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Change to use the more common MTTDL model to predict failures

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This commit is contained in:
Andrea Mazzoleni 2015-01-19 22:10:58 +01:00
parent e3f9a7a866
commit d1363b02f3
1 changed files with 24 additions and 10 deletions
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34
cmdline/device.c
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@ -217,21 +217,35 @@ static double poisson_prob_n_or_more_failures(double rate, unsigned n)
*/
static double raid_prob_of_one_or_more_failures(double array_failure_rate, double replace_rate, unsigned n, unsigned redundancy)
{
(void)n;
unsigned i;
double MTBF;
double MTTR;
double MTTDL;
double raid_failure_rate;
/*
* To compute the RAID probability of failure
* we scale the ARRAY failure rate considering the joint
* probability that after the first failure,
* you need also extra failures during the replace time
* to make the RAID to fail.
* Use the MTTDL model (Mean Time To Data Loss) to estimate the
* failure rate of the array.
*
* See:
* Garth Alan Gibson, "Redundant Disk Arrays: Reliable, Parallel Secondary Storage", 1990
*/
/* probability of the extra 1 failure in the replace time */
double prob_next_failures = poisson_prob_n_or_more_failures(array_failure_rate / replace_rate, redundancy);
/* get the Mean Time Between Failure of a single disk */
/* from the array failure rate */
MTBF = n / array_failure_rate;
/* scaled rate for RAID */
double raid_failure_rate = array_failure_rate * prob_next_failures;
/* get the Mean Time Between Repair (the time that a failed disk is replaced) */
/* from the repair rate */
MTTR = 1.0 / replace_rate;
/* use the approximated MTTDL equation */
MTTDL = pow(MTBF, redundancy + 1) / pow(MTTR, redundancy);
for (i = 0; i < redundancy + 1; ++i)
MTTDL /= n - i;
/* the raid failure rate is just the inverse of the MTTDL */
raid_failure_rate = 1.0 / MTTDL;
/* probability of at least one RAID failure */
/* note that is almost equal at the probabilty of */