Good points, Wanda (especially that about the backup ADSM admin), but they
also apply if your DB is mirrored, don't they? Losing two disks
simultaneously hits you when relying on RAID5, but it might (and according
to Murphy _will_) also hit you when mirroring the DB (except you have two
copies).
My question is, whether DB mirroring really is safer than RAID5. There
are situation conceivable, where DB mirroring helps and RAID5 does not
(e.g. partial page writes), but do they really occur in practice? Has
anybody encountered them? If not, we are spending a lot of disk space for
the seldom case that two disks fail simultaneously in a way, that the
mirrored DB can live on. Which is o.k. if you can afford it, and I would
really prefer to stay with mirroring, but I fear I need a stronger argument
to get an additional SSA subsystem ...
Prather, Wanda writes:
> Well, here's my 2 cents worth.
> I certainly agree with Andy Raibeck that the SAFEST and FASTEST solution is
> to mirror the DB.
>
> ****************************************************************************
> ******************
> NOTE!
> Please do NOT take the following as a recommendation to stop mirroring your
> DB!
> Don't even THINK about it unless you REALLY understand your environment,
> REALLY understand ADSM data base recovery, and are willing to TEST your
> procedures!
> ****************************************************************************
> ******************
>
> However, IF you do careful planning, and your environment is SUITABLE, I
> think some sites can manage with one copy of the DB on RAID5.
>
> In our particular environment, I don't mirror the DB (which is on RAID5)
> because
>
> (1) I have mirrored the log on DIFFERENT disk (also RAID5),
> (2) The log is in ROLLFORWARD mode, a full DB backup is run every midnight,
> our "offsite" vault is only 2 buildings away, and
> (3) an outage to recover the DB would not be a big problem for us.
>
> I have been through the recovery drill several times, and I know that I can
> put the DB back to where it crashed by using my last full backup tapes, and
> my log. So the DB data is SAFE. In my case, what I would lose if my data
> base crashed would be TIME, not data, and in THIS PARTICULAR SITE that is an
> acceptable risk, given also the low liklihood of failure in the RAID5 disk.
> We also have very fast tape, and only a 10GB data base
>
> I would NOT consider running without the DB mirror if we had implemented the
> space management component, which would make ANY outage time unacceptable.
>
> Before deciding you can do without a DB mirror, you should consider:
>
> 1) How long would it take you to recover your DB from tape and roll forward
> from the log? Remember to include:
>
> * The time it takes you to figure out what the problem is and remember
> what you need to do about it
> * The time it takes your hardware people or your vendor to get you
> replacement disk.
> * The time it takes to FORMAT new DB space. (Just running a dsmfmt on
> AIX can take HOURS for many GB...)
> * Where are your DB backups stored? If they are offsite, how long
> would it take you to get the tapes back?
> * How fast are your tape drives? How long to actually do the DB
> restore?
>
> 2) What if it happens while you are on vacation? Does your backup ADSM
> admin know what to do?
>
> 3) Are you running the LOG in ROLLFORWARD mode?
>
> 4) Are you running space management (HSM) clients? This makes ANY DB outage
> unacceptable.
>
> 5) How VISIBLE is an ADSM outage in your shop? What are the political
> impacts of having an outage of several hours?
>
> 6) How willing are you (and do you have a suitable test platform) to set up
> and test your DB recovery procedures until you can do it smoothly and
> quickly?
>
> 7) How much is your TIME worth? Disk is cheap.
>
> You should be confident that you can deal with these non-trivial issues
> before you decide to run without the DB mirror.
> All them become non-issues if you run with DB mirrors -- buying disk will be
> the cheapest solution in most cases!
>
>
> > -----Original Message-----
> > From: Reinhard Mersch [SMTP:mersch AT UNI-MUENSTER DOT DE]
> > Sent: Wednesday, April 21, 1999 7:44 AM
> > To: ADSM-L AT VM.MARIST DOT EDU
> > Subject: DB mirroring vs. RAID
> >
> > Hi,
> >
> > again I am struggling with my ever growing ADSM DB, not knowing where to
> > get the additional disk space I need. An option would be, to give up DB
> > mirroring and rely on RAID5 instead.
> >
> > Since RAID5 protects against media/hardware failures, are there other
> > threats, DB mirroring protects against and RAID5 does not? There has been
> > discussion on this in August 98 and Andy Raibeck mentioned partial page
> > writes. But how probable are they? Has anybody out there ever been in a
> > situation where DB mirroring (would have) helped and RAID5 (would have)
> > not?
> >
> > Regards
> >
> > --
> > Reinhard Mersch Westfaelische Wilhelms-Universitaet
> > Zentrum fuer Informationsverarbeitung - ehemals Universitaetsrechenzentrum
> > Roentgenstrasse 9-13, D-48149 Muenster, Germany Tel: +49(251)83-31583
> > E-Mail: mersch AT uni-muenster DOT de Fax:
> > +49(251)83-31653
--
Reinhard Mersch Westfaelische Wilhelms-Universitaet
Reinhard Mersch Westfaelische Wilhelms-Universitaet
Zentrum fuer Informationsverarbeitung - ehemals Universitaetsrechenzentrum
Roentgenstrasse 9-13, D-48149 Muenster, Germany Tel: +49(251)83-31583
E-Mail: mersch AT uni-muenster DOT de Fax:
+49(251)83-31653
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