iostat -x -d
can display many i/o statistic info. For util of iostat, the explanation is :
Percentage of CPU time during which I/O requests were issued to the device (band-width utilization for the device). Device saturation occurs when this value is close to 100%
I want to know how the util was computed?
I make an experiment, (see following code), start 40 thread to randomly read 40 files. I suppose the disk util should be very high, but I am wrong, the iostat is as follow, anyone can give why? THX
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sdb1 0.01 0.44 0.24 0.57 3.44 8.14 14.34 0.00 2.28 0.66 0.05
Code:
#include <iostream>
#include <fstream>
#include <pthread.h>
using namespace std;
void* work(void* a)
{
int* id = (int*)a;
string file = "sys.partition";
char buf[100];
sprintf(buf, "%d", *id);
file.append(string(buf));
ifstream in(file.c_str());
in.seekg(0, ios_base::end);
size_t len = in.tellg();
cout << "open file : " << file << " , " << len << endl;
srand(time(NULL));
while(true)
{
size_t pos = rand() % len;
in.seekg(pos);
//cout << pos << endl;
in.read(buf, 10);
system("sync");
}
in.close();
}
int main(int argc, char** argv)
{
static const int num = 40;
pthread_t threads[num];
for (int i = 0; i < num; i++) {
pthread_create(&threads[i], NULL, work, &i);
}
for (int i = 0; i < num; i++) {
pthread_join(threads[i], NULL);
}
return 0;
}