Write Amplification plays a critical role in the durability of an SSD . As you already know, the less you write to an SSD the more life it will have, so this technology directly influences this factor and is one of the points where engineers pay the most attention. In this article we are going to explain what Write Amplification is , what it is and why it is important.
Write Amplification is integrated into the controller design of all SSDs, and unlike other factors that collaborate in maximizing the useful life of the device, this is not a technique that will double or triple its useful life, but it is very important for the longevity of the device. Let’s see it.
What is Write Amplification and how does it work?
Writing amplicification is complex enough that we are not going to attempt to define it in this article as such; suffice it to say that an SSD that is trying to manage its free blocks will sometimes need to move data from two blocks that have little data to another block to leave that space free and can be erased. Normally this works as a kind of internal defragmentation of the SSD blocks: if there are two blocks that have little data, that data is moved to a third block to leave the first two free and to reuse them.
Since this operation has not been requested by the SSD’s HOST system, the result is an “unexpected” write that subjects the SSD to more writes than it should. If the number of NAND writes within the SSD matches the number of requests from the HOST system, then the write amplification would be 1.0. If the average number of flash writes were double, then the Write Amplification would be 2.0.
Therefore, if you were to write a piece of system software that measured SSD writes, this measurement would not reach the actual number of times it was written to the flash memory chips due to this write amplification. Therefore, as you will suppose, it is a harmful factor, and what the SSD controller does is, precisely, try to minimize it in order to maximize the life of the SSD.
How is write amplification reduced?
There are many ways to reduce this factor. Some SSDs write data into a kind of log structure, so the data is written to sequential NAND addresses, and maintenance is done during periods of device downtime. This reduces write amplification, but data management becomes more complicated as the SSD fills up.
The approach of SandForce controllers, for example, is to compress the data that enters the SSD memory. If this data is compressed to half its original size, only half the writes actually reach the SSD. This allows SandForce to boast that its Write Amplification is less than 1.0, a figure that is not intuitive at all since it involves other factors such as the detriment of the performance of the SSD, and also the data that the NAND receives is not real.
In summary, an SSD that has been designed to reduce write amplification will reduce the number of times NAND Flash memory sees write cycles, thus taking advantage of the limited number of erase / write operations that NAND chips can support before experiencing bit errors.
