From 6852f299ae377a336900e115489949f0ca42655d Mon Sep 17 00:00:00 2001
From: Vitaliy Filippov <vitalif@yourcmc.ru>
Date: Thu, 24 Sep 2020 23:14:22 +0300
Subject: [PATCH] Add functions to calculate AFR for a cluster

---
 README.md  |  2 +-
 mon/afr.js | 57 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 mon/afr.js

diff --git a/README.md b/README.md
index ec109c0e..a91e893b 100644
--- a/README.md
+++ b/README.md
@@ -200,7 +200,7 @@ Hardware configuration: 4 nodes, each with:
 - 6x SATA SSD Intel D3-4510 3.84 TB
 - 2x Xeon Gold 6242 (16 cores @ 2.8 GHz)
 - 384 GB RAM
-- 1x 25 GbE network interface (Mellanox ConnectX-4 LX)
+- 1x 25 GbE network interface (Mellanox ConnectX-4 LX), connected to a Juniper QFX5200 switch
 
 CPU powersaving was disabled. Both Vitastor and Ceph were configured with 2 OSDs per 1 SSD.
 
diff --git a/mon/afr.js b/mon/afr.js
new file mode 100644
index 00000000..bf8551e4
--- /dev/null
+++ b/mon/afr.js
@@ -0,0 +1,57 @@
+// Functions to calculate Annualized Failure Rate of your cluster
+// if you know AFR of your drives, number of drives, expected rebalance time
+// and replication factor
+// License: VNPL-1.0 (see README.md for details)
+
+module.exports = {
+    cluster_afr,
+    failure_rate,
+    c_n_k,
+};
+
+console.log(100*cluster_afr(100, 0.03, 10, 3), '%');
+console.log(100*cluster_afr(1000, 0.03, 1, 3), '%');
+console.log(100*cluster_afr(5, 0.1, 1, 2), '%');
+console.log(100*cluster_afr(14, 0.01, 1, 2), '%');
+console.log(100*cluster_afr(100, 0.03, 1, 2), '%');
+
+// Estimate AFR of the cluster (not taking failure domains into account)
+// n - number of drives
+// afr - annualized failure rate of a single drive
+// l - expected rebalance time in days after a single drive failure
+// k - replication factor / number of drives that must fail at the same time for the cluster to fail
+function cluster_afr(n, afr, l, k)
+{
+    let p = 0;
+    for (let i = 0; i < n-(k-1); i++)
+    {
+        p += afr * (1-afr)**i * failure_rate(n-i-1, afr*l/365, k-1);
+    }
+    return p;
+}
+
+// Probability of at least <f> failures in a cluster with <n> drives with AFR=<a>
+function failure_rate(n, a, f)
+{
+    if (f <= 0)
+    {
+        return (1-a)**n;
+    }
+    let p = 1;
+    for (let i = 0; i < f; i++)
+    {
+        p -= c_n_k(n, i) * (1-a)**(n-i) * a**i;
+    }
+    return p;
+}
+
+// Combination count
+function c_n_k(n, k)
+{
+    let r = 1;
+    for (let i = 0; i < k; i++)
+    {
+        r *= (n-i) / (i+1);
+    }
+    return r;
+}