Re: NT Oracle database with RAID-5 hardware controller - Is it good?

In article <901610331.4239.0.nnrp-10.c2de712e_at_news.demon.co.uk>,   "MotoX" <rat_at_tat.a-tat.com> wrote:
> Having done a lot of real world testing on a variety of platforms, I'd add
> (and *always test your own system for accurate data*):
>
> iolo_at_my-dejanews.com wrote in message <6pi34l$h59$1_at_nnrp1.dejanews.com>...
> >> So, in layman's terms:
> >>
> >> READING: Fast
> >
> >Read operations will be faster
>
> Reading is generally as good as striped (RAID0), with RAID5.
>
> >
> >> Writing: A little slower.
> >
>
> Writing can be *substantially* slower. I'd read reports of people seeing
> 100-300% slowdown on writes for RAID5 v normal striping. I didn't really
> believe it - until I went away and did my own testing. Be very careful with
> RAID5 in write operations, the penalty can be much worse than you think.
> That's why you test.
>
> >> But don't forget one of the primary purposes of RAID 5: Fault tolerance.
> if
> >> one of the drives fail, your still up and running. And if you have a hot
> >> spare, it will automatically bring it on line in the event of a bad
> drive.
> >>
> >
> >If complete reduncy of data is required you'd have to use RAID 1.
>
> But this is not necessarily any better than a RAID5 array with a hot-spare.
> I've had a couple of RAID5 drives go down on me and the hot-spares have
> rebuilt just fine.

What if another drive goes down while you're re-building ??

>
> >The striping of data across several drives has other consequences besides
> >balancing I/O. One advantage is that logical system files may be created
> >which are larger than the maximum size normally supported by an operating
> >system
>
> Assuming the OS supports those greater file sizes... And with an LVM you
> don't have to stripe just to create larger logical volumes. (BTW, I tend to
> run raw, which I much prefer via a good LVM.)
>
> >the disadvantage is, however, that it is no longer possible to
> >locate a single datafile on a specific physical drive. This may cause the
> >loss of some application tuning possibilities. Database recovery can also
> be
> >more time-consuming. If one physical disk needs recovery - all the disks
> >which are part of the logical RAID device must be involved in the recovery.
>
> Yep, which is where the RAID1 bit comes in handy (with RAID0).
>
> >
> >Datafiles and archives can be placed on RAID arrays since they are accessed
> >randomly.
>
> Most datawarehouses use nothing but sequential reading and writing in large
> batches. Again, I'd suggest a bit of caution here.
>
> >Redo logs should *NOT* be put on RAID arrays since they are accessed
> > sequentially and therefore performance is enhanced having the disk head
> near
> >the last write location. Mirroring of redo logs, however is strongly
> >recommended.
>
> Not on RAID5, I think you mean.
>

Yes

> >
> >
> >Oracle recommend RAID 1 rather than RAID 5.
>

I meant for redo log files.

> No they don't. *All* the Oracle White Papers and books (and testing) I've
> read/done show the pro's and con's of each RAID level set-up. You make your
> own decision based on your needs and budget. Try convincing a customer with
> a 10TB datawarehouse that RAID1 might be a good idea for all their drives -
> quite an expensive, and maybe unecessary, move.
>
> >
> >
> >Having said all that, there don't seem to be any perceptible performance
> >problems with our Oracle servers under RAID 5. (DIGITAL PRIORIS / AIX)
> >where each individual disk size is 2.5 Gbyte.
>
> If you don't have much heavy write activity then that can be the case. But
> then have you ever compared and thoroughly tested your system on alternative
> set-ups (RAID0, 1 or a mix)? That's the only real way you'd know the
> difference. Most customers don't do that much upfront benchmarking of
> various configurations of their kit, in my experience.
>
> MotoX.
>
>

I was comparing performance under RAID5 to the previous setup without any RAID. Database activity is similar ( with seasonal variations) - compared to the old setup we didn't experience any *significant* deterioration.

Regards

--
Oliver Willandsen
European Commission
http://europa.eu.int
All remarks are my own and do not necessarily reflect offical European Commission policy.

-----== Posted via Deja News, The Leader in Internet Discussion ==----- http://www.dejanews.com/rg_mkgrp.xp Create Your Own Free Member Forum Received on Tue Jul 28 1998 - 09:11:10 CDT