Phase-change random access memory (PRAM) is a promising storage-class memory technology that has the potential to replace flash memory and DRAM in many applications. Because individual cells in a PRAM can be written independently, only data cells whose current values differ from the corresponding bits in a write request need to be updated. Furthermore, when a block write request is received, the PRAM may contain many free blocks that are available for overwriting, and these free blocks will generally have different contents. For this reason, the number of bit programming operations required to write new data to the PRAM (and consequently power consumption and write bandwidth) depends on the location that is chosen to be overwritten. This paper describes a block placement algorithm for reducing PRAM bit writes based on the idea of indexing free blocks using a content-based signature; computing the signature value of a new block of data to be written allows a free block with similar contents to be located quickly. While the benefit that can be realized by the use of any block placement algorithm is heavily dependent on the workload, our evaluation results show that block placement using content-based signatures is able to reduce the number of PRAM bit programming operations by as much as an order of magnitude. 1
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