Replication in the harp file system
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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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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

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


Overall organization i

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)


Overall organization ii

A

B

C

Overall organization (II)

UPS

UPS

UPS

Primary

Secondary

Witness


The role of the ups i

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


The role of the ups ii

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


The role of the ups iii

The role of the UPS (III)

  • HARP uses replication and UPS to implement a stable storage in the main memory of its two servers


The role of the witness

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


After a failure i

A

B

C

After a failure (I)

UPS

UPS

UPS

Primary

Secondary

Witness


After a failure ii

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


After a failure iii

A

B

C

After a failure (III)

UPS

UPS

UPS

Temporary

Secondary

Old Primary

New Primary


After a failure iv

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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