Trevor Foley wrote:
> Hi Kenneth,
>
> Yes, I was talking about ADSM mirroring.
>
> Trevor
>
> -----Original Message-----
> From: Kenneth Sparks [mailto:ken_x_sparks AT AMOCO DOT COM]
> Sent: Saturday,24 April 1999 1:05
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: Re: DB mirroring vs. RAID
>
> Can I assume this is talking only about the ADSM mirroring and not the
> OS mirroring?
>
> ______________________________ Reply Separator
> _________________________________
> Subject: Re: DB mirroring vs. RAID
> Author: Trevor.Foley (Trevor.Foley AT BANKERSTRUST.COM DOT AU) at unix,mime
> Date: 4/22/99 6:25 PM
>
> Hi Reinhard,
>
> I can confirm absolutely that partial page writes do occur in the real world.
> We
> had to instances of this on one server within a few days. Unfortunately, we
> wer
> en't able to recover. We had the server option MIRRORWRITE LOG set to the
> defaul
> t of PARALLEL at the time, and the ADSM server did not recover as it should
> have
> . If we had had MIRRORWRITE LOG set to SEQUENTIAL, the database would have
> recov
> ered, I believe. And there have been changes made to the recovery logic
> (sorry,
> I can't remember which version) that should enabled the database to recover
> even
> if set to PARALLEL.
>
> regards,
>
> Trevor
>
> -----Original Message-----
> From: Reinhard Mersch [mailto:mersch AT UNI-MUENSTER DOT DE]
> Sent: Thursday,22 April 1999 19:35
> To: ADSM-L AT VM.MARIST DOT EDU
> Subject: Re: DB mirroring vs. RAID
>
> 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
> 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
|
