Date: Wed, 28 Jun 2006 01:03:52 +0200 From: Ivan Voras <ivoras@fer.hr> To: Leo Huang <leo.huang.gd@gmail.com> Cc: freebsd-fs@freebsd.org Subject: Re: Is the fsync() fake on FreeBSD6.1? Message-ID: <44A1B958.4030204@fer.hr> In-Reply-To: <a0cd7c070606270032h3a42de6ahf21cd11abedb6400@mail.gmail.com> References: <a0cd7c070606270032h3a42de6ahf21cd11abedb6400@mail.gmail.com>
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Leo Huang wrote:
> The result is followed:
> OS Clients Result(queries per second) TPS(got from
> iostat)
> FreeBSD6.1 50 516.1 about 2000
> Debian3.1 50 49.8 about 200
>
> I know that MySQL uses fsync() to flush both the data and log files at
I tried to see the effects from fsync() with this little program:
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#define BUF_SIZE 512
#define COUNT 50000
int main() {
int fd;
char buf[BUF_SIZE];
int i;
fd = open("test.file", O_CREAT|O_TRUNC|O_WRONLY, 0600);
if (fd < 0) {
printf("cannot open\n");
exit(1);
}
for (i = 0; i < COUNT; i++) {
if (write(fd, buf, BUF_SIZE) != BUF_SIZE) {
printf("error writing\n");
exit(1);
}
if (fsync(fd) < 0) {
printf("error in fsync\n");
exit(1);
}
}
close(fd);
unlink("test.file");
return 0;
But I see strange results with iostat. It shows 16KB transactions, ~2900
tps and 46 MB/s. On the other hand, the program runs for ~36 seconds,
which gives ~1390 tps (this is a single desktop drive). Since 36 seconds
of 46MB/s would result in a file 1.6 GB in size, while it's clearly
50000*512=25MB, iostat is lying.
I think it's a too valuable tool to be lying. For what it's worth, gstat
is also lying in the same way.
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