IDS Forum

Re: question about mirroring logical logs

Posted By: Martin Fuerderer
Date: Friday, 23 April 2004, at 7:22 a.m.

In Response To: question about mirroring logical logs (Jim Cramer)

Hi,

this might be a dumb question ... :

Why don't you utilize the IDS's built in mirroring ?

- put all your logical log files into a dedicated dbspace,
- use IDS's mirroring to mirror all chunks of that dedicated
dbspace to any disk/device you like ...

This will make recovery much easier, without all this hassle
of OS mirroring, etc.

One additional advice:
There's a file named
$INFORMIXDIR/etc/oncfg_<servername>.<servernum>
It is a small text file, containing some information about
dbspaces, chunks, log files. This info is needed/useful when
looking for log files to be salvaged before a restore.

Create a backup of this file on a regular basis, e.g. when you
are doing your nightly archive ...
(You do not need to backup this "continuously", but I'd recommend
to backup the file after "serious administration changes", e.g. newly
mirroring, adding chunks, dbspaces, log files, etc. The info in the
file is
primarily used to locate things on disks/in chunks, not so much to
determine the state of them. So nightly backup of the file should
suffice ...)

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

forum.subscriber@iiug.org wrote on 22.04.2004 23:10:06:

> Hi,
> I have a question about something that I would like to try, and am
> wondering
> if anyone else has tried it and/or knows for sure if it will work or
not.
>
> The goal is to: avoid losing *any* transactions in the logical logs if
the
> database
> disk (a raid) fails. I am using IDS 7.31.
>
> I cannot use ER or HDR to achieve this. That is not an option.
>
> I do onbar nightly, incremental backups of data and transactions logs.
> We have, as most sites go, a fairly low transaction rate. So, I have
> set the size of the log files fairly small so that they tend to fill
> up as fast as possible during the day, between nightly backups, and
> thus get backed up by continuous log backup.
> However, if the disk fails there will always be some log not yet backed
up.
>
> So, I came up with this idea and spoke to IBM Informix IDS Tech Support
> today.
> One person thought this would work. The other thought that it probably
> would
> not but was not sure. Can anyone state definitely whether it will or
will
> not?
>
> Thanks in advance,
> Jim Cramer
> University of Iowa
>
> The plan:
> - use O.S. mirroring (HPUX 11.11 64-bit) to mirror the logical log
chunks
> to
> a separate disk than what the rest of the database is on.
>
> - if the raid with the database fails (it has happened to a couple of
our
> raids), replace it.
>
> - then use the OS Logical Volume Manager to create LVs of the same size
> as what I had on the failed disk.
>
> - bring IDS up empty.
>
> - add the chunks back, with same pathnames (actually symbolic links in
> /dev)
> as what the crashed instance used.
>
> Recover to the raid logical log chunks the data from all transactions
since
> the last continuous logical backup to the point in time when the crash
> happened:
>
> - using the OS mirroring commands, re-establish the mirror between the
> external
> disk, containing logical log contents reflecting everything up to the
crash,
> and then tell the empty half of the logical log mirror, on the raid, to
sync
> to the
> other, external disk, half containing the data.
>
> - do an onbar cold restore, which will try to salvage the logical logs,
> which came
> from the external mirror disk, and back them up before performing the
> restore.
>
> - onbar will then restore the archive and replay the transactions in
the
> backed up,
> salvaged logs which came from the mirror.
>
> One support person felt that it would not work due to some sort of
internal
> consistency
> check failure, probably when trying to replay the salvaged logs. The
other
> person,
> who checked with a 3rd person, thought that it would.
>
> Thanks for reading and for any comments that you have.
>
>
>
>
>
>
>

Messages In This Thread

• question about mirroring logical logs
  Jim Cramer -- Thursday, 22 April 2004, at 5:10 p.m.
  • Re: question about mirroring logical logs
    Martin Fuerderer -- Friday, 23 April 2004, at 7:22 a.m.
  • Re: question about mirroring logical logs
    ART KAGEL, BLOOMBERG/ 499 PARK -- Friday, 23 April 2004, at 9:13 a.m.

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