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Solid-state storage challenges
Reliability: NAND flash has been hampered by concerns related to wear out. A flash-memory cell permits only a finite number of writes before it becomes unusable. "100,000 live cycles for an SLC flash cell is quite typical, although in reality you are likely to get a significantly higher number of writes before a cell wears out," says Bob Wambach, EMC's senior director of product marketing for Symmetrix. This is an order of magnitude above the number of writes cited for consumer-level MLC flash, but it's still limited.
While 100,000 writes per flash cell appears to be small, flash drive vendors like STEC Inc. have been able to warrant their drives for three-plus years with better MTBF specs (2 million to 4 million hours) than enterprise hard drives (typically 1 million hours) through a series of techniques. From front-ending the flash storage with a small DRAM cache, using wear-leveling algorithms that evenly distribute writes across blocks of cells to sophisticated bad-block management techniques and continuous proactive drive monitoring, NAND flash media is able to meet enterprise requirements. Additionally, storage array vendors are deploying solid-state drives in RAID configurations to further
| reduce the probability of data loss.
Read-write performance gap: The substantially slower write performance of flash cells vs. reads has been another area of concern. The limitations in a flash cell's lifecycle and its slow write performance can be attributed to the way flash cells are written: they're accessed in blocks of cells. To write to a block, cells in the block need to be opened, existing content needs to be erased and cells need to be closed. This adds significant overhead when writing and updating data.
Through techniques such as the use of a small DRAM cache in the drive, enterprise-level flash drive vendors have been trying to close the performance gap between reads and writes. "Our enterprise-level Zeus drives support 18,000 random write IOPS and 52,000 random read IOPS," reports Pat Wilkinson, STEC's VP of marketing and business development. This is several orders of magnitude above the few hundred IOPS supported by high-end disk drives, where IOPS can only be scaled by increasing the number of spindles (see "Performance comparison," below).
Array interoperability issues: Array vendors need to ensure that the solid-state option doesn't adversely impact their array's reliability and performance. The array must be able to deal with the high performance of solid-state drives, which will likely push the array to its limits. From replication and mirroring to thin provisioning, all features need to work with the solid-state option in place. Most importantly, vendors need to ensure that their array architecture can cope with the peculiarities of NAND flash. A case in point for the latter is NetApp and the Write Anywhere File Layout (WAFL). WAFL was designed to reduce disk head movements and eliminate random writes. For that, WAFL continuously moves data in an attempt to serialize access. This makes it more difficult for NetApp to just replace disk drives with flash drives, as flash drives would wear out much faster than in more traditional storage arrays where data on the spindles is more static. While NetApp won't comment on it, this is likely one of the reasons why the company opted for a cache-based, solid-state option for its first-generation solid-state offering rather than replacing disk drives with solid-state drives.
This was first published in July 2008