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Thread: help

  1. #1
    Join Date
    Jun 2012

    Unanswered: help


    i want to understand , what is a Asynchronous I/O and synchronous I/O ?

    anyone is here that can help me

  2. #2
    Join Date
    May 2003
    There are a several definitions of that:

    • Synchronous I/O occurs at the same time as the SQL statement executes if DB2 must go to disk to get the data. Asynchronous I/O is when the data is already in memory (bufferpools), so the actual I/O occurred at some earlier point, such as when the data was requested previously by some other SQL statement.
    • When data is updated (insert/update/delete), the page in the bufferpool is updated, but the page is not written to the tablespace on disk until some later period when DB2 determines it is more efficient to do so, and when it does happen later it is asynchronous with the SQL Commit statement. However, DB2 must write to the transaction recovery log synchronously with the commit statement to make sure DB2 can recover the updated data if DB2 crashes.
    • During certain kinds of SQL queries, typically those that involve table scans, DB2 will do look-ahead reads and put data in the bufferpools before the actual page is actually requested, so that it will already be in memory when needed. This is called sequential prefetch. DB2 does this when it knows that eventually all the pages of the table will need to accessed to satisfy the query.
    M. A. Feldman
    IBM Certified DBA on DB2 for Linux, UNIX, and Windows
    IBM Certified DBA on DB2 for z/OS and OS/390

  3. #3
    Join Date
    Jun 2003
    Toronto, Canada
    Provided Answers: 1
    Quote Originally Posted by masoud.sh28 View Post

    i want to understand , what is a Asynchronous I/O and synchronous I/O ?
    Surely, you're not the only one curious about that.
    "It does not work" is not a valid problem statement.

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