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Re: are redo records always flushed in order?

From: Jeremy Paul Schneider <>
Date: Wed, 25 Apr 2007 22:00:47 -0500
Message-ID: <>

Not sure I can completely answer your question but here's a start.

  1. Are records written sequentially? Although I have no concrete proof at the moment I'm pretty sure the answer is yes. I don't think that Oracle will ever do a "seek" backwards in a redo log; that seems pretty problematic for many reasons.
  2. The most important thing to remember is that when a transaction issues a COMMIT statement, the COMMIT does not return to the client (and is not successful) until the redo data is written to disk.

First, if all eight I/O operations were being written together then TX1 does not succesfully COMMIT until the 8th I/O is complete. However I don't think that it works this way; a COMMIT will always cause an immediate log buffer flush so when TX1 COMMITs it will probably cause an I/O for the first 5 blocks and when TX2 COMMITs it will cause an I/O for the last 3.

Second, all of the transactions will ALWAYS be replayed from the redolog regardless of whether the transaction committed. After instance startup and after redo replay, uncommitted transactions will be rolled back in the background by SMON (or sooner if a session tries to access a block before SMON gets to it). So replaying the redo log is not conditional on whether or not a transaction was "COMMITTED".

For more detailed info check out Julian Dyke's excellent presentation Transaction Internals here:


On 4/25/07, Jessica Mao <> wrote:
> i'm afraid my questions were not well presented. more descriptions:
> quite often single redo write needs to flush multiple redo records
> and/or commit records to redo log. let's say this time LGWR finds in log
> buffer 10 redo records and 1 commit record for TX1, followed by 3 redo
> records and 1 commit record for TX2, and is going to flush them all in
> single redo write.
> 1.) are the records always flushed in the same order as they were
> generated in log buffer? in this case, are TX1's records always flushed
> no later than TX2's? doesn't matter how TX1 / TX2 committed (immediate,
> wait, batch, nowait)
> 2.) assume answer to 1.) is yes. say it takes 8 physical I/O at OS level
> to serve this redo write, first 5 for TX1's records, then 3 for TX2's.
> what happens if instance crashes at I/O #7? all TX1's records are
> already written to the redo log. TX2's are not. during recovery, would
> db discard TX1's records in redo log and rollback TX1 instead replay? if
> rollback, how does db know that TX1 belonged to a failed redo write?
> through on-disk RBA? if replay, then redo write is not atomic.
> thanks! -Jessica
> Jessica Mao wrote, On 4/25/2007 12:37 PM:
> > Dear Gurus,
> >
> > Could you please help me with my questions below?
> >
> > if TX1 starts and commits (no matter what mode: immediate, wait,
> > batch, nowait). after that TX2 starts and commits (whatever mode). are
> > TX1's redo records (including the commit record) always flushed no
> > later than TX2's? is there ANY chance that TX2's records (including
> > the commit record) would be flushed first? i can't think of any but i
> > could be missing something.
> >
> > a related question: (maybe i'm worrying too much) is redo write always
> > atomic? when single redo write size is bigger than max OS I/O write
> > size, 1 redo write may take several physical writes to finish. if
> > there's instance crash in the middle of the several physical writes,
> > is the db able to discard the already written to disk commit records
> > and roll back ALL transactions associated with this redo write? if
> > yes, how does the db achieve that? using on-disk RBA?
> >
> > would the zero-copy redo and private redo strands features introduce
> > new issue on this matter?
> >
> > the DB is 10gR2 (since i mentioned the different commit methods), OS
> > are mainly hp-ux 11, solaris 10, windows xp.
> >
> > thanks a lot!
> >
> > Jessica Mao
> >
> --

Jeremy Schneider
Chicago, IL

Received on Wed Apr 25 2007 - 22:00:47 CDT

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