ELF: An Efficient LogStructured Flash File System For Micro Sensor Nodes

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ELF An Efficient Log-Structured Flash File
System For Micro Sensor Nodes

• Hui Dai, Michael Neufeld, Richard Han
• Presenter
• Yamuna Krishnamurthy

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Sensor Node Memory Usage
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Existing File Systems
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Flash Memory Characteristics

• Most common storage medium for micro sensors
• Flash is divided into sectors (sector n
  264byte page)
• On-chip cache (264 bytes)
• Concurrent read/writes prohibited
• Flash pages have limited life
• Reliability mechanisms like logging can either
  use Flash or EEPROM as is done for ELFs
  implementation on Mica2 mote

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Sensor Data Types and Their Characteristics

• Sensor Data
• Form major part of the data stored
• Data written sequentially without modifying
  earlier records
• Cleared periodically
• Configuration Data
• Changes infrequently after deployment
• Require high reliability for proper sensor
  functioning
• Binary Program Images
• Dynamic reprogramming of sensor nodes with either
  a complete new binary image or patches
• Store the binary image in flash memory before
  rebooting the system
• Requires high reliability

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ELF Design Goals

• Allow access to flash memory with simple file
  operations like write-modify,write-append,reading,
  creation and deletion
• Extend the operational lifetime of the flash with
  wear leveling
• Achieve a small memory footprint
• Optimize common sensor file operations
• Avoid excessive energy consumption
• Provide optional best-effort data reliability

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ELF File System Architecture

• Resource Abstraction
• Run-time memory
• In-memory representation of open files
• Configuration data like cleaning policy
• General File Operations
• Logical abstraction of file/dir operation
• ELF Maintenance Tasks
• System maintenance tasks like snapshot of dir
  structure and file meta data in EEPROM

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Data Structures in RAM, EEPROM and FLASH
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Flash Data Structures

• Physical Nodes
• Meta-Data
• Unique 16-bit node identifier
• 16-bit version number indicating age
• 32-bit field storing length of the node and
  included data
• Types
• ELF_DIR Represents a dir entry in the file
  system
• ELF_FILE Includes meta-data to operate on file
• ELF_COMMON Represents changes to the file

• Per-Page Meta Data

struct page_info uint16_t crc uint16_t
nextPage11 uint16_t flags5 uint16_t
writeEncounter uint16_t magicNumber
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In-Memory Data Structures

• File Abstraction
• Node Abstraction
• File Descriptor

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In-EEPROM Data Structures

• Cache dir structure for fast file access
• Store system snapshot to enable file consistency
• Allows fast startup or system reboot
• Allows crash recovery

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File Operations in ELF
Open/Create
Append
Modify
Read/Seek
Rename
Delete
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ELF Flash Memory Operations

• Read
• Flash-gtData
• Flash-gtCache-gtData
• Write
• Data-gtCache-gtFlash
• Page contents erased before writing
• Modify
• Page -gtCache-gtModify-gtFlash

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Resource Management

• Bitmap approach to maintain free/dirty used
  blocks
• Garbage Collection
• Cleaner called when number of free pages drop
  below threshold

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Crash Recovery

• Optional
• Checkpoint
• Roll Forward
• Maintains snapshots of current operation
  inodeNum,action,curr_version,highest_version

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Performance Results

• Sequential Read Performance
• Elf reads data directly from flash whereas
  matchbox reads from flash to cache and then reads
  the data

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Performance Results

• M corresponds to maximum write throughput or
  memory consumption of matchbox
• Experiments performed with varying RAM buffer
• E1 and E1 correspond to 0 byte buffer, E2 and
  E2 correspond to 32 byte buffer and so on
• E1 has the whole bitmap stored in RAM whereas E1
  has only first 64 bytes stored in RAM

Sequential Write Throughput
Sequential Write Memory Consumption
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Performance Results

• Random Write Performance

• Random Read Performance

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Performance Results

• Wear Leveling Performance

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Conclusion

• Provides complete,efficient and reliable file
  system for micro sensor nodes
• Uses atomic write operations for operating with
  file metadata
• Support for simple garbage collection
• Optional best effort crash recovery mechanism
• Achieves wear leveling
• Allows random access of data

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Limitations and Future Work

• Cannot handle frequent occurrences of reboot,
  file open and creation
• Loss of reliability due to write buffering
• Energy and latency cost of ensuring reliability
  not explored
• Using compression mechanisms to reduce memory
  usage

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THANK YOU