Abstract: With the rapid increasing capacity of flash memory, flash-aware indexing techniques are highly desirable for flash devices. The unique features of flash memory, such as the erase-before-write constraint and the asymmetric read/write cost, severely deteriorate the performance of the traditional B+-tree algorithm. In this paper, we propose an optimized indexing method, called \textitlazy-update B+-tree, to overcome the limitations of flash memory. The basic idea is to defer the committing of update requests to the B+-tree by buffering them in a segment of main memory. They are later committed in groups so that the cost of each write operation can be amortized by a bunch of update requests. We identify a victim selection problem for the \textitlazy-update B+-tree and develop two heuristic-based commit policies to address this problem. Simulation results show that the proposed \textitlazy-update method, along with a well-designed commit policy, greatly improves the update performance of the traditional B+-tree while preserving the query efficiency.