From: Gene Sais (Gsais@CO.PALM-BEACH.FL.US)
Date: Wed Nov 27 2002 - 14:00:45 EST
A very good analogy posted on the Oracle list, faq at www.orafaq.com.
That is true, up to a point.
Think of the cache as a water tank. You have a garden hose
filling up the tank. You can keep increasing the water
pressure for a while.
But the outlet at the other end of the tank has a fixed
capacity. It flows 10 GPM, and no more.
What happens when you increase the flow at the intake to
20 GPM?
The tank fills up.
When the tank fills up, your intake flow will need to decrease,
because you can only flow 10 GPM at the outlet.
Now, think of the outlet as writing to disk, the RAID5 cache
is the water tank, and your database is the inlet that wants
to run at 20 GPM.
If your database activity will never be intensive enough to
stress the cache like this, no problem. But 'never' is a
very long time.
If any of this sound familiar, Cary Millsap posted a very similar
explanation a few weeks ago.
>>> hhass@FRESNO.CA.GOV 11/27/02 12:00PM >>>
Well, depends on what "suitable" means. RAID-5 has a built in penalty of
about 4 to 1 on writes as compared to no RAID or striping (RAID-0.) Fast
write cache helps throughput, but doesn't fix the inherent 4-1 write issues.
And, like most things, money can help fix that problem. With enough
spindels, RAID-5 can be as fast as RAID-0 with fewer spindels. But, given
all that, if performance is the primary issue, stripe the data accross as
many spindles as you can (RAID-0) and mirror if you want redundancy
(RAID-1;) do them both and you get what many call RAID-10. And separating
your indexes, db's, and logs are database issues and will help depending on
a whole bunch of db stuff.
Myself, I wouldn't even consider putting a production db on a set of disks
that don't have some level of redundancy, I've had to replace too many disks
to risk my job on non-redundant drives. RAID-5 if you must; RAID-10 if you
can.
Harold
-----Original Message-----
From: Gene Sais [mailto:Gsais@CO.PALM-BEACH.FL.US]
Sent: Wednesday, November 27, 2002 8:30 AM
To: aix-l@Princeton.EDU
Subject: Re: How many SSA disks to configure in a RAID array ?
Actually, a myth Raid-5 is suitable for intensive I/O. Nothing beats
Raid-1. If nothing else, put your redo logs on Raid-1. Check out this
link, membership is free and loads of good documentation from some of the
best in the field.
http://www.hotsos.com/dnloads/1.Millsap2000.01.03-RAID5.pdf
hth,
Gene
>>> James.Jackson@MAIL.STATE.AR.US 11/27/02 09:34AM >>>
These days, with fast disk drives, big disk capacities and write-caching
controllers, RAID level is less of a factor than it used to be. As I
understand it, a more critical aspect to DB performance is separation of
workload; i.e., isolating disk I/O for data files vs indexes vs redo
logs. You also have to weigh cost, as well, and you need to consider
how much SSA bandwidth you have and gauge your opportunities for future
growth. If you choose RAID-5, you'd be best served to have fast-write
cache on your SSA card.
Regards,
James Jackson
-----Original Message-----
From: Wesley Joyce [mailto:Wesley.Joyce@UVI.EDU]
Sent: Wednesday, November 27, 2002 8:01 AM
To: aix-l@Princeton.EDU
Subject: Re: How many SSA disks to configure in a RAID array ?
I would like to add a related question. I am obtaining a 7133 with 6
18GB
drives. I remember when it was best to setup a different RAID level
based
on the type of files that would be stored. Is this still recommend or
can
I go with a RAID 5? Data that will be stored will be Oracle data,
Content
Manager and user home directories.
At 04:09 AM 11/27/2002, you wrote:
>Hi sachin
>
>In case you have a single SSA controller , I suppose you would be
better off
>putting all your disk in one raid , so you can stripe across five disks
>instead of 3 or 2 as the case maybe.
>
>Regards
>Shirish joshi
>
>
> -----Original Message-----
>From: Ganu Sachin, IBM [mailto:Sachin.Ganu@KWA2.SIEMENS.CO.IN]
>Sent: 27 November 2002 10:41
>To: aix-l@Princeton.EDU
>Subject: How many SSA disks to configure in a RAID array ?
>
>Hi
>
>I am going to install 5 x 72 GB SSA disks (10K RPM) in RAID 0
configuration
>in a 7133 box. I have a SSA controller with onboard 128 MB SDRAM & 32
MB
>FastWrite cache. Do I configure 1st RAID 0 set with 3 disks in loop A
and
>2nd RAID 0 set with 2 disks in loop B or do I configure these 5 SSA
disks in
>single RAID 0 array on loop A ? I am not bothered redundancy / fault
>tolerance.
>
>Your expert comments please ....
>
>Thanks & regards
>
>Sachin Ganu
>IBM Global Services
>
>
>
>
>---
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Wesley Joyce, Systems Administrator
Center for Administrative Computing (CAC), IT
University of the Virgin Islands
#2 John Brewers Bay, St. Thomas, USVI 00802-9990
(340) 693-1469 (voice) / (340) 693-1465 (fax)
http://www.uvi.edu
"If you can't explain it simply, than you don't know it well enough. -
Unknown."
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