Hi,
The code below may be useful for doing simple testing of the O_SYNC
and f[data]sync code in the kernel. It times various combinations of
updates-in-place and appends under various synchronisation mechanisms,
making it possible to see clearly whether fdatasync is skipping inode
updates for updates-in-place, for example.
--Stephen
synctest.c:
#include <assert.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>
#include <signal.h>
#include <sys/fcntl.h>
#include <sys/stat.h>
#include <sys/vfs.h>
#include <sys/resource.h>
int iterations = 1000;
struct timeval last_stopwatch_time;
void stopwatch()
{
struct timeval now;
int delta;
gettimeofday(&now, 0);
delta = (now.tv_sec - last_stopwatch_time.tv_sec) * 1000000 +
(now.tv_usec - last_stopwatch_time.tv_usec);
printf ("Stopwatch: elapsed time %d.%03d seconds\n",
delta / 1000000, (delta / 1000) % 1000);
last_stopwatch_time = now;
}
int main()
{
int fd, syncfd;
int i;
int err;
char c;
fd = open("syncfile", O_CREAT|O_TRUNC|O_RDWR, 0666);
if (fd < 0) {
perror("open");
exit(1);
}
syncfd = open("syncfile", O_RDWR|O_SYNC, 0666);
if (syncfd < 0) {
perror("open");
exit(1);
}
c = -1;
err = pwrite(fd, &c, 1, 0);
if (err != 1) {
perror("pwrite");
exit(1);
}
gettimeofday(&last_stopwatch_time, 0);
/* First pass: do nothing but fsync */
printf ("Performing %d fsyncs:\n", iterations);
for (i=0; i<iterations; i++)
fsync(fd);
printf ("done. \n");
stopwatch();
/* Next pass: do nothing but fdatasync */
printf ("Performing %d fdatasyncs:\n", iterations);
for (i=0; i<iterations; i++)
fdatasync(fd);
printf ("done. \n");
stopwatch();
/* Next pass: do fsync with a file read in each pass to force an
* atime timestamp update */
printf ("Performing %d atime fsyncs:\n", iterations);
for (i=0; i<iterations; i++) {
pread(fd, &c, 1, 0);
fsync(fd);
}
printf ("done. \n");
stopwatch();
/* Next pass: do fdatasync with a file read in each pass to
* force an atime timestamp update */
printf ("Performing %d atime fdatasyncs:\n", iterations);
for (i=0; i<iterations; i++) {
pread(fd, &c, 1, 0);
fdatasync(fd);
}
printf ("done. \n");
stopwatch();
/* Next pass: do fdatasync with a file write in each pass: this
* should do a real sync of the data, but not of the inode. */
printf ("Performing %d write fdatasyncs:\n", iterations);
for (i=0; i<iterations; i++) {
pwrite(fd, &c, 1, 0);
fdatasync(fd);
}
printf ("done. \n");
stopwatch();
/* Next pass: do fdatasync with a file append in each pass: this
* should do a real sync of the data, and should also sync the
* inode for the new i_size. */
printf ("Performing %d append fdatasyncs:\n", iterations);
for (i=0; i<iterations; i++) {
pwrite(fd, &c, 1, i);
fdatasync(fd);
}
printf ("done. \n");
stopwatch();
/* Next pass: do fsync with a file write in each pass: this
* should do a real sync of the data and inode in each round,
* resulting in much head banging. */
printf ("Performing %d write fsyncs:\n", iterations);
for (i=0; i<iterations; i++) {
pwrite(fd, &c, 1, 0);
fsync(fd);
}
printf ("done. \n");
stopwatch();
/* Next pass: do O_SYNC writes. */
printf ("Performing %d O_SYNC writes:\n", iterations);
for (i=0; i<iterations; i++) {
pwrite(syncfd, &c, 1, 0);
}
printf ("done. \n");
stopwatch();
/* Next pass: do O_SYNC appends. */
ftruncate(syncfd, 0);
fsync(syncfd);
printf ("Performing %d O_SYNC appends:\n", iterations);
for (i=0; i<iterations; i++) {
pwrite(syncfd, &c, 1, i);
}
printf ("done. \n");
stopwatch();
return 0;
}