Why if i write to a raw hard disk (without FS) the kernel also makes reads.
$ sudo dd if=/dev/zero of=/dev/sda bs=32k count=1 oflag=direct status=none $ iostat -xc 1 /dev/sda | grep -E "Device|sda" Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util sda 45,54 0,99 1053,47 31,68 0,00 0,00 0,00 0,00 1,17 3071,00 3,04 23,13 32,00 66,38 308,91 Is it readahead?
Instead of dd i wrote a c program that does the same, i even used posix_fadvise to hint the kernel that i do not want read ahead.
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <unistd.h> #include <stdlib.h> #include <stdio.h> #define _GNU_SOURCE #define BLOCKSIZE 512 #define COUNT 32768 int main(void) { // read COUNT bytes from /dev/zero int fd; mode_t mode = O_RDONLY; char *filename = "/dev/zero"; fd = openat(AT_FDCWD, filename, mode); if (fd < 0) { perror("Creation error"); exit (1); } void *pbuf; posix_memalign(&pbuf, BLOCKSIZE, COUNT); size_t a; a = COUNT; ssize_t ret; ret = read(fd, pbuf, a); if (ret < 0) { perror("read error"); exit (1); } close (fd); // write COUNT bytes to /dev/sda int f = open("/dev/sda", O_WRONLY|__O_DIRECT); ret = posix_fadvise (f, 0, COUNT, POSIX_FADV_NOREUSE); if (ret < 0) perror ("posix_fadvise"); ret = write(f, pbuf, COUNT); if (ret < 0) { perror("write error"); exit (1); } close(f); free(pbuf); return 0; } But the result is the same
$ iostat -xc 1 /dev/sda | grep -E "Device|sda" Device r/s w/s rkB/s wkB/s r_await w_await aqu-sz rareq-sz wareq-sz svctm %util sda 46,00 1,00 1064,00 32,00 10,78 1,00 0,43 23,13 32,00 10,55 49,60 It does not matter if it is a spindel disk or ssd , the result is the same.
Also tried different kernels.
