write is faster whan seek?

[Dmitri Monakhov <dmonakhov@xxxxxxxxxx>]

I've found what any non continious sequence violation  result in significant
pefrormance drawback. I've two types of requests:
1)Ideally sequential  writes:
   for(i=0;i<num;i++) {
       write(fd, chunk, page_size*32);
   } 
   fsync(fd);

2) Sequential writes with dgap for each 32'th page
   for(i=0;i<num;i++) {
       write(fd, chunk, page_size*31);
       lseek(fd, page_size, SEEK_CUR);
   }
   fsync(fd);

I've found what second IO pattern is about twice times slower whan the first 
one regardless to ioscheduler or HW disk. It is not clear to me why this
happen. Is it linux speciffic or general hardware behaviour speciffic.
I've naively expected what disk hardware cat merge several 31-paged
requests in to continious one by filling holes by some sort of dummy activity.
        
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