SSDs Ready For The Enterprise

Because the storage subsystem is the performance bottleneck for most commercial applications, system designers have sought to speed the flow of data from disk to main memory using a variety of schemes. The latest move relies on solid-state disk technology. Over the past year, flash-memory-based solid-state devices have emerged that deliver data in a few microseconds; are significantly less expensive than RAM on a per-gigabyte basis; and, like disks, store data reliably when powered off.

Enterprise-class flash devices are still pricey--averaging more than $100 per gigabyte. But for those with deep pockets and a real need for speed, these systems can deliver up to 45,000 read I/O operations per second, or 16,000 write IOPS, compared with the 170 IOPS typical of a 15-000 RPM drive. A single mirrored pair of SSDs can outperform 100 spinning disks that would cost several times as much after drive enclosures, array features, software that's licensed by capacity, and other so-called slot costs are figured in. Flash devices also can help you go green because that pair of high-end SSDs will use much less power, and generate less heat that the data center cooling system must remove, compared with a group of 15,000-RPM drives delivering the same IOPS.

Vendors from EMC to Xiotech are debuting innovative SSD technologies suitable for enterprise data centers and branch offices. But there are a few downsides. Long-term reliability is unproven, the bottom-line cost on a per-gigabyte basis could send your CFO into cardiac arrest, and advances in data classification and tiering are still needed to gain maximum benefit. IT groups need to be aware of the trade-offs.

How Flash Works
While flash is semiconductor memory, like the RAM that makes up a computer's main memory, it doesn't allow direct read and write access to each byte, the way RAM does. Just as a disk drive is divided into sectors, the NAND flash chips typically used in SSDs are organized into pages, typically of 4 KB each. These pages are in turn collected into blocks of 256 KB to 1 MB.

Data can be read from the flash memory on a page-by-page basis and written to empty pages. However, to overwrite data in a previously used page, the entire block containing that page must be erased--a relatively slow process. If other pages in that block contain valuable data, that information must either be relocated to pages in another block or loaded into RAM cache in the SSD and written back once the block has been erased. This results in flash-memory devices being three to 10 times slower to write data than to read it. In addition, the high voltages needed to erase blocks cause wear on the drives' microscopic transistors and connections, eventually wearing them out.

Flash memory comes in two basic varieties, differentiated mainly by the ability to handle multiple erase cycles. As the name implies, single-level cell, or SLC, flash stores 1 bit in each memory cell, while multilevel cell memory stores 2 or 3 bits of data in each cell.

Storing 2 or 3 bits per cell increases density, and therefore reduces cost, but it also slows access and reduces the longevity of the device. Therefore, most server and array flash systems use SLC technology.

Even among SSDs using the same flash technologies--or even the same flash chips--performance can vary significantly, however. A main variable is that individual vendors design the controllers that put a storage interface on their flash chips.