MySQL: why undo log is required in the processes of database recovery?












0















Let's imagine that an incident happens when you updates.



start transaction;

update users set gender = 1 where id = 3;



  1. Undo log space is allocated.

  2. Previous values are copied to undo log

  3. Record of undo log modifications are written to redo log.

  4. page is modified in buffer pull.

  5. Record of page modification are written to redo log, and this page is marked as dirty.

  6. then, an incident happen, MySQL crashes!


we restart MySQL, then recover from an incident, there are three steps:




  1. Analysis

  2. Redo, which will restore the state when MySQL crashed.

  3. Undo.


For step3, we are undo uncommitted transactions through undo log in backwards order.



My questions are:




  1. Can we do step3 by undo redo log that caused by uncommitted transaction, rather than apply undo log?


  2. Why undo log is required in the step3?


  3. Is undo log for recover from an crash that happens within recovery?







mysql transaction-log transaction recovery







share|improve this question





























    0















    Let's imagine that an incident happens when you updates.



    start transaction;
    
update users set gender = 1 where id = 3;



    1. Undo log space is allocated.

    2. Previous values are copied to undo log

    3. Record of undo log modifications are written to redo log.

    4. page is modified in buffer pull.

    5. Record of page modification are written to redo log, and this page is marked as dirty.

    6. then, an incident happen, MySQL crashes!


    we restart MySQL, then recover from an incident, there are three steps:




    1. Analysis

    2. Redo, which will restore the state when MySQL crashed.

    3. Undo.


    For step3, we are undo uncommitted transactions through undo log in backwards order.



    My questions are:




    1. Can we do step3 by undo redo log that caused by uncommitted transaction, rather than apply undo log?


    2. Why undo log is required in the step3?


    3. Is undo log for recover from an crash that happens within recovery?







    mysql transaction-log transaction recovery







    share|improve this question



























      0












      0








      0








      Let's imagine that an incident happens when you updates.



      start transaction;
      
update users set gender = 1 where id = 3;



      1. Undo log space is allocated.

      2. Previous values are copied to undo log

      3. Record of undo log modifications are written to redo log.

      4. page is modified in buffer pull.

      5. Record of page modification are written to redo log, and this page is marked as dirty.

      6. then, an incident happen, MySQL crashes!


      we restart MySQL, then recover from an incident, there are three steps:




      1. Analysis

      2. Redo, which will restore the state when MySQL crashed.

      3. Undo.


      For step3, we are undo uncommitted transactions through undo log in backwards order.



      My questions are:




      1. Can we do step3 by undo redo log that caused by uncommitted transaction, rather than apply undo log?


      2. Why undo log is required in the step3?


      3. Is undo log for recover from an crash that happens within recovery?







      mysql transaction-log transaction recovery







      share|improve this question
















      Let's imagine that an incident happens when you updates.



      start transaction;
      
update users set gender = 1 where id = 3;



      1. Undo log space is allocated.

      2. Previous values are copied to undo log

      3. Record of undo log modifications are written to redo log.

      4. page is modified in buffer pull.

      5. Record of page modification are written to redo log, and this page is marked as dirty.

      6. then, an incident happen, MySQL crashes!


      we restart MySQL, then recover from an incident, there are three steps:




      1. Analysis

      2. Redo, which will restore the state when MySQL crashed.

      3. Undo.


      For step3, we are undo uncommitted transactions through undo log in backwards order.



      My questions are:




      1. Can we do step3 by undo redo log that caused by uncommitted transaction, rather than apply undo log?


      2. Why undo log is required in the step3?


      3. Is undo log for recover from an crash that happens within recovery?







      mysql transaction-log transaction recovery




      mysql transaction-log transaction recovery






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited 1 min ago







      Ryan Lv

















      asked 15 mins ago









      Ryan LvRyan Lv

      6318




      6318






















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