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WAL Question

From: Harry Yau <harry(at)regaltronic(dot)com>
To: PGH <pgsql-hackers(at)postgresql(dot)org>, PGG <pgsql-general(at)postgresql(dot)org>
Subject: WAL Question
Date: 2001-07-04 01:43:22
Message-ID: 3B4274BA.AB002BFF@regaltronic.com (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackers
Hi Dear:

    I am using Postgres 7.0 on RedHat Linux 6.3 right now. I wanna know
is there transaction log in PostGres 7.0? I know there is WAL in 7.1. I
am also thinking to upgrade to 7.1.2 too. I have read a lot of message
in the PostgreSQL mailing list archive which is about the WAL.
    The following is some conclusion of what I learn from there:

   * checkpoint is made according to the config setting of
     checkpoint_segment and checkpoint_timeout.
   * after a checkpoint has been made, the log flush to the disk drive.
   * after a checkpoint has been made, any log segments writtend before
     the redo record ( the record in the log whick the REDO operation
     begin) are REMOVE to free disk space in WAL directory.
   * seems to me, everything is automatic.

Please, point me out if there anything wrong.
    However, there is still something really confusing me. In PostgreSQL

7.1 Documentation Chapter 9 WAL , 9.2.1 Database Recovery with WAL, it
tell me about the flow of the process to recover from WAL: 1. reads
pg_control, 2. reads checkpoint record, 3. reads redo record, and then
finally 4. REDO operation. I wonder do we need to run all the above
procedures manually, after a crash?? or They will REALLY be
automatically run when re-start the postgres after a crash?? It sounds
like a miracle for me!!

Thank You very Much
Harry Yau







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