IDS Forum

Re: question about mirroring logical logs

Posted By: ART KAGEL, BLOOMBERG/ 499 PARK
Date: Friday, 23 April 2004, at 9:13 a.m.

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

Jim,

Here's my take and suggestions:

OK, so the schene is: all of your dbspaces are on the same structure which is
RAID5 AAAAAAAHHHHHHHH!!!!! NO RAID5! NO RAID5! NO RAID5! NO RAID5!

OK, I'm better now...Had to get that our of my system. I'll comment on the
initial setup later.

But, you want to use a std mirror of the logical log dbspace to another disk
structure which would presumably not crash together with this array. Have I got
that right?

> - 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.

Assuming the mirror is broken, ahh but the primary was part of the crashed
array. OK, I was confused for a sec...

> - 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.

If you just put the logical log dbspace and the root dbspace on a COMPLETELY
separate mirrored pair separate from the main RAID containing the data dbspaces,
then the whole recovery becomes trivial! Since the rootdb did not crash and
the logs did not crash, when you lose the main array you can just perform a WARM
restore of the lost dbspaces and the logical logs will be fine! You would only
have a major problem then if both of the mirror drives containing the rootdb
and logical log dbspace were to crash together. As I point out in my NO RAID5
Rant (reposted on CDI just this week!) if you build that mirror from drives with
different manufacturers lot numbers the probablility of that happening is
vanishingly small!

> - 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.

PLEASE PLEASE go to the CDI archive on the IIUG web site and search for "RAID5
Rant", read my periodic posting, and take it to heart. NO RAID5! NO RAID5!
NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO
RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO RAID5! NO
RAID5! For more ammunition see the BAARF (Battle Against Any RAID Five...) web
site (www.baarf.com). I do not agree with them that RAID3 and RAID4 are as bad
as RAID5, but I do believe that RAID10 is the way to go for ALL arrays and
especially for database systems.

Art S. Kagel

Art S. Kagel

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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