Re: ORA-01555: snapshot too old

Oh go on, I'll add my 2 cents'-worth...

Dave's right in saying that increasing minextents will help cure 1555's. But that's frankly a daft way to increase the size of rollback segments (well, OK, not daft, but not entirely wholesome, either).

If you're using dictionary managed tablespace, then no segment should (ideally) have more than around half a dozen extents or so, and that goes for rollback segments, too.

Why not simply recreate your rollback segments with the same number of extents, but make the extent sizes bigger? In other words, play around with INITIAL and NEXT, not MINEXTENTS.

Regards
HJR "Dave Wotton" <Dave.Wotton_at_dwotton.nospam.clara.co.uk> wrote in message news:8Qa57.85596$Do6.3950598_at_nnrp4.clara.net...
>
> "Vincent Ventrone" <vav_at_brandeis.edu> wrote in message
 news:9ivkms$qni$1_at_new-news.cc.brandeis.edu...
> >>"Guang Mei" <gmei_at_proteome.com> wrote in message
 news:kcI47.30B6.4620_at_news.shore.net...
> >> We have a cron job every night to exp some schemas(oracle 8.05 on Sun).
 We
> >> got the following error during last night's exp for the first time:
> >>
> >> EXP-00008: ORACLE error 1555 encountered
> >>
> > I'm figuring that you did, indeed, have active transactions going on in
 the
> > database during the export & one or more of these transactions issued a
> > COMMIT before the export ended. When a transaction COMMITs, Oracle makes
 the
> > undo records available to be overwritten by other transactions even
 though
> > it also considers these undo records to be "inactive, in-use" if some
 other
> > operation needs them for read-consistency. The text of the error
 message --
> > "rollback segment too small" -- is misleading. Once a transaction
 commits,
> > the space it was using in the rollback segment is now up for grabs --
 size
> > is not the issue. n other words it's a scheduling problem. You have
 two
> > options I think:
> >
> > 1. Find a different time for the export or figure out some way to
 ensure
> > that it has exclusive use of the database (ould be hard to do.)
> >
> > 2. Run the export with the parameter CONSISTENT=Y. This parameter sets
 a
> > consistency point for the *entire* export operation and it will then
 have
> > its own undo records to maintain consistency as of the begining of the
> > export operation regardless of what else is going on in the database. I
> > haven't tested this proposition myself, but I think this would solve
 your
> > rpoblem. Just make sure that your rollback segments can grow becuase
 the
> > export itself will end up generating a lot of undo if there is a lot of
 DML
> > going on while it is running, since the entire export operation is now
 one
> > big transaction.
>
> Vincent's explanation of why snapshot too old has occurred is correct, but
> his second recommendation is wrong.
>
> Using the consistent=y option will indeed make the whole of the export
> internally consistent, but it won't avoid the snapshot too old message -
> in fact it will make it more likely. The export is basically only running
> selects, it isn't doing any DML and so doesn't generate any undo records
 to
> maintain consistency - it is relying on the undo records created by other
> DML statements not being overwritten, just as it does when the
 consistent=y
> option is not used. Since consistent=y means the database has to maintain
 a consistent view of the entire database from
> the time the export starts
> (rather than just a consistent view of each table, from the time each
> table is exported), it is much more likely that a required undo record
 will
> be overwritten in that time. Using consistent=y is a good idea,
 particularly
> if you hope to be able to import more than one table from the export and
> have them consistent, but it won't help your snapshot too old problem.
>
> As you know, increasing max-extents for your rollback segments won't help,
> because they're not extending anyway, for the reasons Vincente pointed
 out.
> However, recreating your rollback segments with *min-extents*
 significantly
> bigger *will* help because you are pre-allocating them, so there is more
> space in the rollback segment to use for undo records before they are
> overwritten. If there is sufficient space in the rollback segment to
> accommodate all the undo generated by any DML running during the export,
> no undo records will be overwritten, so no snapshot too old will occur.
>
> Dave.
> --
> If you reply to this newsgroup posting by email, remove the "nospam"
> from my email address first.
>
>
>
>
>
Received on Wed Jul 18 2001 - 23:38:41 CDT