Messages in this thread | | | Date | Mon, 22 May 2006 17:40:07 -0700 | From | fitzboy <> | Subject | Re: tuning for large files in xfs |
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> > 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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