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Packet buffers are an essential part of routers. In high-end routers, these buffers need to store a large amount of data at very high speeds. To satisfy these requirements, we need a memory with the the speed of SRAM and the density of DRAM. A typical solution is to use hybrid packet buffers built from a combination of SRAM and DRAM, where the SRAM holds the heads and tails of per-flow packet FIFOs and the DRAM is used for bulk storage. The main challenge then is to minimize the size of the SRAM while providing reasonable performance guarantees. We analyze a commonly used hybrid architecture from a statistical perspective, and investigate how small the SRAM can get if the packet buffer designer is willing to tolerate a certain drop probability. We introduce an analytical model to represent the SRAM buffer occupancy, and derive drop probabilities as a function of SRAM size under a wide range of statistical traffic patterns. By our analysis, we show that, for low drop probability, the required SRAM size is proportional to the number of flows.