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Re: Fwd: RE: RAID or NOT to RAID?

From: Paul Drake <>
Date: Wed, 08 Aug 2001 23:20:41 -0700
Message-ID: <>

Hi all.
I've been disconnected for awhile - found myself out in Nebraska. Installing 81715/Win2000 on a new Dell PowerEdge 2500. Only had 6 drives on 1 IO channel configured as 3 "containers" - RAID 1. The front grille reminds me of an electric shaver. It was the kinda trip that ended with me pulling a hamstring while unplugging a power cord for my notebook. Gotta stay hydrated and get that daily banana for potassium.

repeat after me ...

RAID 10 != RAID 0+1
RAID 0+1 != RAID 10 Even with only 4 drives, when they might seem the same, RAID 0+1 is stripe first, then mirror.
RAID 10 is mirror first, then stripe.
As someone once said, the best way to tell how its configured, is to pull a drive out of a hot swap bay, put it back in and see how many drives re-silver.
A corollary would be - pull one drive - and then pull another non-adjacent drive (e.g. in the other cage). If its RAID 01 - you're completely hosed.
So much for non-destructive testing. :)

oh yeah, and "it depends".

If you only need 8KB or 64 KB blocks at one time, go for neither, and just separate files onto different RAID 1 volumes of 2 disks each. If you're daring, don't even bother to use hardware RAID for the online redo logs - and just duplex them with multiple log members of a redo log group.

After you locate what your point of contention is - either move the hot spots out to dedicated drives, or add more drives to the volume that has the most I/O.

If you need massive amounts of data from full table scans - go for deeper stripes.
Even numbers of drives in a volume are preferable for RAID 0 stripes, odd for RAID 3,5.
This makes it easy to calculate the stripe depths as a multple of the db_block_size and OS io_size in your head.

Gaja wrote a great section on this topic in the Performance Tuning 101 Book.

And you can "fix" the RAID configuration by simply deleting the existing RAID config - and starting from scratch. I had a site where a Dell Tech took a perfectly good 4 x RAID 1 (8 drives) config and turn it into a single RAID 0+1 config. <Ctrl-A> at boot gets you into where you can wipe it clean and start from scratch - assuming that you can wipe the slate clean.

And I'll beat Joe T to the punch - since you're going to have to re-install the OS - Dell boxes run Linux pretty well. Just that Dell still sucks badly for calling RedHat Linux "Linux 7" on their store website.
That still pisses me off.

Linux != RedHat.

Paul wrote:
> In a previous job I had to deal with this issue. WinNT 4.0 on a dual
> processor Dell box with 24G of RAID. I had specified RAID 0 + 1 but
> "someone" knew better and got it with RAID 5 (5 is obviously better than
> 0). The SA wouldn't or couldn't reconfigure and the job needed to get
> done.
> It was for a decision support system (basically read-only) and it may be
> that sort of a system is less impacted. I abandoned any thought of OFA.
> Just stick all the datafiles out on one directory branch (makes cold
> backups easier) and let the RAID sort out the contention.
> I didn't like it because I was basically trusting to someone else's
> decisions but performance was adequate and the task was successful. I'm
> not sure that this would be true with an OLTP system.
> I've seen the notation RAID 10 (which is RAID 0 + 1). Perhaps we should
> standardize on that. Obviously RAID 10 has to be twice as good as RAID 5.
> Right?
> "Denmark Weatherburne"
> <denmark_weatherburne_at_ho To: Multiple recipients of
>list ORACLE-L
>> <>
> Sent by: cc:
> Subject: Fwd: RE: RAID or
> 08/08/2001 02:47 PM
> Please respond to
> Hi DBA's,
> I hope I'm not opening a can of worms, but I'd like your feedback on the
> issue of using RAID 5 on NT 4.0 with Oracle 8.0.5.

Please see the official ORACLE-L FAQ:
Author: Paul Drake

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Received on Thu Aug 09 2001 - 01:20:41 CDT

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