# Synopsis:

Amount of data that is not available when M nodes go down

Use the following formulas to estimate the amount of data loss when more than the replication factor number of nodes leave the cluster:

```
Given –
N = number of nodes in cluster
M = number of nodes lost
r = replication factor
```

# Formulas

```
What portion of data is not available?
m! (n-r)!
-—- -——
(m-r)! n!
For Replication Factor of 2, this simplifies to:
m (m-1)
-———
n (n-1)
if r > m no data loss is expected
```

# Explanation

```
The formula is actually the permutations of having all replicas on the m nodes that go down, divided by all the permutations for replicas across all n nodes in the cluster.
```

# Example

```
If two nodes leave a 10 node cluster
2(1)/10(9) = 0.0222 = 2.2%
2.2% of data is lost.
```