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OCZ RevoDrive X2 240GB – PCIe SSD Review

Posted 30 November 2010 20:31 CET by Wendy Robertson

 


The SandForce SSD controller

The SSD controllers used in the RevoDrive X2 are the SandForce SF-1200 series, designated SF1222TA3-SBH on the RevoDrive X2, and as we have said, there are four SandForce SF-1200 series SSD controllers on the RevoDrive X2.

The SandForce SF-1200 series is a brand new SSD controller and has brought a number of new technologies to SSD, some are designed to increase performance, others designed to improve durability and increase the lifespan of the SSD. Let’s take a brief look at a few of these technologies.

From the above screenshot we can see at least 2 unique technologies that SandForce are using on this new SSD controller under a technology they call DuraClass which we look at below.

DuraWrite

There are two fundamental things that cannot be ignored when using flash memory for storage. That is, the flash memory type currently used is NAND, which is (a) expensive and (b) has a limited amount of total write cycles. I’m pretty sure that all SSD manufactures would love to only use SLC (single level cell) NAND on their SSDs: it’s faster and has 10x the total write cycles of multi-level cell (MLC) NAND. Unfortunately though, the cost of SLC is enormous, and this is the main reason that SLC is generally reserved for enterprise class SSDs.

The other fundamental is MLC NAND is less durable compared to SLC NAND. SLC NAND is reckoned to have 100,000 total write cycles, whereas 34nm MLC NAND has a typical 5,000 total write cycles. The curse of NAND is write amplification: put simply, this means that the more data that has to be written, the faster those write cycles are used up. SandForce has looked at write amplification in great detail, and come up with a new technology called DuraWrite. The SandForce SSD controller is smart enough and fast enough to compress data on the fly, and then write a compressed version of the data to the NAND itself. By doing this, some writes can be 20 times smaller than they would be if they remained uncompressed. This also has an added bonus, if you have to write less of the data to NAND, then the write process also becomes much faster.

So, as well as lowering write amplification you also increase performance, which is very clever, allowing the much cheaper MLC NAND to be used in an enterprise class environment. Of course, not all data can be compressed in this fashion, ZIP and RAR files for example are already heavily compressed so won’t benefit much by this process, but overall DuraWrite works extremely well as we will see later on in this article.


RAISE

Another new technology found in the SandForce SF1200 is RAISE (Redundant Array of Independent Silicon Elements).

As I mentioned earlier, our OCZ RevoDrive X2 SSD review sample has a user capacity of 240GB. The actual amount of NAND that the drive has is 256GB of MLC NAND. So what is the other 16GB used for?

At some point during an SSD’s life a NAND cell or block of NAND cells will fail, which is why all SSDs do not make the full amount of onboard NAND available to the user for storage. All SSDs have a pool of reserved NAND to map into use if a cell or block of cells fails. This can be as low as 1GB or 2GB of NAND in reserve. RAISE sets aside a good deal more than this in its pool of reserved NAND. In the case of the OCZ RevoDrive X2 240GB, the amount of reserved NAND is approximately 16GB. By keeping so much NAND in reserve, this should ensure the SSD can last for the expected minimum lifespan of 5 years.

As a result of this large pool of reserved NAND, drive failure is much less likely to happen due to just simply running out of NAND that has no more write cycles available.

Once again, as an added bonus of reserving so much NAND, the wear levelling and garbage collection algorithms can be enhanced, thus ensuring there are always fresh NAND blocks to write to.

Now let’s head to the next page where we will look at our test PC and testing procedures...

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