SQL SERVER数据页checksum校验算法
在SQL SERVER2005以上版本中，数据页默认开启checksum，标识为m_flagBits & 0x200 == True，其值m_tornBits位于页头0x3C，4字节。
其算法概述如下：
读8KB 进BUF
将BUF头部 CHECKSUM的4字节值清0
uint32 checksum = 0 //初始checksum
for i in range(0,15):
//每扇区的初始checksum
overall = 0;
for ii in range(0,127):
//对当前扇区的每个4字节做累加异或
overall = overall ^ BUF;
//对每扇区的checksum进行移位，方法为向左移位15-i位，
//左边移出的15-i位补到最低位。
checksum = checksum ^ rol(overall, 15- i);
return checksum; //Gets checksum
c源码如下：
//***CODE***//
#include <stdio.h>
#include <stdlib.h>
#define seed 15 //Initial seed(for first sector)
#define CHAR_BIT 8
//***PROTOTYPES***//
unsigned int page_checksum(int page_id, unsigned int *ondisk);
unsigned int rol(unsigned int value, unsigned int rotation);
int main(int argc, char *argv[]) {
unsigned int computed_checksum; //Var to retrieve calculated checksum
unsigned int ondisk_checksum; //Var to retrieve checksum on disk
computed_checksum = page_checksum(152, &ondisk_checksum); //page_checksum call to retrieve stored and calculated checksum for page 152
//***PRINTS***//
printf("Calculated checksum: 0x%08x\n", computed_checksum);
printf("On disk checksum: 0x%08x\n", ondisk_checksum);
}
unsigned int page_checksum(int page_id, unsigned int *ondisk)
{
FILE *fileptr;
unsigned int i;
unsigned int j;
unsigned int checksum;
unsigned int overall;
unsigned int *pagebuf; //A pointer to describe 2d array
fileptr = fopen("C:\\Users\\andre\\Desktop\\teste.mdf", "r+b"); //Open dummy data file for binary read
fseek(fileptr, page_id * 8192, SEEK_SET); //Calculate page address on data file and points to it
fread(pagebuf, 4, 2048, fileptr); //Read page buffer
fclose(fileptr);
checksum = 0;
overall = 0;
*ondisk = pagebuf; //This means that torn bits is stored on first sector in 15th element, Internals researches understand this
pagebuf = 0x00000000; //Fill checksum field with zeroes (this field will be discarded in algorithm)
for (i = 0; i < 16; i++) //Loop through sectors
{
overall = 0; //Reset overall sum for sectors
for (j = 0; j < 128; j++) //Loop through elements in sector i
{
overall = overall ^ (unsigned int)pagebuf; //XOR operation between sector i elements
}
checksum = checksum ^ rol(overall, seed - i); //Current checksum is overall for sector i circular shifted by seed (15 - i)
}
return checksum; //Gets checksum
}
unsigned int rol(unsigned int value, unsigned int rotation)
{
return (value) << (rotation) | (value) >> (sizeof(int) * CHAR_BIT - rotation) & ( (1 << rotation) -1);
}
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