write amplification

Write amplification is an issue that occurs in solid state storage devices that can decrease the lifespan of the device and impact performance.

Write amplification is an issue that occurs in solid state storage devices that can decrease the lifespan of the device and impact performance. Write amplification occurs because solid state storage cells must be erased before they can be rewritten to. This operation requires data and metadata to be written multiple times and increases the number of program/erase cycles required over the life of the solid state storage device.

Program/erase cycles are managed by the device's flash controller, which uses a logical to physical mapping system known as logical block addressing (LBA) to manage data. When data is rewritten, the flash controller writes the new data in a different location, and then updates the LBA with the new location. Invalid data continues to reside in the old location, and those cells must be erased before they can be written to again. Because each cell can tolerate a finite number of program/erase cycles before becoming unreliable, write amplification can reduce the lifespan of the device. The overhead associated with performing additional program/erase cycles can impact performance.

Write amplification SSD is addressed in a number of different ways. One method is known as over-provisioning. Over-provisioning presents the user with less capacity than what is actually physically available on the storage device. This additional space is used to lower the write amplification when the controller writes to the storage among other things. Another way solid state devices reduce write amplification is to separate static and dynamic data. This reduces the amount of garbage collection the controller must perform, thus reducing the overall number of writes.

This was first published in January 2012

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