关于字节对齐的问题

[不闻窗外事]

/*
 * Structures for I/O stats.
 * These are now dynamically allocated.
 */
struct io_stats {
	/* # of sectors read */
	unsigned long rd_sectors	__attribute__ ((aligned (8)));
	/* # of sectors written */
	unsigned long wr_sectors	__attribute__ ((packed));
	/* # of sectors discarded */
	unsigned long dc_sectors	__attribute__ ((packed));
	/* # of read operations issued to the device */
	unsigned long rd_ios		__attribute__ ((packed));
	/* # of read requests merged */
	unsigned long rd_merges		__attribute__ ((packed));
	/* # of write operations issued to the device */
	unsigned long wr_ios		__attribute__ ((packed));
	/* # of write requests merged */
	unsigned long wr_merges		__attribute__ ((packed));
	/* # of discard operations issued to the device */
	unsigned long dc_ios		__attribute__ ((packed));
	/* # of discard requests merged */
	unsigned long dc_merges		__attribute__ ((packed));
	/* # of flush requests issued to the device */
	unsigned long fl_ios		__attribute__ ((packed));
	/* Time of read requests in queue */
	unsigned int  rd_ticks		__attribute__ ((packed));
	/* Time of write requests in queue */
	unsigned int  wr_ticks		__attribute__ ((packed));
	/* Time of discard requests in queue */
	unsigned int  dc_ticks		__attribute__ ((packed));
	/* Time of flush requests in queue */
	unsigned int  fl_ticks		__attribute__ ((packed));
	/* # of I/Os in progress */
	unsigned int  ios_pgr		__attribute__ ((packed));
	/* # of ticks total (for this device) for I/O */
	unsigned int  tot_ticks		__attribute__ ((packed));
	/* # of ticks requests spent in queue */
	unsigned int  rq_ticks		__attribute__ ((packed));
};

看代码的时候看到这种结构体，我不太明白这能起到什么作用，为什么要在这里设置字节对齐，以及在这个结构体中这个设置起到的作用是什么？？ 有什么特殊的意义吗。。。 平时写程序使用结构体基本上没有用过这种字节对齐，不太理解

[注视着]

应该是为了方便移植的吧。

[帅得不敢出门]

多平台数据交互， 比如windows上一个程序用这个结构与另一linux上的程序交互此结构，如果不指定，双方结构位置有可能不是一一对应。

[moquan1100]

你这个是IO结构体，request在文件系统中，在page cache中都是4KB, 8个sectors。