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# 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
X! = X [factorial](https://en.wikipedia.org/wiki/Factorial)
```

# 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](https://en.wikipedia.org/wiki/Permutation) 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 with RF=2:
2(1)/10(9) = 0.0222 = 2.2%
2.2% of data is lost.
```

# Timestamp

October 2020