IDS Forum

question about mirroring logical logs

Posted By: Jim Cramer
Date: Thursday, 22 April 2004, at 5:10 p.m.

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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