非易失性内存编程模型NVM Programming Model

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"NVMProgrammingModel_v1.2.pdf" 非易失性内存编程模型（NVM Programming Model, NPM）是SNI（存储网络行业协会）发布的一份技术文档，版本1.2。该文档旨在为支持非易失性内存（Non-Volatile Memory, NVM）的软件提供推荐的行为规范。非易失性内存是一种在断电后仍能保持数据的存储技术，它结合了传统内存的高速读写能力与硬盘的持久性，对现代计算架构带来了革命性的变化。 NVM编程模型的核心目标是确保软件开发者能够充分利用NVM的特性，同时保持系统的稳定性和兼容性。文档中可能涵盖了以下几个关键知识点： 1. **内存模型**：NVM的内存模型描述了如何在程序中访问和管理NVM，包括如何分配、释放和访问NVM空间，以及如何处理并发访问时的同步问题。 2. **持久性模型**：NVM的一个关键特性是其持久性，即数据在断电后依然存在。编程模型将详细解释如何保证数据的持久性，包括事务处理、错误恢复和崩溃一致性。 3. **性能优化**：由于NVM的性能介于DRAM和磁盘之间，文档可能会提供关于如何优化NVM读写速度的策略，例如使用适当的I/O调度算法和缓存策略。 4. **错误处理和容错机制**：在NVM系统中，硬件故障或数据损坏的可能性仍然存在。NPM会定义如何设计健壮的错误处理机制，以确保数据的完整性和系统的可用性。 5. **编程接口和API**：为了方便开发者使用NVM，文档可能会定义一组标准的编程接口和API，这些接口允许开发者安全地操作NVM，而无需深入理解底层硬件细节。 6. **兼容性和向后兼容性**：随着NVM技术的发展，新版本的NVM设备可能会引入新的特性和功能。NPM需要考虑如何保持软件与不同版本NVM设备的兼容性。 7. **案例研究和最佳实践**：文档可能会包含实际应用中的案例研究，以及针对特定场景的最佳实践，帮助开发者理解和应用NVM编程模型。 8. **知识产权和使用许可**：SNI允许个人和企业内部使用这份文档，但要求在复制或引用时尊重其版权，并对SNI进行适当署名。 NVM编程模型对于理解和利用非易失性内存技术至关重要，它为软件开发者提供了指导，帮助他们在应用程序中正确且高效地使用NVM，从而实现高性能、低延迟的数据存储和处理。

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4 Overview of the NVM Programming Model (informative)

4.1 How to read and use this specification

Documentation for I/O programming typically consists of a set of OS-specific Application

Program Interfaces (APIs). API documentation describes the syntax and behavior of the API.

This specification intentionally takes a different approach and describes the behavior of NVM

programming interfaces, but allows the syntax to integrate with similar operating system

interfaces. A recommended approach for using this specification is:

1. Determine which mode applies (read 4.3 NVM programming modes).

2. Refer to the mode section to learn about the functionality provided by the mode and

how it relates to native operating system APIs; the use cases provide examples. The mode

specific section refers to other specification sections that may be of interest to the developer.

3. Determine which mode actions and attributes relate to software objectives.

4. Locate the vendor/OS mapping document (see 5.2) to determine which APIs map to the

actions and attributes.

For an example, a developer wants to update an existing application to utilize persistent

memory hardware. The application is designed to bypass caches to assure key content is

durable across power failures; the developer wants to learn about the persistent memory

programming model. For this example:

1. The NVM programming modes section identifies NVM.PM.FILE mode (see 10

NVM.PM.FILE) as the starting point for application use of persistent memory.

2. The NVM.PM.FILE mode text describes the general approach for accessing PM (similar

to native memory-mapped files) and the role of PM aware file system.

3. The NVM.PM.FILE mode identifies the NVM.PM.FILE.MAP and NVM.PM.FILE.SYNC

actions and attributes that allow an application to discover support for optional features.

4. The operating system vendor’s mapping document describes the mapping between

NVM.PM.FILE.MAP/SYNC and API calls, and also provides information about supported PM-

aware file systems.

4.2 NVM device models

4.2.1 Overview

This section describes device models for NVM to help readers understand how key terms in

the programming model relate to other software and hardware. The models presented here

generally apply across operating systems, file systems, and hardware; but there are

differences across implementations. This specification strives to discuss the model generically,

but mentions key exceptions.

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One of the challenges discussing the software view of NVM is that the same terms are often

used to mean different things. For example, between commonly used management

applications, programming interfaces, and operating system documentation, volume may refer

to a variety of things. Within this specification, NVM volume has a specific meaning.

An NVM volume is a subset of one or more NVM devices, treated by software as a single

logical entity. For the purposes of this specification, a volume is a container of storage. A

volume may be block capable and may be persistent memory capable. The consumer of a

volume sees its content as a set of contiguous addresses, but the unit of access for a volume

differs across different modes and device types. Logical addressability and physical allocation

may be different.

In the examples in this section, “NVM block device” refers to NVM hardware that emulates a

disk and is accessed in software by reading or writing ranges of blocks. “PM device” refers to

NVM hardware that may be accessed via load and store operations.

4.2.2 Block NVM example

Consider a single drive form factor SSD where the entire SSD

capacity is dedicated to a file system. In this case, a single NVM

block volume maps to a single hardware device. A file system

(not depicted) is mounted on the NVM block volume.

The same model may apply to NVM block hardware other than an SDD (including flash on

PCIe cards).

4.2.3 Persistent memory example

This example depicts a NVDIMM and PM volume. A PM-aware

file system (not depicted) would be mounted on the PM volume.

The same model may apply to PM hardware other than an

NVDIMM (including SSDs, PCIe cards, etc.).

4.2.4 NVM block volume using PM hardware

In this example, the persistent memory implementation includes

a driver that uses a range of persistent memory (a PM volume)

and makes it appear to be a block NVM device in the legacy

block stack. This emulated block device could be aggregated or

de-aggregated like legacy block devices. In this example, the

emulated block device is mapped 1-1 to an NVM block volume and non-PM file system.

Note that there are other models for connecting a non-PM file system to PM hardware.

Figure 1 Block NVM example

Figure 2 PM example

Figure 3 Block volume using PM HW

NVM block volume

SSD hardware

NVDIMMs

PM volume

PM hardware

NVM block volume

PM volu me

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4.4 Introduction to actions, attributes, and use cases

4.4.1 Overview

This specification uses four types of elements to describe NVM behavior. Use cases are the

highest order description. They describe complete scenarios that accomplish a goal. Actions

are more specific in that they describe an operation that represents or interacts with NVM.

Attributes comprise information about NVM. Property Group Lists describe groups of related

properties that may be considered attributes of a data structure or class; but the specification

allows flexibility in the implementation.

4.4.2 Use cases

In general, a use case states a goal or trigger and a result. It captures the intent of an

application and describes how actions are used to accomplish that intent. Use cases illustrate

the use of actions and help to validate action definitions. Use cases also describe system

behaviors that are not represented as actions. Each use case includes the following

information:

• a purpose and context including actors involved in the use case;

• triggers and preconditions indicating when a use case applies;

• inputs, outputs, events and actions that occur during the use case;

• references to related materials or concepts including other use cases that use or extend the

use case.

4.4.3 Actions

Actions are defined using the following naming convention:

The actions in this specification all have a context of “NVM”. The mode refers to one of the

NVM models documented herein (or “COMMON” for actions used in multiple modes). The verb

states what the action does. Examples of actions include “NVM.COMMON.GET_ATTRIBUTE”

and “NVM.FILE.ATOMIC_WRITE”. In some cases native actions that are not explicitly

specified by the programming model are referenced to illustrate usage.

The description of each action includes:

• parameters and results of the action

• details of the action’s behavior

• compatibility of the action with pre-existing APIs in the industry

A number of actions involve options that can be specified each time the action is used. The

options are given names that begin with the name of the action and end with a descriptive term

that is unique for the action. Examples include NVM.PM.FILE.MAP_COPY_ON_WRITE and

NVM.PM.FILE.MAP_SHARED.

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