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REPLICATION IN THE HARP FILE SYSTEM

REPLICATION IN THE HARP FILE SYSTEM. B. Liskov, S. Ghemawat, R. Gruber, P. Johnson, L. Shrira, M. Williams MIT. Paper highlights. HARP is a fault-tolerant file system using Uninterruptible power supplies and replication to support non-blocking writes

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REPLICATION IN THE HARP FILE SYSTEM

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  1. REPLICATION IN THE HARP FILE SYSTEM B. Liskov, S. Ghemawat, R. Gruber,P. Johnson, L. Shrira, M. Williams MIT

  2. Paper highlights • HARP is a fault-tolerant file system using • Uninterruptible power supplies and replication to support non-blocking writes • Upgradable witnesses to handle site failures • Focus on key ideas • The paper was not covered in detail

  3. Overall organization (I) • A HARP system consists of three machines • a primary server • a secondary server • a server holding a witness • All three servers are on separate Uninterruptible Power Supplies (UPS)

  4. A B C Overall organization (II) UPS UPS UPS Primary Secondary Witness

  5. The role of the UPS (I) • Most file servers use non-blocking writes • Reply to clients before the data are actually written to disk • Not acceptable for a fault-tolerant file server • Must write data in stable storage before replying to the client • Major drawback is additional delay

  6. The role of the UPS (II) • The HARP system reply to clients as soon as the data have written in the main memories of both its primary and secondary server • Replicating data on two servers protect them from a single software or hardware failure • The two UPS protect them from a power failure

  7. The role of the UPS (III) • HARP uses replication and UPS to implement a stable storage in the main memory of its two servers

  8. The role of the witness • The witness does not do anything as long as both primary and secondary servers are operational • When a failure occurs, the server that can contact the witness becomes the new primary • Ensures the consistency of the data in the presence of both site failures and network partitions

  9. A B C After a failure (I) UPS UPS UPS Primary Secondary Witness

  10. After a failure (II) • Site B can communicate with the witness • It becomes the new primary • HARP cannot operate with a single site • Site C becomes a temporary secondary:witness is said to be promoted • In practice C does not get all replicated data from C • Keeps instead a log of all updates

  11. A B C After a failure (III) UPS UPS UPS Temporary Secondary Old Primary New Primary

  12. After a failure (IV) • When A recovers • A is brought up to date by B and C • A and B act again as primary and secondary servers • Temporary secondary server on C is demoted to its previous status of witness

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