6.4 Data and File Replication

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6.4 Data and File Replication

• Gang Shen

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Why replicate

• Performance
• Reliability
• Resource sharing
• Network resource saving

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Challenge

• Transparency
• Replication
• Concurrent control
• Failure recovery
• Serialization

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Atomicity

• In database systems, atomicity is one of the ACID
  transaction properties. An atomic transaction is
  a series of database operations which either all
  occur, or all do not occur1.
• All or nothing

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Atomicity

• In DFS (Distributed File System), replicated
  objects (data or file) should follow atomicity
  rules, i.e., all copies should be updated
  (synchronously or asynchronously) or none.

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Goal

• One-copy serializability The effect of
  transactions performed by clients on replicated
  objects should be the same as if they had been
  performed one at a time on a single set of
  objects.2

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Architecture

• FSA , File service agent, client interface
• RM, replica manager, provide replication
  functions 3

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Architecture3
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Read operations 3

• Read-one-primary, FSA only read from a primary
  RM, consistency
• Read-one, FSA may read from any RM, concurrency
• Read-quorum, FSA must read from a quorum of RMs
  to decide the currency of data

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Write Operations3

• Write-one-primary, only write to primary RM,
  primary RM update all other RMs
• Write-all, update to all RMs
• Write-all- available, write to all functioning
  RMs. Faulty RM need to be synched before bring
  online.

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Write Operations

• Write-quorum, update to a predefined quorum of
  RMs
• Write-gossip, update to any RM and lazily
  propagated to other RMs

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Read one primary, write one primary

• Other RMs are backups of primary RM
• No concurrency
• Easy serialized
• Simple to implement
• Achieve one-copy serializability
• Primary RM is performance bottleneck

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Read one, Write all

• Provides concurrency
• Concurrency control protocol needed to ensure
  consistency (serialization)
• Achieve one-copy serializability
• Difficult to implement (there will be failed TM
  to block any updates)

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Read one, Write all available

• Variation of Read one, Write all
• May not guarantee one-copy serializability
• Issue of loss conflict in transactions

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Loss of Conflicts

• Assume to RMs, (a,b), object X,Y replicated to
  both.
• Two transactions
• T1R(X),W(Y),commit
• T2R(Y),W(X),commit

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Loss of Conflict

• If Xa,Yb failed, transaction as follows
• T1R(Xa),(Yb failed),W(Ya),commit
• T2R(Yb),(Xa failed),W(Xb),commit
• There is no conflict since no object is shared.
  Thus loss conflict. This can introduce error.

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Read quorum,Write quorum

• Version number attached to replicated object
• Highest version numbered object is the latest
  object in read.
• Write operation advances version by 1
• Write quorum gt half of all object copies
• Write quorumread quorum gt all object copies

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Gossip Update

• Applicable for frequent read, less update
  situations
• Increased performance
• Typical read one, write gossip
• Use timestamp

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Basic Gossip Update

• Used for overwrite
• Three operations, read, update, gossip arrive
• Read, if TSfsaltTSrm, RM has recent data, return
  it, otherwise wait for gossip, or try other RM
• Update, if TsfsagtTSrm, update. Update TSrm send
  gossip. Otherwise, process based on application,
  perform update or reject
• Gossip arrive, update RM if gossip carries new
  updates.

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Causal Order Gossip Protocol3

• Used for read-modify
• In a fixed RM configuration
• Using vector timestamps
• Using buffer to keep the order

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Windows Server 20034

• Support DFS
• State based, multi master scheduled replication
• Use namespace for transparent file sharing
• Use Remote Differential Compression to propagate
  change only to save bandwidth

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Continued5

• If replication detects a conflict, last update
  wins. No merge files, but copies are kept for
  reference.

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Reference

• 1 Wikipedia http//en.wikipedia.org/wiki/Atomic
  ity
• 2 M. T. HarandiJ. Hou (modified I.
  Gupta)"Transactions with Replication"http//www.
  crhc.uiuc.edu/nhv/428/slides/repl-trans.ppt
• 3 Randy Chow,Theodore Johnson, Distributed
  Operating Systems Algorithms, 1998
• 4 "Overview of the Distributed File System
  Solution in Microsoft Windows Server 2003
  R2"http//technet2.microsoft.com/WindowsServer/en
  /library/d3afe6ee-3083-4950-a093-8ab748651b761033.
  mspx?mfrtrue
• 5 "Distributed File System Replication
  Frequently Asked Questions"http//technet2.micros
  oft.com/WindowsServer/en/library/f9b98a0f-c1ae-4a9
  f-9724-80c679596e6b1033.mspx?mfrtrue