Solution for blocking fsync in 0.8

[Jay Kreps]

One issue with using the filesystem for persistence is that the
synchronization in the filesystem is not great. In particular the fsync and
fsyncdata system calls block appends to the file, apparently for the entire
duration of the fsync (which can be quite long). This is documented in some
detail here:
  http://antirez.com/post/fsync-different-thread-useless.html

This is a problem in 0.7 because our definition of a committed message is
one written prior to calling fsync(). This is the only way to guarantee the
message is on disk. We do not hand out any messages to consumers until an
fsync call occurs. The problem is that regardless of whether the fsync is
in a background thread or not it will block any produce requests to the
file. This is buffered a bit in the client since our produce request is
effectively async in 0.7, but it can lead to weird latency spikes
nontheless as this buffering gets filled.

In 0.8 with replication the definition of a committed message changes to
one that is replicated to multiple machines, not necessarily committed to
disk. This is a different kind of guarantee with different strengths and
weaknesses (pro: data can survive destruction of the file system on one
machine, con: you will lose a few messages if you haven't sync'd and the
power goes out). We will likely retain the flush interval and time settings
for those who want fine grained control over flushing, but it is less
relevant.

Unfortunately *any* call to fsync will block appends even in a background
thread so how can we give control over physical disk persistence without
introducing high latency for the producer? The answer is that the linux
pdflush daemon actually does a very similar thing to our flush parameters.
pdflush is a daemon running on every linux machine that controls the
writing of buffered/cached data back to disk. It allows you to control the
percentage of memory filled with dirty pages by giving it either a
percentage of memory, a time out for any dirty page to be written, or a
fixed number of dirty bytes.

The question is, does pdflush block appends? The answer seems to be mostly
no. It locks the page being flushed but not the whole file. The time to
flush one page is actually usually pretty quick (plus I think it may not be
flushing just written pages anyway). I wrote some test code for this and
here are the results:

I modified the code from the link above. Here are the results from my
desktop (Centos Linux 2.6.32).

We run the test writing 1024 bytes every 100 us and flushing every 500 us:

$ ./pdflush-test 1024 100 500
21
4
3
3
9
6
Sync in 20277 us (0), sleeping for 500 us
19819
7
7
8
38
Sync in 19470 us (0), sleeping for 500 us
19048
7
4
3
8
4
Sync in 19405 us (0), sleeping for 500 us
19017
6
6
10
6
Sync in 19410 us (0), sleeping for 500 us
19025
7
7
11
6

$ cat /proc/sys/vm/dirty_writeback_centisecs
100
$ cat /proc/sys/vm/dirty_expire_centisecs
500

Now run the test with the background flush disabled (rarely running):
$ ./pdflush-test 1024 100 5000000000000 > times.txt

I ran this for 298,028 writes. The 99.9th percentile for this test is 17 us
and the max time was 2043 us (2ms).

Here is the test code:

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <pthread.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/time.h>
#include <stdlib.h>

static long long microseconds(void) {
    struct timeval tv;
    long long mst;

    gettimeofday(&tv, NULL);
    mst = ((long long)tv.tv_sec)*1000000;
    mst += tv.tv_usec;
    return mst;
}

void *IOThreadEntryPoint(void *arg) {
    int fd, retval;
    long long start;
    long sleep = (long) arg;

    while(1) {
        usleep(sleep);
        start = microseconds();
        fd = open("/tmp/foo.txt",O_RDONLY);
        retval = fsync(fd);
        close(fd);
        printf("Sync in %lld us (%d), sleeping for %ld us\n",
microseconds()-start, retval, sleep);
    }
    return NULL;
}

int main(int argc, char* argv[]) {
    if(argc != 4) {
      printf("USAGE: %s size write_sleep fsync_sleep\n", argv[0]);
      exit(1);
    }

    pthread_t thread;
    int fd = open("/tmp/foo.txt",O_WRONLY|O_CREAT,0644);
    long long start;
    long long ellapsed;
    int size = atoi(argv[1]);
    long write_sleep = atol(argv[2]);
    long fsync_sleep = atol(argv[3]);
    char buff[size];

    pthread_create(&thread,NULL,IOThreadEntryPoint, (void*) fsync_sleep);

    while(1) {
        start = microseconds();
        if (write(fd,buff,size) == -1) {
            perror("write");
            exit(1);
        }
        ellapsed = microseconds()-start;
        printf("%lld\n", ellapsed);
        usleep(write_sleep);
    }
    close(fd);
    exit(0);
}

Cheers,

-Jay