyte of storage has been falling. However, SSDs will change the face of enterprise storage over the next 10 years but this technology has its own limitations and one would need to modify the file system to gain the required level of performance.”

Let us take a closer look at some of the limitation of flash-based SSDs vis-à-vis DRAM-based SSDs and the approach vendors are taking to improve the performance of flash-based SSDs.

Performance issues

Writes to flash SSDs require that the cell first be ‘erased’ after which it can be programmed with new data. The erase procedure (typically ranges from 1.5-2.0 milliseconds) is quite long compared to everything else in a memory-based system and nearly as long as a seek operation on a HDD. While SSD Read performance can be sustained at 20-40 MBps, Write performance significantly lags at only 1-5 MBps.

Ramachandran Narayanaswamy, Vice-President and Head-Networking and Storage Industry Group, MindTree Ltd., added, “In a typical SSD, if you want to write 4 KB of data, the system first needs to copy a 128 KB block of data from the flash drive to main memory. Then the system modifies the data, now in the system DRAM, with the 4 KB of new data. Finally, it needs to write the entire 128 KB block back to the flash drive. Typical write amplification multipliers are in the neighborhood of 20–40X, on average.”

Sivasankaran L, Director-Storage Practice, Sun Microsystems India, said, “Ramping up disk performance in a square-inch is just impossible and there is a clear mismatch between the speeds at which current server CPUs and memory operates. Further flash SSDs cannot match the performance of DRAM-based SSDs that are widely used in OLTP and database intensive environments. DRAM SSDs would typically operate or give a million IOPS while a 15,000 RPM HDD would give 350-450 IOPS.”

Herman Yiu, Regional Marketing Manager, for Ultra Mobile Group and NAND Solutions Group, Intel Asia Pacific., explained this performance gap saying, “DRAM is inherently faster than NAND or flash-based SSD in random reads and writes. Put into an array (they are not a typical SSD), they generally have greater random I/O performance than even Intel’s X25-E. However, they are very expensive (DRAM itself is approx 10x NAND), with system price points reaching up to $250,000–in most applications, a DRAM-based SSD array with battery backup costs 20-25x more than a comparable NAND SSD.” DRAM SSDs have actually been around for decades and yet the penetration of this technology into enterprise computing is miniscule due to its price and volatile nature.

High endurance

Flash-based SSDs, on account of the design, wear out after repetitive data writes due to charges that get trapped in the dielectric oxide layer between the substrate and the floating gate of the cell. Sen added, “The endurance problem in SSDs can lead to failure of disks as SSDs are prone to wear leveling.”

Yiu mentioned that Intel can handle the wear-leveling effectively so that SSDs do not fail. According to him, Intel’s wear-leveling technology is designed as Active Wear-Leveling technology, which is combined with dynamic wear-leveling and static wear-leveling technology. A traditional wear leveling scheme has a wear leveling efficiency of about 3X, which means they have about three times as many writes on the block that has the most writes to it compared with the blocks that have the average number of writes to it. To compare with traditional wear-leveling technologies, we could probably safely put about three times more NAND writes on our SSD while safely remaining within the NAND cycling capabilities because of the great efficiency that exists in the company’s active wear-leveling algorithms.

Saving power

DRAM-based SSDs usually incorporate internal battery and backup storage systems to ensure data persistence. Sivashankaran said, “DRAM SSDs would increase the operational costs such as power and cooling. Further, while a spinning disk would consume 12 Watts of power, a flash SSD would consume only 2.5 Watts of power. So, flash-based SSDs would help save on operations costs in the long run.”

Sivashankaran said, “The ratio of cost of per gigabyte of storage between HDD and flash SSD is $5:$35 but when SSDs are pitched against DRAM SSDs the cost equation looks like this—$100 for DRAM SSD as against $35 in the case of flash SSDs. The cost of DRAM SSDs are prohibitively high and hence its applications are very niche.”

Nirmal Puranik, Brand Manager–Storage, Systems and Technology Group, IBM India/SA agreed, “NAND-based SSDs would consume lesser power when compared to spinning disk. DRAM-based SSDs performance today does not justify its high costs and hence we see higher and faster adoption of NAND SSDs as we are seeing a consistent drop in the prices and it’s getting productized.”

Developments in flash SSDs

Yiu explained the advantage of Intel SSDs in enterprise computing is that an infrastructure already exists using standard SATA and SAS HDDs. Intel’s SSDs can drop into these sockets and provide up to a 115x improvement in performance (which is fast enough for most server acceleration requirements), is more reliable, and consumes less power than the technology they are replacing (HDD). RAM based arrays are more complex and require OS and hardware modifications and optimizations, and again because they are volatile, are only used for accelerating performance and not for storage.

Sun’s approach towards increasing the performance of flash SSD is different and relies on its ZFS file system that combines Solaris Hybrid Storage Pool technology. Using this technology, which is a part of ZFS, the 7000 family intelligently and transparently moves data between tiers of read and write optimized SSDs, DRAM, and disk storage, while managing all of the tiers as a single pool of storage. Shivashankaran said, “A Hybrid Storage Pool approach provides the benefits of high performance SSDs while still saving money with low cost high capacity disk drives. Using Hybrid Storage Pool with ZFS, storage architects can move data from SSDs to SAS or SATA drives seamlessly.” He however, of the opinion that, while SSD performance and operating costs are appealing, clearly it is not cost-effective in every case to substitute SSDs for mechanical drives in a storage array. Sun Microsystems is using NAND-based SSD for its open storage product, Sun Storage 7000 series.

IBM is using two different types of single-level cell (SLC) SSDs, including drives from STEC and multi-level cell (MLC) technology. In SLC technology, the limit on the number of writes can be one million, which under some stressed usage cases could be exceeded in days. For MLC, the number of writes can range from 10,000 to 100,000 making MLC even 10 to 100 times less durable than SLC. Puranik said, “SLC technology is reliable through wear-leveling, over provisioning and error correction technologies, but MLC still requires more work and innovation to bring it into enterprise-class applications.”

Narayanaswamy said, “I believe NAND SSD offers high scalability and it can be a fabricated technology in the road ahead for enterprise storage. Give NAND SSDs another 5-8 years and it would emerge as the default storage technology, while DRAM SSDs would a remain special purpose, highly scalable technology used in mission critical application.” He added that to get the performance out from NAND SSDs vendors need to build transactional file system such as Solaris ZFS that can access multiple discs seamlessly. MindTree too is working towards developing a new transactional file system that can access and move data from FCs, to SSDs to SATA/SAS drives.

akhtar.pasha@expressindia.com