A Primer on Memory Persistency

Description

This book introduces readers to emerging persistent memory (PM) technologies that promise the performance of dynamic random-access memory (DRAM) with the durability of traditional storage media, such as hard disks and solid-state drives (SSDs). Persistent memories (PMs), such as Intel's Optane DC persistent memories, are commercially available today. Unlike traditional storage devices, PMs can be accessed over a byte-addressable load-store interface with access latency that is comparable to DRAM. Unfortunately, existing hardware and software systems are ill-equipped to fully avail the potential of these byte-addressable memory technologies as they have been designed to access traditional storage media over a block-based interface. Several mechanisms have been explored in the research literature over the past decade to design hardware and software systems that provide high-performance access to PMs.Because PMs are durable, they can retain data across failures, such as power failures and program crashes. Upon a failure, recovery mechanisms may inspect PM data, reconstruct state and resume program execution. Correct recovery of data requires that operations to the PM are properly ordered during normal program execution. Memory persistency models define the order in which memory operations are performed at the PM. Much like memory consistency models, memory persistency models may be relaxed to improve application performance. Several proposals have emerged recently to design memory persistency models for hardware and software systems and for high-level programming languages. These proposals differ in several key aspects; they relax PM ordering constraints, introduce varying programmability burden, and introduce differing granularity of failure atomicity for PM operations.This primer provides a detailed overview of the various classes of the memory persistency models, their implementations in hardware, programming languages and software systems proposed in the recent research literature, and the PM ordering techniques employed by modern processors. Cover Copyright Page Title Page Contents Preface Acknowledgments Persistent Memories Introduction Persistent Memory Technologies Intel Optane DC Persistent Memory Performance of Optane DC Persistent Memory Modeling Persistent Memory Systems SYSTEM CONFIGURATIONS Traditional Storage Systems Persistent Memory System Persistent Memory Support in Existing File Systems Persistent Memory File Systems Direct Access to Persistent Memories Data Persistence Ensuring Data Durability Persistence at CPU Caches Persistence at PM Controller Persistence at Persistent Memory Device ARMv8.2 ISA Extensions Ordering Memory Operations to PM Why is Ordering Required? Sources of PM Operation Reordering Applying Correct Memory Order to PM Memory Persistency Models Persistency Models Recovery Observer Strict Persistency Buffered Strict Persistency Model Formalizing the Strict Persistency Model Persist Ordering Examples Hardware Implementations of Strict Persistency Naive Implementation Bulk Persistence Delegated Persistence Drawbacks of Strict Persistency Epoch Persistency Model Strong Persist Atomicity Buffered Epoch Persistency Model Formalizing the Epoch Persistency Model Persist Ordering Examples Hardware Implementations of Epoch Persistency Naive Implementation Buffered Epoch Persistency in Hardware Offline Conflict Resolution Separate Ordering and Durability Barriers Drawbacks of Epoch Persistency Strand Persistency Model Strong Persist Atomicity Formalizing the Strand Persistency Model Persist Ordering Examples Hardware Implementations of Strand Persistency Hardware Mechanisms for Atomic Durability Failure Atomicity Write-Ahead Logging Log Structuring Shadow Paging Failure-Atomic Mechanisms Hardware Undo Logging Naive Implementation Optimized Undo Logging Software-Assisted Hardware Logging Hardware Redo Logging Optimized Redo Logging Durable Hardware Transactions Hardware Checkpointing Mechanisms Coarse-Grained Checkpointing Checkpointing on Power Failure Programming Persistent Memory Systems File Systems Ext4 DAX NOVA File System Splitfs Strata Programming PM Systems Transactional Failure Atomicity Intel’s PMDK libraries Deferred Commit Transactions Failure Atomicity for Outer-Critical Section Failure Atomicity for Synchronization Free Region Testing PM Applications Conclusion Bibliography Authors’ Biographies