Understanding Solid State Disk and the Oracle Database Flash Cache (older version)

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Presentation given at the Victorian Oracle User group.

Presentation given at the Victorian Oracle User group.

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  • Apologies, I’m a database type.....Quest is best known for toad, but we also have enterprise monitoring across all levels of the stackIn Melbourne, SQL Navigator + the spotlights. It’s not a complete co-incidence about the star trek theme.
  • The higher logical read rate overwhelms the HDD

Transcript

  • 1. Understanding Solid State Disk and the 11gR2 DB flash cache
    Guy Harrison
    Director R&D, Melbourne
    www.guyharrison.net
  • 2. Introductions
  • 3. Agenda
    Recap of Traditional Disk drive technologies
    Solid State Disk (SSD) technologies
    SSD performance
    SSD economics
    Flash SSD Write IO
    Oracle DB flash cache architecture
    Performance comparisons
  • 4. 5MB HDD circa 1956
  • 5. 28MB HDD - 1961
    1800 RPM
  • 6. Plus ca Change....
  • 7. Moore’s law
    Transistor density doubles every 18 months
    Exponential growth is observed in most electronic components:
    CPU clock speeds
    RAM
    Hard Disk Drive storage density
    But not in mechanical components
    Service time (Seek latency) – limited by actuator arm speed and disk circumference
    Throughput (rotational latency) – limited by speed of rotation, circumference and data density
  • 8. Disk trends 2001-2009
  • 9. SSD to the rescue?
  • 10. Power consumption
  • 11. Economics of SSD
  • 12. Hierarchical storage management
    $/GB
    $/IOP
  • 13. Flash Technology concepts
    Storage Hierarchy:
    Cell: One (SLC) or Two (MLC) bits
    Page: Typically 4K
    Block: Typically 128-512K
    Writes:
    Read and first write require single page IO
    Overwriting a page requires an erase & overwrite of the block
    Wear endurance:
    100,000 cycles for SLC before failure
    5,000 – 10,000 cycles for MLC
  • 14. Flash SSD performance
  • 15. Flash write mitigation algorithms
    • Garbage collection and “cleaning” maintains free blocks for writes.
    • 16. “Write Amplification” measures the efficiency of this algorithm
    “Wear Levelling” ensures that writes are evenly spread across the drive
    The TRIM API allows OS to invoke cleanup on file delete
    Some SSD have extra blocks for write optimization
    You want an SSD with SLC, TRIM (?) and low write amplification
  • 17. Oracle DB flash cache
    Introduced in 11GR2 for OEL and Solaris only
    OEL patch 8974084
    Secondary cache maintained by the DBWR, but only when idle cycles permit
    Architecture is tolerant of poor flash write performance
  • 18. Buffer cache architecture
    Buffer
    cache
    Free
    Buffer
    Waits
    Oracle process
    DBWR
    Database
    files
  • 19. Db Flash Cache Architecture
    Buffer
    cache
    Oracle process
    Flash cache
    DBWR
    Database
    files
  • 20. Configuration
    Create filesystem from flash device
    Apply OEL patch 8974084
    SetDB_FLASH_CACHE_FILEand DB_FLASH_CACHE_SIZE.
    Consider Filesystemio_options=setall
  • 21. Oracle Db flash cache statistics
    http://guyharrison.squarespace.com/storage/flash_insert_stats.sql
  • 22. Flash Cache Efficiency
    http://guyharrison.squarespace.com/storage/flash_time_savings.sql
  • 23. Flash cache Contents
    http://guyharrison.squarespace.com/storage/flashContents.sql
  • 24. Performance: Read only
  • 25. Performance: Read/Write
  • 26. Write complete waits
    Flash cache architecture avoids ‘free buffer waits’ due to flash IO, but write complete waits can still occur on hot blocks.
    Free buffer waits are still likely against the database files, due to high physical read rates created by the flash cache
  • 27. Recommendations
    Consider a hierarchy of storage – memory, flash, disk, tape (don’t wait for SSD “Nirvana”)
    Use SLC flash SSD with low write amplification.
    Consider flash SSD datafiles for small tablespaces especially read only.
    Partitioning a large table might allow more frequently accessed data to reside on flash
    Consider db flash cache to reduce read IO for large tablespaces (OEL/Solaris only).
  • 28. Spotlight
  • 29.
  • 30.
  • 31. References
    Guy Harrison blog (guyharrison.net) postings:
    http://bit.ly/6yKhlh
    http://bit.ly/6mrX3k
    http://bit.ly/dmTFYh
    Kevin Closson:
    http://kevinclosson.wordpress.com/2009/12/15/pardon-me-where-is-that-flash-cache-part-ii/
    General articles on SSD:
    http://www.anandtech.com/storage/showdoc.aspx?i=3631
    http://en.wikipedia.org/wiki/Flash_memory