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SSD product review: Intel, OCZ, Samsung, Silicon Power, SuperTalent
by Marc Prieur
Published on October 6, 2008

SSD drives - against (cont)
With the aim of avoiding writing errors so as to minimize cell wear, SSD drives have built-in ECC correction: if the ECC algorithm detects an error the block is marked as defective and the data is written elsewhere. Of course, it’s even simpler to limit cell wear in the first place.

To achieve this, SSD drives use a technique known as wear leveling. The controller has a number of writes and the date of the last write of the data for each block. This allows it to arrange data so that erasures and re-writes are distributed evenly so as to share transistor wear evenly, even if this results in putting the oldest data on the most worn blocks. This can of course have a negative impact on performances during intensive writing on an SSD drive that has already had a fair amount of wear.

The other particularity of SSD drives is in terms of the data retention span in memory cells. In fact, data is not stocked permanently and indeed the retention span varies depending on how worn a cell is. According to the JEDEC specifications, a “new” cell ought to have a 10-year span, whereas at the end of a cell’s life, retention drops to around a year.

All this means that the life expectancy of an SSD drive can vary enormously depending on the technology used. In the best-case scenario, an MLC drive may be filled every day for 27 years before being worn out. In practice we’re far from this figure and Intel, new to the market and naturally claiming to be on the cutting edge in terms of reliability, gives a 5-year life expectancy for 100 GB/day. This isn’t bad but it remains to be seen what the actual life span of SSD-drives will be and, above all, what the differences between different models will be, something it’s very difficult to know in advance.

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