Abstract
Log-structured merge (LSM) trees typically employ Bloom Filters (BFs) to prevent unnecessary disk accesses for point queries. The size of BFs can be tuned to navigate a memory vs. performance tradeoff. State-of-the-art memory allocation strategies use a worst-case model for point lookup cost to derive a closed-form solution. However, existing approaches have three limitations: (1) the number of key-value pairs to be ingested must be known a priori, (2) the closed-form solution only works for a perfectly shaped LSM tree, and (3) the model assumes a uniform query distribution. Due to these limitations, the available memory budget for BFs is sub-optimally utilized, especially when the system is under memory pressure (i.e., less than 7 bits per key).
In this paper, we design Mnemosyne, a BF reallocation framework for evolving LSM trees that does not require prior workload knowledge. We use a more general query cost model that considers the access pattern per file, and we find that no system accurately maintains access statistics per file, and that simply maintaining a counter per file significantly deviates from the ground truth for evolving LSM trees. To address this, we propose Merlin, a dynamic sliding-window-based tracking mechanism that accurately captures these statistics. The upgraded Mnemosyne^+ combines Merlin with our new cost model. In our evaluation, Mnemosyne reduces query latency by up to 20% compared to RocksDB under memory pressure, and Mnemosyne^+ further improves throughput by another 10% when workloads exhibit higher skew.