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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 ... – PowerPoint PPT presentation

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Title: REPLICATION IN THE HARP FILE SYSTEM


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
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
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 secondarywitness is
    said to be promoted
  • In practice C does not get all replicated data
    from C
  • Keeps instead a log of all updates

11
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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