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The Google File System

The Google File System. (GFS). Introduction. Special Assumptions Consistency Model System Design System Interactions Fault Tolerance (Results). Assumptions. The system will always be broken Files are BIG Large streaming reads / small random reads Large sequential writes (appends)

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The Google File System

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  1. The Google File System (GFS)

  2. Introduction • Special Assumptions • Consistency Model • System Design • System Interactions • Fault Tolerance • (Results)

  3. Assumptions • The system will always be broken • Files are BIG • Large streaming reads / small random reads • Large sequential writes (appends) • Lots of multiple appends • High sustained bandwidth

  4. Consistency Model • Consistent: All readers see same thing. • Defined: You see exactly what you write. • Undefined: Consistent, but might not be exactly as expected.

  5. How do Apps Deal? • Parts of files are inconsistent • Must do some checking of data: • Application level checksums

  6. Single Master Architecture • Good: • Has global knowledge • Can make intelligent placement/replication decisions. • Bad: • Becomes a bottleneck • Must limit it’s involvement in read/write

  7. Architecture • Master • Keeps track of everything • Chunk Servers • Where the data lives • Each chunk is 64MB • On other file systems ~8KB

  8. Let The Master Rule • Namespace Locking • Replica placement • Creation • (Garbage Collection)

  9. Metadata • In Memory • Fast • Limited space • Chunk Locations • No persistent record • Op Log • Every change to metadata

  10. System Interactions • To write: • Ask master for chunk locations (cache) • Push data to all chunks (to a buffer) • Send write request to primary • Primary writes changes (in order received) • Primary forwards to secondaries (in order received) • Secondaries write changes, confirm.

  11. Record Append • Atomic • Allows for multiple writers • May cause inconsistent states between successful appends.

  12. Fault Tolerance • Restore state fast • Copies, Copies, Copies • Checksums for data integrity

  13. Results summary • When you build a file system around the specific applications which use the system, it works well.

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