IDS Forum

Re: Ontape/logical logs

Posted By: Martin Fuerderer
Date: Thursday, 30 September 2004, at 5:54 a.m.

In Response To: Ontape/logical logs (Peter Skipworth)

Hi,

no, you are not. You can let the running "ontape -r" command apply
as many logs as you want (or have), but once it has finished there is
no way to start applying logs again.
The reason for this is, at the point where you tell the "ontape -r"
command that you have no more logical logs, the database is
(at least generally) in an inconsistent state. That means there will be
some open transactions. With no more logs available (as you told
the "ontape -r" that there are no more logs), there is "no hope" that
these transactions will ever be committed. To avoid that these
transactions stay open for the remaining eternety, they are rolled
back. This is the so-called "transaction clean-up phase" of the restore.
Now the database is logically consistent.
Applying new log files would mean that you would want to continue
where you were before the "transaction clean-up", i.e. you would want
to continue those open transactions. But as they have already been
rolled back, there's no way to continue them. Therefore it's not
possible to apply more logical logs after a restore has finished.

You can split the restore into the physical restore ("ontape -p")
followed by the logical restore ("ontape -l") but the same applies,
as soon as you have finished that "ontape -l" command there was
a "transaction clean-up" and no return to log applying is possible.

Having said all this, it becomes clear that Norma Jean Sebastian
is absolutely correct (as always :) , because HDR (High-availability
Data Replication) is doing exactly what you are trying to achieve
manually. It keeps the secondary server (the slave) in this "log
recovery phase", i.e. after a physical restore (and possibly restoring
some logical logs from tape) it will not do the "transaction clean-up".
Instead it will always wait for new logical log records to arrive so that
they can be applied. This is done automatically, i.e. the primary (master)
and secondary server "talk to each other" to figure out, when log
records are to be transfered from primary to secondary and applied
there. This is ongoing at the discretion of transactions, rather than
a whole log file at the time it becomes full. So it gives you a hot
standby that is much more up-to-date. It also gives you the
added benefit that you can still use this secondary server for
querying data (obviously you can't modify data on the slave).

There's more benefit to gain, so I recommend reading the
manual "Administrator's Guide", especially the chapter
about "High-Availability Data Replication".

Regards,
Martin
--
Martin Fuerderer
IBM Informix Development Munich
Data Management Solutions

forum.subscriber@iiug.org wrote on 30.09.2004 02:21:09:

> Hi all,
>
> I'm reasonably new when it comes to Informix, so I hope this isn't an
often-
> asked question. I've set up a disaster recovery server for our main
Informix
> server, to which I automatically send my level 0/level 1 backups and
logical
> logs, as they're written to disk. I've performed a restore with 'ontape
-r'
> successfully, but have a question. Once I finish performing the ontape
> command, am I able to apply each logical log in turn, as it's sent to
the
> backup server, to bring it up to date ? I thought 'ontape -l' might be
what
> I
> needed, but it doesn't seem to work...
>
> I realise 'ontape -r' is able to apply logical logs, but I'd obviously
> prefer
> not to have to restore the entire database if we ever have a severe
crash.
>
> Any help appreciated,
>
> Cheers,
>
> Peter

Messages In This Thread

• Ontape/logical logs
  Peter Skipworth -- Wednesday, 29 September 2004, at 8:21 p.m.
  • RE: Ontape/logical logs
    Sebastian, Norma J. -- Wednesday, 29 September 2004, at 11:37 p.m.
  • Re: Ontape/logical logs
    Martin Fuerderer -- Thursday, 30 September 2004, at 5:54 a.m.
    • RE: Ontape/logical logs
      Sebastian, Norma J. -- Thursday, 30 September 2004, at 9:34 a.m.

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