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SubjectRe: tuning for large files in xfs
 >
> What's your testing methodology?
>

here is my code... pretty simple, opens the file and reads around to a
random block, 32k worth (note, doing a 2k read doesn't make much of a
difference, only marginal, from 6.7ms per seek down to 6.1).

int main (int argc, char* argv[]) {
char buffer[256*1024];
int fd = open(argv[1],O_RDONLY);
struct stat s;
fstat(fd,&s);
s.st_size=s.st_size-(s.st_size%256*1024);
initTimers();
srandom(startSec);
long long currentPos;
int currentSize=32*1024;
int startTime=getTimeMS();
for (int currentRead = 0;currentRead<10000;currentRead++) {
currentPos=random();
currentPos=currentPos*currentSize;
currentPos=currentPos%s.st_size;
if (pread(fd,buffer,currentSize,currentPos) != currentSize)
std::cout<<"couldn't read in"<<std::endl;
}
std::cout << "blocksize of
"<<currentSize<<"took"<<getTimeMS()-startTime<<" ms"<<std::endl;
}

> You can try to measure the amount of seeks going to the disk by using
> iostat, and see if that matches your test program.
>

I used iostat and found exactly what I was expecting: 10,000 rounds x 16
(number of 2k blocks in a 32k read) x 4 (number of 512 blocks per 2k
block) = 640,000 reads, which is what iostat told me. so now my question
remains, if each seek is supposed to average 3.5ms, how come I am seeing
an average of 6-7ms?

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