Bonsai Merkle Forests: Efficiently Achieving Crash Consistency in Secure Persistent Memory

ABSTRACTDue to its durability, the security of persistent memory (PM) needs to be ensured. Recent works have identified the requirements for correctly architecting secure PM to achieve crash recoverability. A key performance bottleneck, however, lies in the integrity tree update, which needs to be consistent with the memory persistency model and incurs a very high performance overhead. In this paper, we aim to drastically reduce this performance overhead. First, we propose to leverage a small on-chip non-volatile metadata cache (nvMC) for keeping a small portion of the integrity tree. We show that nvMC cannot be managed like a regular cache due to violating crash recoverability, and hence derive a set of invariants to be satisfied for the nvMC to work properly. Then, we propose the idea of Bonsai Merkle Forests (BMF), which splits an integrity tree into multiple trees, leading to a forest, with the tree roots maintained in the nvMC. We propose and analyze different ways of BMF management. Our experimental results show that our proposed BMF schemes drastically reduce the performance overhead of BMT root updates, from 426% to just 3.5%.