fsdax-pmem.md

基于FSDAX模式使用PMEM

FSDAX模式下，PMEM会以文件系统的方式承现，使用者通过使用文件系统中的文件来达到读写PMEM的目的。本文以libpmem来说明在评测环境中如何使用。

文件系统目录

评测环境中有两个FSDAX PMEM的挂载点，分别为/mnt/pmem0和/mnt/pmem1，其文件系统类型为ext4，并且打开了dax选项。

示例代码

#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
#include <libpmem.h>

/* using 1k of pmem for this example */
#define PMEM_LEN 1024

// Maximum length of our buffer
#define MAX_BUF_LEN 30


/****************************
 * This function writes the "Hello..." string to persistent-memory.
 *****************************/
void write_hello_string (char *buf, char *path)
{
	char *pmemaddr;
	size_t mapped_len;
	int is_pmem;
	
	/* create a pmem file and memory map it */
	if ((pmemaddr = (char *)pmem_map_file(path, PMEM_LEN, PMEM_FILE_CREATE,
				0666, &mapped_len, &is_pmem)) == NULL) {
		perror("pmem_map_file");
		exit(1);
	}
	/* store a string to the persistent memory */
	strcpy(pmemaddr, buf);

	/* flush above strcpy to persistence */
	if (is_pmem)
		pmem_persist(pmemaddr, mapped_len);
	else
		pmem_msync(pmemaddr, mapped_len);

	/* output a string from the persistent memory to console */
	printf("\nWrite the (%s) string to persistent memory.\n",pmemaddr);	
			
	return;	
}

/****************************
 * This function reads the "Hello..." string from persistent-memory.
 *****************************/
void read_hello_string(char *path)
{
	char *pmemaddr;
	size_t mapped_len;
	int is_pmem;

		/* open the pmem file to read back the data */
		if ((pmemaddr = (char *)pmem_map_file(path, PMEM_LEN, PMEM_FILE_CREATE,
					0666, &mapped_len, &is_pmem)) == NULL) {
			perror("pmem_map_file");
			exit(1);
		}  	
		/* Reading the string from persistent-memory and write to console */
		printf("\nRead the (%s) string from persistent memory.\n",pmemaddr);

	return;
}

/****************************
 * This main function gather from the command line and call the appropriate
 * functions to perform read and write persistently to memory.
 *****************************/
int main(int argc, char *argv[])
{
	char *path = argv[2];
	
	// Create the string to save to persistent memory
	char buf[MAX_BUF_LEN] = "Hello Persistent Memory!!!";
	
	if (strcmp (argv[1], "-w") == 0) {
	
		write_hello_string (buf, path);
		
	}   else if (strcmp (argv[1], "-r") == 0) {

		read_hello_string(path);
	}	
	else { 
		fprintf(stderr, "Usage: %s <-w/-r> <filename>\n", argv[0]);
		exit(1);
	}
}

本示例代码取自PMDK Example工程，在评测环境，确保filename参数设定的文件路径在/mnt/pmem0或是/mnt/pmem1即可使用PMEM能力。

示例代码中有几个关键点需要注意：

• pmem_map_file进行文件映射，如果映射后的地址区域为PMEM，则is_pmem为true；
• 对于pmem地址，需要使用pmem_persist来保证持久化；
• fsdax模式下，pmem_map_file需要指定映射空间的长度。