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fileSystemDesign.md

File metadata and controls

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File system

Scenario

  • Write a file
  • Read a file
  • Use multiple machines to store these files

Storage

How to save a file in one machine

  • Metadata
    • FileInfo
      • Name = dengchao.mp4
      • CreatedTime = 201505031232
      • Size = 2044323
    • Index
      • Block 11 -> diskOffset1
      • Block 12 -> diskOffset2
      • Block 13 -> diskOffset3
  • Block
    • 1 block = 1024 Byte
    • Advantages
      • Error checking
      • Fragmenting the data for storage

How to save a much larger file in one machine

  • Change chunk size
    • 1 chunk = 64M = 64 * 1024K
    • Advantages
      • Reduce size of metadata
    • Disadvantages
      • Waste space for small files

Scale

Architecture style

  • Peer 2 Peer (BitComet, Cassandra)
    • Advantage: No single point of failure
    • Disadvantage: Multiple machines need to negotiate with each other
  • Master slave
    • Advantage: Simple design. Easy to keep data consistent
    • Disadvantage: Master is a single point of failure
  • Final decision
    • Master + slave
    • Restart the single master

How to save an extra large file on several machines

  • One master + many chunk servers

Move chunk offset from master to slaves

  • Master don't record the disk offset of a chunk
    • Advantage: Reduce the size of metadata in master; Reduce the traffic between master and chunk server

Write process

  1. The client divides the file into chunks. Create a chunk index for each chunk
  2. Send (FileName, chunk index) to master and master replies with assigned chunk servers
  3. The client transfer data with the assigned chunk server.

Do not support modification

Read process

  1. The client sends (FileName) to master and receives a chunk list (chunk index, chunk server) from the master
  2. The client connects with different server for reading files

Master task

  • Store metadata for different files
  • Store Map (file name + chunk index -> chunk server)
    • Find corresponding server when reading in data
    • Write to more available chunk server

Failure and recovery

Single master

  • Double master (Apache Hadoop Goes Realtime at Facebook)
  • Multi master (Paxos algorithm)

What if a chunk is broken

  • Check sum 4bytes = 32 bit
  • Each chunk has a checksum
  • Write checksum when writing out a chunk
  • Check checsum when reading in a chunk

Avoid loss of data when chunk server is down

  • Replica: 3 copies
    • Two copies in the same data center but on different racks
    • Third copy in a different data center
  • How to choose chunk servers
    • Find servers which are not busy
    • Find servers with lots of available disk space

How to recover when a chunk is broken

  • Ask master for help

How to find whether a chunk server is down

  • Heart beat message

How to solve client bottleneck

  • Client only writes to a leader chunk server. The leader chunk server is responsible for communicating with other chunk servers.
  • How to select leading slaves

How to solve chunk server failure

  • Ask the client to retry