Re: are redo records always flushed in order?
- From: Jessica Mao <jessica.mao@xxxxxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Wed, 25 Apr 2007 16:06:41 -0700
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
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