Wednesday, November 28, 2012

What is SSD Write Amplification?

If you're considering the change to a SSD drive, you might be scared by the fact that flash memory has  a limited number of write/erase cycles. Though there are special kinds of Flash memory that can endure over 100 thousand cycles, common chips usually have a 10 thousand write cycle per cell. And if that scares you, what to say about the new 20nm chips, that have just 3000 program/erase cycles?

So, how can we trust a SSD that can only withstand 3000 write cycles? The key here is that the SSD controllers are now smart enough to compress the data and rearrange it internally in order to make the most out of every single cell - more so, most SSDs have a few gigabytes of spare area that is used for this exact purpose alone (in enterprise SSD drives, that spare area can be bigger than the actual stated capacity).

That's why most manufactures can guarantee their SSDs to be good even if you write 20GB of data each and every day for over 5 years, on it.

But what about write amplification? Write amplification is what happens when some amount of data you send to the SSD ends up causing a large number of write cycles. For instance, if you write just a single byte to the drive, it will end up using at least a 4KB block, and as flash memory can only be erased in "pages" consisting of several blocks, that single byte of data might end up causing the drive to write lots of KB - not to mention associated metadata relating to the file system, such as last access timestamps, metadata, search indexes, etc.

For the most part, this is something that happens behind the scenes and that you'll never have to worry about. But if you like knowing how things work... it's nice to have a generic understanding of the process. If you're a bit like me, then you'll know that you can also try and minimize write cycles as much as possible, using a RAM drive for temporary files, for instance, instead of using the SSD for that.

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