strange raid5

[Ming Zhang <mingz@ele.uri.edu>]

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i created a 32KB chunk size 3 disk raid5. then write this disk with a
small code i wrote. i found that even i write it with 1048756 in unit,
which is multiple of stripe size, it still has a lot of read when seen
from iostat. 

any idea? thanks!

i attached the code for reference.

[root@bakstor2u root]# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6]
[raid10] [faulty]
md0 : active raid5 sdc[2] sdb[1] sda[0]
      781422592 blocks level 5, 32k chunk, algorithm 2 [3/3] [UUU]

unused devices: <none>
[root@bakstor2u root]# ./write /dev/md0 1048576 1000
 1048576Bytes *     1000 :    34.745MB/s


avg-cpu:  %user   %nice    %sys %iowait   %idle
           0.00    0.00   17.17   82.83    0.00

Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read   Blk_wrtn
hda               0.00         0.00         0.00          0          0
hdc               0.00         0.00         0.00          0          0
md0            8791.92         0.00     70335.35          0      69632
sda             605.05       387.88     35143.43        384      34792
sdb             611.11       323.23     35143.43        320      34792
sdc             602.02       387.88     35143.43        384      34792
sdd               0.00         0.00         0.00          0          0
sde               0.00         0.00         0.00          0          0
sdf               0.00         0.00         0.00          0          0
sdg               0.00         0.00         0.00          0          0
sdh               0.00         0.00         0.00          0          0



[-- Attachment #2: write.c --]
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/*
 * I need a small program to write in various mode.
 */
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <fcntl.h>

int main(int argc, char *argv[])
{
	int size, cnt;
	char *buf;
	char *fn;
	int fid, i;
	struct timeval tv1, tv2, ttv1, ttv2;
	double x;
	int sync = 1;
	int timing = 0;
	unsigned long *t;

	if (argc < 4) {
		printf("%s <fn> <strip size> <strip count> [s/a [t]]\n", argv[0]);
		exit(1);
	}
	fn = argv[1];
	size = atoi(argv[2]);
	cnt = atoi(argv[3]);
	if ((argc >=5) && (argv[4][0] == 'a'))
		sync = 0;
	if ((argc >=6) && (argv[5][0] == 't'))
		timing = 1;
	if (timing) {
		t = (unsigned long *)malloc(sizeof(double) * cnt);
		if (!t) {
			printf("fail to get mem for t\n");
			exit(1);
		}
	}
	buf = malloc(size * sizeof(char));
	if (!buf) {
		printf("fail to get memory\n");
		exit(1);
	}
	fid = open(fn, O_CREAT|O_WRONLY|(sync ? O_SYNC : 0), S_IRWXU);
	if (fid == -1) {
		printf("open file fail\n");
		exit(1);
	}
	gettimeofday(&tv1, NULL);
	for (i = 0; i < cnt; i++) {
		if (timing) {
			gettimeofday(&ttv1, NULL);
			write(fid, buf, size);
			gettimeofday(&ttv2, NULL);
			t[i] = (ttv2.tv_sec - ttv1.tv_sec) * 1000000 + ttv2.tv_usec - ttv1.tv_usec;
		} else
			write(fid, buf, size);
	}
	close(fid);
	gettimeofday(&tv2, NULL);
	x = (tv2.tv_sec - tv1.tv_sec) + ((double)(tv2.tv_usec - tv1.tv_usec)) 
		* 0.000001;
	x = ((double)(size * cnt) / 1048576.0) / x;
	printf("%8dBytes * %8d :%10.3fMB/s\n", size, cnt, x);
	if (timing) {
		for (i = 0; i < cnt; i++) {
			printf("%8ld", t[i]);
			if (!(i % 10))
				printf("\n");
		}
		printf("\n");
	}
}