on den 31.08.2005 Klokka 08:55 (-0600) skreiv Rob Sims:
> We have noticed when changing from kernel 2.4.23 to 2.6.8 that
> timestamps of files are not changed if opened for a write and nothing is
> written. When using 2.4.23 timestamps are changed. When using a local
> filesystem (reiserfs) with either kernel, timestamps are changed.
> Symptoms vary with the client, not the server. See the script below.
>
> When run on a 2.4.23 machine in an NFS mounted directory, output is
> "Good." When run on a 2.6.8 or 2.6.12-rc4 machine in an NFS directory,
> output is "Error."
>
> Is this a bug? How do we revert to the 2.4/local fs behavior?
This is a consequence of 2.6 NFS clients optimising away unnecessary
truncate calls. Whereas this is correct behaviour for truncate(), it
appears to be incorrect for open(O_TRUNC).
In fact, local filesystems like xfs and ext3 appear to have the opposite
problem: they change ctime if you call ftruncate(0) on the zero-length
file, as the attached test shows.
Cheers,
Trond
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <time.h>
#include <unistd.h>
int main(int argc, char *argv[])
{
struct stat buf1, buf2, buf3;
int fd;
if (argc != 2) {
printf("syntax: %s filename\n", argv[0]);
exit(1);
}
fd = open(argv[1], O_CREAT|O_EXCL|O_WRONLY, 0644);
if (fd == -1) {
perror("open(%s, O_CREAT|O_EXCL|O_WRONLY) failed\n", argv[1]);
exit(1);
}
if (fstat(fd, &buf1) == -1) {
perror("fstat() failed\n");
exit(1);
}
printf("File: %s, st_size = %lu, st_ctime = %s\n", argv[1],
buf1.st_size,
asctime(localtime(&buf1.st_ctime)));
close(fd);
sleep(2);
fd = open(argv[1], O_TRUNC|O_WRONLY);
if (fd == -1) {
perror("open(%s, O_TRUNC|O_WRONLY) failed\n", argv[1]);
exit(1);
}
if (fstat(fd, &buf2) == -1) {
perror("fstat() failed\n");
exit(1);
}
printf("File: %s, st_size = %lu, st_ctime = %s\n", argv[1],
buf2.st_size,
asctime(localtime(&buf2.st_ctime)));
if (buf1.st_ctime == buf2.st_ctime)
printf("Bad behaviour in open(%s, O_TRUNC)!\n", argv[1]);
sleep(2);
if (ftruncate(fd, 0) == -1) {
perror("ftruncate(0) failed\n");
exit(1);
}
if (fstat(fd, &buf3) == -1) {
perror("fstat() failed\n");
exit(1);
}
printf("File: %s, st_size = %lu, st_ctime = %s\n", argv[1],
buf3.st_size,
asctime(localtime(&buf3.st_ctime)));
if (buf2.st_ctime != buf3.st_ctime)
printf("Bad behaviour in ftruncate(0)!\n");
close(fd);
exit(0);
}
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