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Availability in Globally Distributed Storage Systems
Daniel Ford, Franc¸ois Labelle, Florentina I. Popovici, Murray Stokely, Van-Anh Truong,Luiz Barroso, Carrie Grimes, and Sean Quinlan Presented By Ala`a Ibrahim
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OUTLINE Markov Model Findings Conclusions Introduction Disks failures
Correlated Failures Fault Tolerance MechanismsMarkov Model of Stripe Availability Markov Model Findings Conclusions
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Data Center
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Data Center Components
Server Components Interconnects Racks Cluster of Racks
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Data Center Components
ALL THESE COMPONENTS CAN FAIL Server Components Interconnects Racks Cluster of Racks
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Cell, Stripe and Chunk Stripe 1 Stripe 2 Stripe 1 Stripe 2
GFS Instance 1 GFS Instance 2 Chunks Chunks Chunks Chunks CELL 2 CELL 1
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Failure Sources Failure Sources Availability
Hardware – Disks, Memory etc. Software – chunk server process Network Interconnect Power Distribution Unit Availability Reasons of unavailable Overloaded Crash or restart Hardware error Automated repair processes
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Disks failures Node restarts Planned machine reboots
Unplanned machine reboots Unknown
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Fault Tolerance Mechanisms
Replication (R = n) ‘n’ identical chunks (replication factor) are placed across storage nodes in different rack/cell/DC Erasure Coding ( RS (n, m)) ‘n’ distinct data blocks and ‘m’ code blocks Can recover utmost ‘m’ blocks from the remaining ‘n-m’ blocks
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Replication Fast Encoding / Decoding Very Space Inefficient 5 replicas
1 Chunk Fast Encoding / Decoding Very Space Inefficient
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Erasure Coding ‘n’ data blocks ‘m’ code blocks Encode ‘n + m’ blocks
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Erasure Coding ‘n’ data blocks ‘m’ code blocks Encode ‘n + m’ blocks
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Erasure Coding Highly Space Efficient Slow Encoding / Decoding
‘n’ data blocks ‘m’ code blocks Encode ‘n’ data blocks ‘n + m’ blocks Decode Highly Space Efficient Slow Encoding / Decoding
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Correlated Failures Failure Domain Failure Burst
Set of machines that simultaneously fails from a common source of failure Failure Burst Sequence of node failures each occurring within a time window ‘w’ of the next Window 120 s
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Correlated Failures… Failure Burst (Window Size)
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Markov Model Chunk placement policy Cell Simulation
trace-based simulation Priority queue
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Markov Chain
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Conclusion The findings provides a feedback for improving
Replication and encoding schemes Recovery rate
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