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Simplify failure probability computation

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This commit is contained in:
Andrea Mazzoleni 2026-01-20 00:44:54 +01:00
parent ede99cbe80
commit 4aa644c8d9
1 changed files with 11 additions and 38 deletions
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49
cmdline/device.c
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@ -461,40 +461,15 @@ static double smart_afr(uint64_t* smart, const char* model)
}
/**
* Factorial.
* Calculates the probability of at least one failure occurring
* within a year using a Poisson distribution.
*
* @param afr The aggregate Annual Failure Rate (lambda).
* @return The probability as a double between 0 and 1.
*/
static double fact(unsigned n)
static double poisson_prob_at_least_one_failure(double rate)
{
double v = 1;
while (n > 1)
v *= n--;
return v;
}
/**
* Probability of having exactly ::n events in a Poisson
* distribution with rate ::rate in a time unit.
*/
static double poisson_prob_n_failures(double rate, unsigned n)
{
return pow(rate, n) * exp(-rate) / fact(n);
}
/**
* Probability of having ::n or more events in a Poisson
* distribution with rate ::rate in a time unit.
*/
static double poisson_prob_n_or_more_failures(double rate, unsigned n)
{
double p_neg = 0;
unsigned i;
for (i = 0; i < n; ++i)
p_neg += poisson_prob_n_failures(rate, n - 1 - i);
return 1 - p_neg;
return 1.0 - exp(-rate);
}
/**
@ -538,9 +513,7 @@ static double raid_prob_of_one_or_more_failures(double array_failure_rate, doubl
raid_failure_rate = 1.0 / MTTDL;
/* probability of at least one RAID failure */
/* note that is almost equal at the probability of */
/* the first failure. */
return poisson_prob_n_or_more_failures(raid_failure_rate, 1);
return poisson_prob_at_least_one_failure(raid_failure_rate);
}
static void state_info_log(devinfo_t* devinfo)
@ -575,7 +548,7 @@ static void state_smart_log(devinfo_t* devinfo, double afr)
log_tag("attr:%s:%s:flags:%" PRIu64 ":%" PRIx64 "\n", devinfo->file, devinfo->name, devinfo->smart[SMART_FLAGS], devinfo->smart[SMART_FLAGS]);
if (afr != 0)
log_tag("attr:%s:%s:afr:%g:%g\n", devinfo->file, devinfo->name, afr, poisson_prob_n_or_more_failures(afr, 1));
log_tag("attr:%s:%s:afr:%g:%g\n", devinfo->file, devinfo->name, afr, poisson_prob_at_least_one_failure(afr));
for (j = 0; j < 256; ++j)
if (devinfo->smart[j] != SMART_UNASSIGNED)
@ -712,7 +685,7 @@ static void state_smart(struct snapraid_state* state, unsigned n, tommy_list* lo
if (devinfo->parent != 0 || !have_parent)
array_failure_rate += afr;
printf("%4.0f%%", poisson_prob_n_or_more_failures(afr, 1) * 100);
printf("%4.0f%%", poisson_prob_at_least_one_failure(afr) * 100);
}
}
@ -762,7 +735,7 @@ static void state_smart(struct snapraid_state* state, unsigned n, tommy_list* lo
* The probability of one and of at least one failure is computed assuming
* a Poisson distribution with the estimated array failure rate.
*/
p_at_least_one_failure = poisson_prob_n_or_more_failures(array_failure_rate, 1);
p_at_least_one_failure = poisson_prob_at_least_one_failure(array_failure_rate);
printf("Probability that at least one disk is going to fail in the next year is %.0f%%.\n", p_at_least_one_failure * 100);
log_tag("summary:array_failure:%g:%g\n", array_failure_rate, p_at_least_one_failure);