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

Flash memory SSD drives have been around for over two years now. With ridiculously low capacity and performances at first, they have started moving forward in many respects and are now cutting out a real advantage across the board with the arrival of 60 GB drives at less than €300. And this is only the start!
SSD – in favour
Compared to classic hard disks, the advantage of the SSD drive is of course the absence of mechanical parts. This means that that they are completely silent and have greater resistance to being knocked about. Seek times are also incredibly fast: while hard disks still have seek times of anywhere between 7 and 17ms, SSD drives measure averages of less than a ms, which is what makes for such a difference in performance for certain operations.

In terms of power consumption, SSD drives are of course very impressive, but it is important to put this in context. 2.5" hard drives are also very economical with, say, a Samsung HM160HI 5400, idling at 0.9W and reading/writing at 2.3W. For anyone who wants to know, from a system point of view, SSD drives function in exactly the same way as a standard hard disk: there is no difference in how they address the SATA controller.
SSD drives - against
SSD drives are unfortunately far from being perfect. Using NAND flash memory, they do suffer its disadvantages, although these are attenuated by the controller. The first limitation with NAND flash memory is that it does not provide a random-access external address bus: data must be read on a block-wise basis.

Each memory chip is divided into blocks, which are then divided into pages. A 2 GB memory is generally divided into blocks of 128 MB, each of which is divided into pages of 2 KB. Pages are then read a page at a time and when it comes to writing, blocks must be written a block at a time. So an SSD takes as much time to read 1 KB as 2 KB, which isn’t a great problem, but as much time is needed to write 4 KB as 128 KB, which is obviously more so..

It is possible to get around this problem by introducing a sort of cache in the controller to intercept data and only write it in blocks of 128 KB. Otherwise, quite apart from the performance problem in this particular case, a proportion of memory cells will be worn out for nothing.

Because, yes, flash memory cells do wear out! And worse, depending on the technology, they wear out at a faster or slower rate. SLC memory, which stocks one bit per transistor, can supposedly be put through 100,000 erase cycles as opposed to 10,000 for MLC memory, which however has the advantage of giving double the capacity for the same physical size.

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SDD drives - against (cont)  

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