NVMe SSD Trim在数据安全中的作用

 

Trim在SCSI 里面的同等命令叫做UNMAP，NVMe里面叫Deallocate，文件系统叫做discard。

在SSD存储系统中，定期主动执行trim指令，能够减少因IO触发的写放大，从而提升IO性能。另外，主动trim后，再次读取返回0，这样使client无法读取已经删除的数据，有利于数据安全（隐私），特别是在公有云中，存储系统主动trim是很有必要的。以下是原理解释和代码验证。

NVMe接收到Deallocate Command后，会标记这些数据块，再次读这些数据块对应的LBA 时返回0。
Zeroout ioctl 通过__blkdev_issue_zeroout发送写0命令，在block层，拆分成128KB blocksize，要求偏移量和长度512B对齐。
NVMe：The NVM Express command set has a generic Dataset Management command, for hinting the host's intent to the storage device on a set of block ranges. One of its operations, deallocate performs trim. It also has a Write Zeroes command that provides a deallocate hint and allows the disk to trim and return zeroes.
 参考：https://en.wikipedia.org/wiki/Trim_(computing)

 

 

代码验证：

 

 

#define _GNU_SOURCE
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

#define BLKDISCARD	_IO(0x12,119)
#define OFFSET (0)
#define BUF_SIZE (512<<10)
static int trim_test(int fd, uint64_t start, uint64_t len)
{
	uint64_t r[2];
	r[0] = start;
	r[1] = len;
	return ioctl(fd, BLKDISCARD, &r);
}

int main()
{
	int fd;
	char* dev_name = "/dev/nvme";
	void* buf = NULL;

	fd = open(dev_name, O_RDWR | O_DIRECT);
	if (fd < 0)
	{
		perror("Failed to open device");
		return errno;
	}
	buf = memalign(4096,BUF_SIZE);
	memset(buf, 0, BUF_SIZE);
	strcpy(buf,"ssd trim test");
	strcpy(buf + BUF_SIZE - 4096,"ssd trim test");
	int len = pwrite(fd, buf, BUF_SIZE, OFFSET);
	if (len < 0)
	{
		perror("write device failed");
		return errno;
	}
	memset(buf, 0, BUF_SIZE);
	len = pread(fd, buf, 4096, OFFSET);
	printf("before trim:%s\n", (char*)buf);
	memset(buf, 0, BUF_SIZE);
	len = pread(fd, buf, 4096, OFFSET+BUF_SIZE-4096);
	printf("without trim:%s\n", (char*)buf);
	if (trim_test(fd,OFFSET,4096) < 0)
	{
		perror("failed to trim");
		return errno;
	}
	memset(buf, 0, BUF_SIZE);
	len = pread(fd, buf, 4096, OFFSET);
	printf("after trim:%s\n", (char*)buf);
	memset(buf, 0, BUF_SIZE);
	len = pread(fd, buf, 4096, OFFSET+BUF_SIZE-4096);
	printf("without trim:%s\n", (char*)buf);
	
	return 0;
}

结果在trim之后数据消失，说明能够保护数据安全，也提供了另外一种删除数据的方法：

before trim:ssd trim test
without trim:ssd trim test
after trim:
without trim:ssd trim test

 

https://blog.csdn.net/WEI_GW2012/article/details/80327778