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Another drawback is long-term reliability or "wear out." Flash SSD may be nonvolatile, but individual flash-memory cells have a limited usage life. "You don't want to hammer one cell too much. If you can avoid writing to a particular cell for days or weeks, you can stretch the life of the cell," explains Niebel. Currently, a flash cell has a write-cycle life of 100,000 writes. That may seem like a lot of writes until you start thinking about a high-volume transaction system.
Flash SSD systems try to overcome this problem by building wear-leveling algorithms into the controllers. These algorithms spread the writes across different cells so no cell gets written to excessively. By combining wear-leveling with RAID-like striping, proponents argue that they can push the effective life of a flash SSD system beyond 100,000 writes.
Flash is delivered in two formats: single-level cell (SLC) and multi-level cell (MLC). The wear-out and cost problem may be partly addressed through a combination of SLC and MLC flash. MLC, which layers more data on each cell, is cheaper than SLC. OEMs can use it to add capacity to SSD systems. However, MLC has a much shorter life than SLC. Because it's the write process and not the read process that shortens cell life, the trick for OEMs becomes "trying to figure what portion needs to be read-only and make that portion out of MLC. They can use SLC for the rest,"
| says Chander. OEMs will have to build intelligence into the controller to direct predominantly read-only data to the MLC cells in a SLC/ MLC SSD storage device.
In addition to write wear out, there's another concern with writes. "NAND flash has a slower write speed," explains IDC's Janukowicz. That and the wear-out problem suggest flash SSD is best suited for applications that write once or occasionally but read many times.
While HDD has an overwhelming advantage in terms of cost/GB, the situation is reversed when it comes to cost per IOPS. But the problem goes beyond comparing one HDD system with one SSD system. "There are several things going on here. First, the hard disk is not keeping up with the performance of the server," says Alan Fitzgerald, CTO at Adtron Corp., a Phoenix-based SSD manufacturer.
This was first published in November 2007