To give a simplified explanation of the how and why of performance degradation, it’s essential to understand that an SSD’s controller will not really delete data until the entire capacity of the drive has been filled; your OS might indicate that the data is gone, but in reality it will still be taking up space on the flash storage – though it will be marked as invalid. The data is only actually changed when it is overwritten. This is the same as how hard drives work but with hard drives this doesn’t affect performance. However, with SSDs it does. The reason why this happens is twofold.
The first issue is that, while SSDs can write files in units of 4KB (called pages), they can only delete in far larger 512KB blocks. This means that if the controller wants to write some data to a certain area that already has data in it, it has to read all the data within that 512KB block, check if any of it needs to be kept, copy the required data to another temporary location, delete the block, then write the new data back into the block. This obviously adds considerably to the amount of time it takes to write data.
The second factor is that the flash memory cells in SSDs degrade over time so that, in the case of MLC drives, they can no longer hold a charge (i.e. data) after around 10,000 write cycles. Thus, to get the optimal lifespan out of any drive, it is imperative that each cell is written to only as much as any other, a process known as ‘wear levelling’. For this reason the controller will fill every block on the drive up before it deletes and rewrites any others. Once the drive has reached its maximum capacity though (remember, this doesn’t mean the drive appears full on your computer, it’s just that all the data from the files you’ve deleted will still be kept until its overwritten), the controller needs to read and delete whole 512KB blocks for every page it wants to write, which obviously leads to significant slowdown. Now, it’s perfectly possible you will go for years without experiencing any problems, as it may take that long to fill your drive with data, but others may notice this within months or even weeks.
To get around this, newer drives support a command called TRIM. This basically tells the drive to actually delete invalid data there and then so that blocks are ready to be written to. Windows 7 will support this command natively so that when you delete a file it tells the controller to delete the data straight away. However, don’t despair if you’re stuck with Vista or XP. Most manufacturers provide a tool that does essentially the same thing (though you have to ‘activate’ it manually, a bit like defragging your hard drive), which in Patriot’s case is aptly called the Performance Restore Utility. Using this after having ‘filled’ the drive does have a dramatic positive effect, bringing performance back to the level of a new drive.
If after the above talk of limited MLC lifespan you’re worried about your expensive drive going the way of the dodo within a few years of purchase, Patriot’s truly class-leading 10-year warranty offers major peace of mind.
As has hopefully become clear, the controller is a very important part of any SSD’s performance. Patriot’s Torqx uses an Indilinx controller with 64MB of cache, which for now seems to suffer fewer problems than those on previous drives we’ve seen and outperforms most rivals (with Intel’s being the notable exception). However, while Intel’s drives are excellent, even the 80GB ‘mainstream’ Intel X25-M will still set you back around £300. Anyway, enough detail; onto performance testing.
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