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SubjectRe: Where is the performance bottleneck?
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join the party. ;)

8 400GB SATA disk on same Marvel 8 port PCIX-133 card. P4 CPU.
Supermicro SCT board.

# cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [multipath] [raid6]
[raid10] [faulty]
md0 : active raid0 sdh[7] sdg[6] sdf[5] sde[4] sdd[3] sdc[2] sdb[1] sda
[0]
3125690368 blocks 64k chunks

8 DISK RAID0 from same slot and card. Stripe size is 512KB.

run oread

# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- ----
cpu----
r b swpd free buff cache si so bi bo in cs us sy
id wa
1 1 0 533216 330424 11004 0 0 7128 1610 1069 77 0 2
95 3
1 0 0 298464 560828 11004 0 0 230404 0 2595 1389 1
23 0 76
0 1 0 64736 792248 11004 0 0 231420 0 2648 1342 0
26 0 74
1 0 0 8948 848416 9696 0 0 229376 0 2638 1337 0
29 0 71
0 0 0 868896 768 9696 0 0 29696 48 1224 162 0 19
73 8
# time ./oread /dev/md0

real 0m6.595s
user 0m0.004s
sys 0m0.151s

run dd

# vmstat 1
procs -----------memory---------- ---swap-- -----io---- --system-- ----
cpu----
r b swpd free buff cache si so bi bo in cs us sy
id wa
2 2 0 854008 2932 17108 0 0 7355 1606 1071 80 0 2
95 3
0 2 0 848888 3112 21388 0 0 164332 0 2985 3564 2
7 0 91
0 2 0 844024 3260 25664 0 0 164040 0 2990 3665 1
7 0 92
0 2 0 840328 3380 28920 0 0 164272 0 2932 3791 1
9 0 90
0 2 0 836360 3500 32232 0 0 163688 100 3001 5045 2
7 0 91
0 2 0 831432 3644 36612 0 0 164120 568 2977 3843 0
9 0 91
0 1 0 826056 3752 41688 0 0 7872 0 1267 1474 1 3
0 96
# time dd if=/dev/md0 of=/dev/null bs=131072 count=8192
8192+0 records in
8192+0 records out

real 0m4.771s
user 0m0.005s
sys 0m0.973s

so the reasonable thing here is because of O_DIRECT, the sys time
reduced a lot.

but the time is longer! the reason i found is...

i attached a new oread.c which allow to set block size of each read and
total read count. so i read full strip once a time,

# time ./oread /dev/md0 524288 2048

real 0m4.950s
user 0m0.000s
sys 0m0.131s

compared to

# time ./oread /dev/md0 131072 8192

real 0m6.633s
user 0m0.002s
sys 0m0.191s


but still, I can get linear speed at 4 DISKS, then no speed gain when
adding more disk into the RAID.

Ming

#include <stdio.h>
#include <stdlib.h>
#define __USE_GNU
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>

#define ALIGN(buf) (char *) (((unsigned long) (buf) + 4095) & ~(4095))

int main(int argc, char *argv[])
{
char *p;
int fd, i;
int BS, BLOCKS;
if (argc < 4) {
printf("%s: <dev> bs cnt\n", argv[0]);
return 1;
}
BS = atoi(argv[2]);
BLOCKS = atoi(argv[3]);
fd = open(argv[1], O_RDONLY | O_DIRECT);
if (fd == -1) {
perror("open");
return 1;
}
p = ALIGN(malloc(BS + 4095));
for (i = 0; i < BLOCKS; i++) {
int r = read(fd, p, BS);
if (r == BS)
continue;
else {
if (r == -1)
perror("read");
break;
}
}
return 0;
}
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