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#define _GNU_SOURCE
/* What each way of making a small commit durable costs on one device: an
* 8 KiB commit plus a 64-byte head, 40 times per way. The ways run
* round-robin so device drift reaches all of them alike.
* probe DIRECTORY -> one JSON object per way (milliseconds). */
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
static double now_ms(void) {
struct timespec t;
clock_gettime(CLOCK_MONOTONIC, &t);
return t.tv_sec * 1e3 + t.tv_nsec / 1e6;
}
static int order(const void *a, const void *b) {
double x = *(const double *)a, y = *(const double *)b;
return x < y ? -1 : x > y;
}
enum { WAYS = 5, ROUNDS = 40, REGION = 8192 };
static const char *names[WAYS] = {
"append, fsync, head, fsync (two syncs)",
"append, head, fdatasync",
"append, head, fsync",
"overwrite written zeros, head, fdatasync",
"overwrite written zeros, head, fsync",
};
int main(int argc, char **argv) {
const char *directory = argc > 1 ? argv[1] : "/var/tmp";
char path[WAYS][512];
int fd[WAYS];
off_t end[WAYS];
static char region[REGION], head[64];
memset(region, 'r', sizeof(region));
for (int w = 0; w < WAYS; ++w) {
snprintf(path[w], sizeof(path[w]), "%s/pure-sync-probe-%d", directory, w);
fd[w] = open(path[w], O_RDWR | O_CREAT | O_TRUNC, 0600);
if (fd[w] < 0) return 1;
end[w] = 4096;
if (w >= 3) {
size_t bytes = (size_t)(ROUNDS + 2) * REGION + 4096;
char *zeros = calloc(1, bytes);
if (!zeros || pwrite(fd[w], zeros, bytes, 0) != (ssize_t)bytes) return 1;
free(zeros);
} else if (pwrite(fd[w], region, 4096, 0) != 4096) return 1;
fsync(fd[w]);
}
double t[WAYS][ROUNDS];
for (int r = 0; r < ROUNDS; ++r)
for (int w = 0; w < WAYS; ++w) {
head[0] = (char)r;
double start = now_ms();
if (pwrite(fd[w], region, REGION, end[w]) != REGION) return 1;
end[w] += REGION;
if (w == 0) fsync(fd[w]);
if (pwrite(fd[w], head, 64, (r % 2) * 64) != 64) return 1;
if (w == 1 || w == 3) fdatasync(fd[w]); else fsync(fd[w]);
t[w][r] = now_ms() - start;
}
for (int w = 0; w < WAYS; ++w) {
qsort(t[w], ROUNDS, sizeof(double), order);
printf("{\"way\":\"%s\",\"p50_ms\":%.2f,\"p90_ms\":%.2f,\"max_ms\":%.2f}\n", names[w], t[w][ROUNDS / 2],
t[w][ROUNDS * 9 / 10], t[w][ROUNDS - 1]);
close(fd[w]);
unlink(path[w]);
}
return 0;
}
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