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Re: RAID Newbie question...

From: <gdas_at_my-deja.com>
Date: Wed, 10 Jan 2001 09:10:39 GMT
Message-ID: <93h8ud$ap2$1@nnrp1.deja.com>

Thanks to everyone for the responses, it makes sense regarding losing the ability to control where the data and indexes are allocated when using RAID 5. What still doesn't make sense is how RAID 5 can provide any performance benefit at all. I know there is a write penalty, but I've also read that read performance is faster. I guess I can't fully grasp how even this could be possible since I'd imagine RAID can't allocate contiguous blocks on disks in conjunction with the oracle extents.

In other words if we have extents on indexes and tables etc and concurrent write activity happening...It seems to me the data would be written to the stripes in a very fragmented way. So I would imagine there would be some read penalty especially when performing table or index scans. Is that right?

I agree, RAID-5 sounds like a very bad idea. However, if we don't have the budget to implement RAID 1 or RAID 0+1. What can we do and still maintain a reasonable failure recovery time? Our application is not mission critical. It doesn't need to be up 24/7. We can deal with downtime, but our threshold is about an hour at most.

Is it wise to go with RAID 0 or no RAID at all and then hope that if a failure occurs, we can simply keep redundant drives on hand and replace them right away and then hope all goes well with our DB recovery from our hot backup onto the new drive or are there any other options that we have?

and we are running this on NT/2000

Thanks very much,
Gavin

In article <3a5aece2_at_news.iprimus.com.au>,   "Howard J. Rogers" <howardjr_at_www.com> wrote:
> The idea of separate disk files for indexes, tables, rollback
 segments and
> so on is simply to try and keep the simultaneous i/o's that inevitably
> occurs during normal DML separate from each other. With a RAID array,
> yes -it looks like one disk, but of course is comprised of multiple
 physical
> disks -so, in fact, the RAID array is separating those conflicting
 i/o's out
> for you already.
>
> Hence, no -you won't need multiple RAID arrays. Effectively, you bung
> everything onto the one array, and let the RAID hardware deal with
 the i/o
> problem.
>
> That said, RAID 5 is a real big no-no when it comes to Oracle (or any
 RDBMS
> for that matter)... because whilst the striping bit of RAID 5 is good
> (separating out those i/os across lots of disks), the calculation of
 parity
> bit of RAID 5 makes writes (relatively speaking) extremely slow.
 Certainly,
> the last place you want to use RAID 5 is for the redo log sub-system,
> because the frequent, intensive and sequential nature of the writes
 makes it
> inappropriate, and if your redo subsystem is running slow, the entire
> database will run slow in sympathy. There's an argument that RAID
 5'ing
> your data files is not nearly so bad, because of the scattered nature
 and
> relative infrequency of the writes.
>
> Personally, I wouldn't touch RAID 5 with a barge pole, but pure
 striping
> without the parity (I can never remember my RAID numbers, but I think
 that's
> RAID 0!) is excellent for performance -though, of course, there's no
> redundancy. You will hear people talk about RAID1+0, but the
 mirroring
> element there seems to me to be of dubious benefit, since a cock-up
 on one
> half of the mirror is invariably reflected onto the other half in the
> twinkling of an i/o. The Redo Log/Archive Log mechanism provides
 sufficient
> redundancy for many situations, without the need for hardware
 mirroring. I
> guess it all depends on your budget.
>
> Regards
> HJR
>
> <gdas_at_my-deja.com> wrote in message news:93efdm$5h4
 $1_at_nnrp1.deja.com...
> > Hi,
> > I'm hoping someone can help. I've been doing alot of reading of
 RAID
> > stripping levels and everything seems clear (but no hands-on
 experience
> > yet).
> >
> > I'm still somewhat confused as to how Oracle and RAID 5 work
 together.
> > I've read several posts on this newsgroup and even read the white
> > papers at orapub.com, but I'm confused on one thing still.
> >
> > If you are running a raid 5 array, it is my understanding that all
 the
> > disks in the array appear as one single logical disk. Is that true?
> >
> > If so, how does a DBA allocate his/her database on this array?
> >
> > I've got it fixed in my mind that I need to put data on their own
> > physical disks, indexes on their own and temp/system/rollback on
 their
> > own.
> >
> > If I have 4 physical disks, this is easy to do. But if I have a
 RAID 5
> > array where the entire array appears as one logical disk, what do I
> > do?
> >
> > Does this mean that I need 4 separate arrays? I believe you need at
> > least 3 disks for raid 5...so this means 12 disks? Is that correct?
> >
> > I've read alot about when to and when not use different stripping
> > levels. I'm asking this question from a purely academic
 perspective...
> > Let's say for some reason that you have made the choice to implement
> > oracle on a raid 5 array. How is it done?
> >
> > Thanks for any help.
> > Gavin
> >
> >
> > Sent via Deja.com
> > http://www.deja.com/
>
>

Sent via Deja.com
http://www.deja.com/ Received on Wed Jan 10 2001 - 03:10:39 CST

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