## Technical details on the model » History » Version 1

*Jessica Mack, 06/01/2015 08:20 PM *

1 | 1 | Jessica Mack | h1. Technical details on the model |
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3 | 1 | Jessica Mack | h4. Some background about replication and erasure coding |

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5 | 1 | Jessica Mack | _"Erasure codes are a superset of replicated and RAID systems."_ |

6 | 1 | Jessica Mack | The main advantage of erasure codes is that they leverage the statistical stability of large number of components. |

7 | 1 | Jessica Mack | Assuming we have 1 million machines, and 10% are down we can calculate the probability of availability of a block that has 2 replicas using the formula below and we will get two-nines availability: |

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9 | 1 | Jessica Mack | !Screen_Shot_2014-06-27_at_00.15.48.png! |

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11 | 1 | Jessica Mack | Instead, we can use erasure coding with the same storage overhead ratio. For instance, we can use k=32, m=32 (in total we will have 64 blocks). The new formula is: |

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13 | 1 | Jessica Mack | !Screen_Shot_2014-06-27_at_00.54.29.png! |

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15 | 1 | Jessica Mack | This time with obtain over 8-nines availability. Nice, isn't it? |

16 | 1 | Jessica Mack | Source: Weatherspoon, H., Kubiatowicz, J., "Erasure Coding vs. Replication: A quantitative comparison" |

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18 | 1 | Jessica Mack | h4. Rados |

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20 | 1 | Jessica Mack | Both replications methods share assumptions: |

21 | 1 | Jessica Mack | * Parallel I/O recovery operations. |

22 | 1 | Jessica Mack | * Objects are written in the primary OSD of the PG identified by the CRUSH map. The primary daemon contacts other OSDs for replication and recovery purposes. |

23 | 1 | Jessica Mack | * Failure rate is constant and follows a Poisson distribution. |

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26 | 1 | Jessica Mack | h4. State model (v 0.2) |

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28 | 1 | Jessica Mack | !simple_state_model_v0.2.jpg! |