lseek and write question

Hello,

I am going to ask a question regarding
write and lseek. I will provide code at the end of this, but first
some background.

I am trying to identify the cause of some latency in writing to disk.
My user claims that performance is much slower on SAN than on local
disk. The developer provided me a C++ program that performed a write
test that confirmed his suspicions. I modified the code to better
fit
my needs which it does now.

What I found during the test is that fsync is an expensive operation
and will block waiting for a confirmation from the disk device. What
I am trying to understand is the lseek function.

From what I read, it simply moves the pointer in the file descriptor
as directed. When I use this lseek function, writes are faster.

My question is why? When I use the write command, does the pointer
get reset and on each write, it will search for EOF?

This is running Linux sytem.

Thanks in advance:

#include <sys/types.h>
#include <sys/time.h>

#include <errno.h>
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char **argv)
{
        struct timeval start, end;
        double usecs;
        long val;
        int ch, fd, idx, ops, numThreads;
        char *fname= "";
        int filesize = 40000000;
        int bytes = 0;
        bool dosync = true, doSeek=false;

        bytes = 0;
        ops = 0;
        char *buf = new char[bytes];
        fname = argv[1];

        while (( ch = getopt(argc,argv, "b:o:f:sl")) != EOF)
                switch (ch) {
                case 'b' :
                        bytes = atoi(optarg);
                        break;
                case 'o' :
                        ops = atoi(optarg);
                        break;
                case 'f' :
                        fname = (optarg);
                        break;
                case 's' :
                        dosync = false;
                        break;
                case 'l' :
                        doSeek = true;
                        break;
                }
                argc -= optind;
                argv += optind;

        gettimeofday(&start,NULL);

        memset(buf,0,bytes);
        if ( dosync ) {
            printf("Processing %d bytes with %d Operations of fsync :
\t", bytes,ops);
        } else {
            printf("Processing %d bytes with %d Operations of fsync :
\t", bytes,1);
        }

        // unlink(fname);
        if ((fd = open(fname, O_RDWR | O_CREAT, 0666)) == -1)
        {
                int errNum = errno;
                printf("ERROR: failed to open %s: n",fname);
                return(0);
        }

        for ( int idx(0) ; idx < ops ; idx++)
        {

                if (write(fd, buf, bytes) != bytes)
                {
                        printf("write: \n");
                        exit (1);
                }

                if ( dosync ) {
                   if (fsync(fd) != 0)
                   {
                        printf("fsync: \n");
                        exit (1);
                   }
                }
                if ( doSeek )
                {
                        if (lseek(fd, (off_t)0, SEEK_SET) == -1)
                        {
                                printf("lseek: %s\n",
strerror(errno));
                                exit (1);
                        }
                }

        }

        // One last sync

        if (fsync(fd) != 0)
        {
                printf("fsync: \n");
                exit (1);
        }
        gettimeofday(&end,NULL);

        int totalSec = 0;
        long totalUSec = 0;

        if (start.tv_usec > end.tv_usec) {
           end.tv_usec += 1000000;
           end.tv_sec--;
        }

        totalSec = end.tv_sec - start.tv_sec;
        totalUSec = end.tv_usec - start.tv_usec;
        int t = totalSec + (totalUSec / 1000000);

        printf("%ld Hours ",t / ( 60 * 60));
        t %= (60*60);
        printf("%ld Minutes ",t / 60);
        t %= 60;
        printf("%ld.%ld Seconds ",t ,totalUSec);
        printf("%ld.%ld Seconds\n ",totalSec ,totalUSec);
}