At 11:59 07/11/16, Andrew Morton wrote:
>
>I suppose so. Although one wonders what earthly point there is in syncing
>a file's data if we haven't yet written out the metadata which is required
>for locating that data.
>
>IOW, fdatasync() is only useful if the application knows that it is overwriting
>already-instantiated blocks.
>
>In which case it might as well have used fsync(). For ext2-style filesystems,
>anyway.
>
>hm. It needs some thought.
I did a test to measure the file overwriting performance difference between
original fdatasync and one that skips journal flush.
The test program and obtained result is as follows:
Test program source code:
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <time.h>
#define BUFSIZE 8192
#define LOOP 1024*1024
main(void)
{
int i;
int fd;
char buf[BUFSIZE];
time_t t1,t2;
memset(buf,0,BUFSIZE);
fd = open("testfile", O_CREAT|O_RDWR);
if (fd < 0)
perror("cannot open file\n");
for (i = 0; i < LOOP; i++)
write(fd,buf,BUFSIZE);
fsync(fd);
lseek(fd, 0, SEEK_SET);
time(&t1);
for (i = 0; i < LOOP; i++) {
write(fd,buf,BUFSIZE);
fdatasync(fd);
}
time(&t2);
printf("%d sec\n",t2-t1);
}
Result:
2.6.24-rc3:
264 sec
2.6.23-rc3-fdatasync-skips-journal-flush-patched
253 sec
Hardware environment:
Dell Poweredge 850
CPU Pentium D 3GHz
memory 4GB
HDD Maxtor 6L160M0
I got somewhat better result from the patched ext3 skipping journal flush.
Some DBMS such as PostgreSQL can use fdatasync. So I think skipping
journal flush on overwriting leads to performance improvement for
these application.
I am for the notion that skipping metadata writeout unconditionally is wrong,
and "important metadata" such as i_size, block-bitmap etc should be
synched even if fdatasync is issued , but unimportant meta such as
mtime and ctime update can be ignored when a file is overwritten.
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