In addition to the answer from rndgermandude (which is also a reason why accessing a data structure in RAM, even in a world without cache lines, would be faster and more efficient if stored in a linear array rather than a tree), SSDs are themselves a complex data structure mapping data into erasure blocks in an attempt to minimize wear on the flash memory and increase efficiency. If the size of your fragments is smaller than the size of this block, you are going to take a performance penalty, and the size of this block is large: I did a search just now to find an article for you detailing this, and http://codecapsule.com/2014/02/12/coding-for-ssds-part-5-acc... mentions 32 megabytes, and anything below that leads to a second layer of data structures on the SSD designed to map the high-level blocks to low-level blocks and incurs erasure penalties.